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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><meta name="viewport" content="width=device-width, initial-scale=0.8"/><title>8.6 Attaching Contracts to Values</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="extras.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" src="../local-redirect/local-redirect.js"></script><script type="text/javascript" src="../local-redirect/local-user-redirect.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></head><body id="doc-racket-lang-org"><div class="tocset"><div class="tocview"><div class="tocviewlist tocviewlisttopspace"><div class="tocviewtitle"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_0");">►</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x">The Racket Reference</a></td></tr></table></div><div class="tocviewsublisttop" style="display: none;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="model.html" class="tocviewlink" data-pltdoc="x">Language Model</a></td></tr><tr><td align="right">2 </td><td><a href="notation.html" class="tocviewlink" data-pltdoc="x">Notation for Documentation</a></td></tr><tr><td align="right">3 </td><td><a href="syntax.html" class="tocviewlink" data-pltdoc="x">Syntactic Forms</a></td></tr><tr><td align="right">4 </td><td><a href="data.html" class="tocviewlink" data-pltdoc="x">Datatypes</a></td></tr><tr><td align="right">5 </td><td><a href="structures.html" class="tocviewlink" data-pltdoc="x">Structures</a></td></tr><tr><td align="right">6 </td><td><a href="mzlib_class.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">7 </td><td><a href="mzlib_unit.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">8 </td><td><a href="contracts.html" class="tocviewselflink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">9 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Control Flow</a></td></tr><tr><td align="right">11 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Parallelism</a></td></tr><tr><td align="right">12 </td><td><a href="Macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">13 </td><td><a href="input-and-output.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">14 </td><td><a href="security.html" class="tocviewlink" data-pltdoc="x">Reflection and Security</a></td></tr><tr><td align="right">15 </td><td><a href="os.html" class="tocviewlink" data-pltdoc="x">Operating System</a></td></tr><tr><td align="right">16 </td><td><a href="memory.html" class="tocviewlink" data-pltdoc="x">Memory Management</a></td></tr><tr><td align="right">17 </td><td><a href="unsafe.html" class="tocviewlink" data-pltdoc="x">Unsafe Operations</a></td></tr><tr><td align="right">18 </td><td><a href="running.h
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the same <a href="syntax-model.html#%28tech._phase._level%29" class="techoutside" data-pltdoc="x"><span class="techinside">phase level</span></a> as the <span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._provide%29%29" class="RktStxLink" data-pltdoc="x">provide</a></span> form; for example,
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<span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._contract-out%29%29" class="RktStxLink" data-pltdoc="x">contract-out</a></span> cannot be nested within <span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for-syntax%29%29" class="RktStxLink" data-pltdoc="x">for-syntax</a></span>). Each <span class="RktVar">id</span>
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is provided from the module. In
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addition, clients of the module must live up to the contract specified
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by <span class="RktVar">contract-expr</span> for each export.</div></p><p>The <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._contract-out%29%29" class="RktStxLink" data-pltdoc="x">contract-out</a></span> form treats modules as units of
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blame. The module that defines the provided variable is expected to
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meet the positive (co-variant) positions of the contract. Each module
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that imports the provided variable must obey the negative
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(contra-variant) positions of the contract. Each <span class="RktVar">contract-expr</span>
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in a <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._contract-out%29%29" class="RktStxLink" data-pltdoc="x">contract-out</a></span> form is effectively moved to the end of the
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enclosing module, so a <span class="RktVar">contract-expr</span> can refer to variables
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that are defined later in the same module.</p><p>Only uses of the contracted variable outside the module are
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checked. Inside the module, no contract checking occurs.</p><p>The <span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._rename%29%29" class="RktStxLink" data-pltdoc="x">rename</a></span> form of <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._contract-out%29%29" class="RktStxLink" data-pltdoc="x">contract-out</a></span> exports the
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first variable (the internal name) with the name specified by the
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second variable (the external name).</p><p>The <span class="RktSym"><a href="define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span> form of <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._contract-out%29%29" class="RktStxLink" data-pltdoc="x">contract-out</a></span>
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provides a structure-type definition <span class="RktVar">id</span>, and each field has a contract
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that dictates the contents of the fields. Unlike a <span class="RktSym"><a href="define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span>
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definition, however, all of the fields (and their contracts) must be
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listed. The contract on the fields that the sub-struct shares with its
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parent are only used in the contract for the sub-struct’s constructor, and
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the selector or mutators for the super-struct are not provided. The
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exported structure-type name always doubles as a constructor, even if
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the original structure-type name does not act as a constructor.
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If the <span class="RktPn">#:omit-constructor</span> option is present, the constructor
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is not provided. The second form of <span class="RktVar">id/ignored</span>, which has both
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<span class="RktVar">id</span> and <span class="RktVar">ignored-id</span>, is deprecated and allowed
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in the grammar only for backward compatability, where <span class="RktVar">ignored-id</span> is ignored.
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The first form should be used instead.</p><p>Note that if the struct is created with <span class="RktSym"><a href="serialization.html#%28form._%28%28lib._racket%2Fserialize..rkt%29._serializable-struct%29%29" class="RktStxLink" data-pltdoc="x">serializable-struct</a></span>
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or <span class="RktSym"><a href="serialization.html#%28form._%28%28lib._racket%2Fserialize..rkt%29._define-serializable-struct%29%29" class="RktStxLink" data-pltdoc="x">define-serializable-struct</a></span>, <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._contract-out%29%29" class="RktStxLink" data-pltdoc="x">contract-out</a></span> does not
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protect struct instances that are created via
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<span class="RktSym"><a href="serialization.html#%28def._%28%28lib._racket%2Fprivate%2Fserialize..rkt%29._deserialize%29%29" class="RktValLink" data-pltdoc="x">deserialize</a></span>. Consider using <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._struct-guard%2Fc%29%29" class="RktStxLink" data-pltdoc="x">struct-guard/c</a></span> instead.</p><p>The <span class="RktPn">#:∃</span>, <span class="RktPn">#:exists</span>, <span class="RktPn">#:∀</span>, and <span class="RktPn">#:forall</span>
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clauses define new abstract contracts. The variables are bound in the
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remainder of the <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._contract-out%29%29" class="RktStxLink" data-pltdoc="x">contract-out</a></span> form to new contracts that hide
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the values they accept and ensure that the exported functions are treated
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parametrically. See <span class="RktSym"><a href="parametric-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fparametric..rkt%29._new-~e2~88~83%2Fc%29%29" class="RktValLink" data-pltdoc="x">new-∃/c</a></span> and <span class="RktSym"><a href="parametric-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fparametric..rkt%29._new-~e2~88~80%2Fc%29%29" class="RktValLink" data-pltdoc="x">new-∀/c</a></span> for details
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on how the clauses hide the values.</p><p>If <span class="RktPn">#:unprotected-submodule</span> appears, the identifier
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that follows it is used as the name of a submodule that
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<span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._contract-out%29%29" class="RktStxLink" data-pltdoc="x">contract-out</a></span> generates. The submodule exports all
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of the names in the <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._contract-out%29%29" class="RktStxLink" data-pltdoc="x">contract-out</a></span>, but without
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contracts.</p><p>The implementation of <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._contract-out%29%29" class="RktStxLink" data-pltdoc="x">contract-out</a></span> uses
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<span class="RktSym"><a href="stxprops.html#%28def._%28%28quote._~23~25kernel%29._syntax-property%29%29" class="RktValLink" data-pltdoc="x">syntax-property</a></span> to attach properties to the code it generates
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that records the syntax of the contracts in the fully expanded program.
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Specifically, the symbol <a name="(idx._(gentag._196._(lib._scribblings/reference/reference..scrbl)))"></a><span class="RktVal">'</span><span class="RktVal">provide/contract-original-contract</span>
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is bound to vectors of two elements, the exported identifier and a
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syntax object for the expression that produces the contract controlling
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the export.</p><p class="SHistory">Changed in version 7.3.0.3 of package <span class="stt">base</span>: Added <span class="RktPn">#:unprotected-submodule</span>.<br/>Changed in version 7.7.0.9: Started ignoring <span class="RktVar">ignored-id</span>.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/contract/base..rkt)._recontract-out))"></a><span title="Provided from: racket/contract/base, racket/contract, racket | Package: base"><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._recontract-out%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">recontract-out</a></span></span><span class="hspace"> </span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">A <span class="RktVar">provide-spec</span> for use in <span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._provide%29%29" class="RktStxLink" data-pltdoc="x">provide</a></span> (currently,
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just like <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._contract-out%29%29" class="RktStxLink" data-pltdoc="x">contract-out</a></span>, only for
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the same <a href="syntax-model.html#%28tech._phase._level%29" class="techoutside" data-pltdoc="x"><span class="techinside">phase level</span></a> as the <span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._provide%29%29" class="RktStxLink" data-pltdoc="x">provide</a></span> form).</div></p><p>It re-exports <span class="RktVar">id</span>, but with positive blame associated
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to the module containing <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._recontract-out%29%29" class="RktStxLink" data-pltdoc="x">recontract-out</a></span> instead of the
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location of the original site of <span class="RktVar">id</span>.</p><p>This can be useful when a public module wants to export an
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identifier from a private module but where any contract violations
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should be reported in terms of the public module instead of the
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private one.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="module.html#%28form._%28%28quote._~23~25kernel%29._module%29%29" class="RktStxLink" data-pltdoc="x">module</a></span><span class="hspace"> </span><span class="RktSym">private-implementation</span><span class="hspace"> </span><span class="RktSym">racket/base</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktSym">racket/contract</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">recip</span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2F%29%29" class="RktValLink" data-pltdoc="x">/</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">non-zero?</span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._not%29%29" class="RktValLink" data-pltdoc="x">not</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._~3d%29%29" class="RktValLink" data-pltdoc="x">=</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fprovide..rkt%29._provide%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">provide/contract</a></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">recip</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._and%2Fc%29%29" class="RktValLink" data-pltdoc="x">and/c</a></span><span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._real~3f%29%29" class="RktValLink" data-pltdoc="x">real?</a></span><span cl
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<span class="RktSym">recip</span> would result in a contract violation blaming
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the private module.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/contract/private/provide..rkt)._provide/contract))"></a><span title="Provided from: racket/contract/base, racket/contract, racket | Package: base"><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fprovide..rkt%29._provide%2Fcontract%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">provide/contract</a></span></span><span class="hspace"> </span><span class="RktVar">unprotected-submodule</span><span class="hspace"> </span><span class="RktVar">contract-out-item</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">A legacy shorthand for <span class="RktPn">(</span><span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._provide%29%29" class="RktStxLink" data-pltdoc="x">provide</a></span><span class="stt"> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._contract-out%29%29" class="RktStxLink" data-pltdoc="x">contract-out</a></span><span class="stt"> </span><span class="RktVar">unprotected-submodule</span><span class="stt"> </span><span class="RktVar">contract-out-item</span><span class="stt"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span>,
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except that a <span class="RktVar">contract-expr</span> within <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fprovide..rkt%29._provide%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">provide/contract</a></span>
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is evaluated at the position of the <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fprovide..rkt%29._provide%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">provide/contract</a></span> form
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instead of at the end of the enclosing module.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/contract/base..rkt)._struct-guard/c))"></a><span title="Provided from: racket/contract/base, racket/contract, racket | Package: base"><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._struct-guard%2Fc%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">struct-guard/c</a></span></span><span class="hspace"> </span><span class="RktVar">contract-expr</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Returns a procedure suitable to be passed as the <span class="RktPn">#:guard</span>
|
||
|
|
argument to <span class="RktSym"><a href="define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span>, <span class="RktSym"><a href="serialization.html#%28form._%28%28lib._racket%2Fserialize..rkt%29._serializable-struct%29%29" class="RktStxLink" data-pltdoc="x">serializable-struct</a></span> (and related forms).
|
||
|
|
The guard procedure ensures that each contract protects the
|
||
|
|
corresponding field values, as long as the struct is not mutated.
|
||
|
|
Mutations are not protected.</div></p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">snake</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">weight</span><span class="hspace"> </span><span class="RktSym">hungry?</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:guard</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._struct-guard%2Fc%29%29" class="RktStxLink" data-pltdoc="x">struct-guard/c</a></span><span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._real~3f%29%29" class="RktValLink" data-pltdoc="x">real?</a></span><span class="hspace"> </span><span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._boolean~3f%29%29" class="RktValLink" data-pltdoc="x">boolean?</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">snake</span><span class="hspace"> </span><span class="RktVal">1.5</span><span class="hspace"> </span><span class="RktVal">"yep"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">snake, field 2: contract violation</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">expected: boolean?</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">given: "yep"</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: boolean?</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">contract from: top-level</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">blaming: top-level</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">(assuming the contract is correct)</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">at: eval:2:0</span></p></td></tr></table></blockquote></div></p><h5 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Nested_Contract_Boundaries"">8.6.1<tt> </tt><a name="(part._.Nested_.Contract_.Boundaries)"></a><a name="(mod-path._racket/contract/region)"></a>Nested Contract Boundaries</h5><p><table cellspacing="0" cellpadding="0" class="defmodule"><tr><td align="left"><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="stt"> </span><a href="attaching-contracts-to-values.html#%28mod-path._racket%2Fcontract%2Fregion%29" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/contract/region</span></a><span class="RktPn">)</span></td><td align="right"><span class="RpackageSpec"><span class="Smaller"> package:</span> <a href="https://pkgs.racket-lang.org/package/base" title="Install this package using `raco pkg install base`"><span class="stt">b
|
||
|
|
All names defined within the first <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._with-contract%29%29" class="RktStxLink" data-pltdoc="x">with-contract</a></span> form are
|
||
|
|
visible externally, but those names listed in the <span class="RktVar">wc-export</span>
|
||
|
|
list are protected with the corresponding contract. The <span class="RktVar">body</span> of
|
||
|
|
the form allows definition/expression interleaving if its context does.</p><p>The second <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._with-contract%29%29" class="RktStxLink" data-pltdoc="x">with-contract</a></span> form must appear in expression position.
|
||
|
|
The final <span class="RktVar">body</span> expression should return the same number of values
|
||
|
|
as the number of contracts listed in the <span class="RktVar">result-spec</span>, and each
|
||
|
|
returned value is contracted with its respective contract. The sequence
|
||
|
|
of <span class="RktVar">body</span> forms is treated as for <span class="RktSym"><a href="let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span>.</p><p>The <span class="RktVar">blame-id</span> is used for the positive positions of
|
||
|
|
contracts paired with exported <span class="RktVar">id</span>s. Contracts broken
|
||
|
|
within the <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._with-contract%29%29" class="RktStxLink" data-pltdoc="x">with-contract</a></span> <span class="RktVar">body</span> will use the
|
||
|
|
<span class="RktVar">blame-id</span> for their negative position.</p><p>If a <span class="RktVar">free-var-list</span> is given, then any uses of the free variables
|
||
|
|
inside the <span class="RktVar">body</span> will be protected with contracts that
|
||
|
|
blame the context of the <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._with-contract%29%29" class="RktStxLink" data-pltdoc="x">with-contract</a></span> form for the positive
|
||
|
|
positions and the <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._with-contract%29%29" class="RktStxLink" data-pltdoc="x">with-contract</a></span> form for the negative ones.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/contract/region..rkt)._define/contract))"></a><span title="Provided from: racket/contract/region, racket/contract, racket | Package: base"><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._define%2Fcontract%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">define/contract</a></span></span><span class="hspace"> </span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">contract-expr</span><span class="hspace"> </span><span class="RktVar">free-var-list</span><span class="hspace"> </span><span class="RktVar">init-value-expr</span><span class="RktPn">)</span></p></blockquote></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._define%2Fcontract%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">define/contract</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">head</span><span class="hspace"> </span><span class="RktVar">args</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">contract-expr</span><span class="hspace"> </span><span class="RktVar">free-var-list</span><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></td></tr></table></blockquote></div><div class="SIntrapara">Works like <span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span>, except that the contract
|
||
|
|
<span class="RktVar">contract-expr</span> is attached to the bound value. For the
|
||
|
|
definition of <span class="RktVar">head</span> and <span class="RktVar">args</span>, see <span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span>.
|
||
|
|
For the definition of <span class="RktVar">free-var-list</span>, see <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._with-contract%29%29" class="RktStxLink" data-pltdoc="x">with-contract</a></span>.</div></p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._define%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">define/contract</a></span><span class="hspace"> </span><span class="RktSym">distance</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._~3e~3d%2Fc%29%29" class="RktValLink" data-pltdoc="x">>=/c</a></span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">43.52</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._define%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">define/contract</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">furlongs->feet</span><span class="hspace"> </span><span class="RktSym">fr</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._real~3f%29%29" class="RktValLink" data-pltdoc="x">real?</a></span><span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._real~3f%29%29" class="RktValLink" data-pltdoc="x">real?</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2A%29%29" class="RktValLink" data-pltdoc="x">*</a></span><span class="hspace"> </span><span class="RktVal">660</span><span class="hspace"> </span><span class="RktSym">fr</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">a contract violation expected here:</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">furlongs->feet</span><span class="hspace"> </span><span class="RktVal">"not a furlong"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">furlongs->feet: contract violation</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">expected: real?</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">given: "not a furlong"</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: the 1st argument of</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">(-&g
|
||
|
|
a contract region. The definition itself is responsible for positive
|
||
|
|
(co-variant) positions of the contract, and references to
|
||
|
|
<span class="RktVar">id</span> outside of the definition must meet the negative
|
||
|
|
positions of the contract. Since the contract boundary is
|
||
|
|
between the definition and the surrounding context, references to
|
||
|
|
<span class="RktVar">id</span> inside the <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._define%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">define/contract</a></span> form are not checked.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">an unsual predicate that prints when called</span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">printing-int?</span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="Writing.html#%28def._%28%28lib._racket%2Fprivate%2Fmisc..rkt%29._displayln%29%29" class="RktValLink" data-pltdoc="x">displayln</a></span><span class="hspace"> </span><span class="RktVal">"I was called"</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._exact-integer~3f%29%29" class="RktValLink" data-pltdoc="x">exact-integer?</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._define%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">define/contract</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">fact</span><span class="hspace"> </span><span class="RktSym">n</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktSym">printing-int?</span><span class="hspace"> </span><span class="RktSym">printing-int?</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="if.html#%28form._%28%28quote._~23~25kernel%29._if%29%29" class="RktStxLink" data-pltdoc="x">if</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._zero~3f%29%29" class="RktValLink" data-pltdoc="x">zero?</a></span><span class="hspace"> </span><span class="RktSym">n</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">1</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2A%29%29" class="RktValLink" data-pltdoc="x">*</a></span><span class="hspace"> </span><span class="RktSym">n</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">fact</span>
|
||
|
|
inside the <span class="RktVar">body</span> will be protected with contracts that
|
||
|
|
blame the context of the <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._define%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">define/contract</a></span> form for the positive
|
||
|
|
positions and the <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._define%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">define/contract</a></span> form for the negative ones.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">integer->binary-string</span><span class="hspace"> </span><span class="RktSym">n</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._number-~3estring%29%29" class="RktValLink" data-pltdoc="x">number->string</a></span><span class="hspace"> </span><span class="RktSym">n</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._define%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">define/contract</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">numbers->strings</span><span class="hspace"> </span><span class="RktSym">lst</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._listof%29%29" class="RktValLink" data-pltdoc="x">listof</a></span><span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._number~3f%29%29" class="RktValLink" data-pltdoc="x">number?</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._listof%29%29" class="RktValLink" data-pltdoc="x">listof</a></span><span class="hspace"> </span><span class="RktSym"><a href="strings.html#%28def._%28%28quote._~23~25kernel%29._string~3f%29%29" class="RktValLink" data-pltdoc="x">string?</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:freevar</span><span class="hspace"> </span><span class="RktSym">integer->binary-string</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._exact-integer~3f%29%29" class="RktValLink" data-pltdoc="x">exact-integer?</a></span><span class="hspace"> </span><span class="RktSym"><a href="strings.html#%28def._%28%28quote._~23~25kernel%29._string~3f%29%29" class="RktValLink" data-pltdoc="x">string?</a></span><span
|
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accessors, and mutators are protected by contracts. For the definitions of
|
||
|
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<span class="RktVar">field</span> and <span class="RktVar">struct-option</span>, see <span class="RktSym"><a href="define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span>.</div></p><p>The <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._struct%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">struct/contract</a></span> form only allows a subset of the
|
||
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<span class="RktVar">struct-option</span> keywords: <span class="RktPn">#:mutable</span>, <span class="RktPn">#:transparent</span>,
|
||
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<span class="RktPn">#:auto-value</span>, <span class="RktPn">#:omit-define-syntaxes</span>, and <span class="RktPn">#:property</span>.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._struct%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">struct/contract</a></span><span class="hspace"> </span><span class="RktSym">fruit</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">seeds</span><span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._number~3f%29%29" class="RktValLink" data-pltdoc="x">number?</a></span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">fruit</span><span class="hspace"> </span><span class="RktVal">60</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#<fruit></span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">fruit</span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">fruit: contract violation</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">expected: number?</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">given: #f</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: the 1st argument of</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">(-> number? symbol? any)</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">contract from: (struct fruit)</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">blaming: top-level</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">(assuming the contract is correct)</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._struct%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">struct/contract</a></span><span class="hspace"> </span><span class="RktSym">apple</span><span class="hspace"> </span><span class="RktSym">fruit</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">type</span><span class="hspace"> </span><span class="RktSym"><a href="strings.html#%28def._%28%28quote._~23~25kernel%29._string~3f%29%29" class="RktValLink" data-pltdoc="x">string?</a></span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">apple</span><span class="hspace"> </span><span class="RktVal">14</span><span class="hspace"> </span><span class="RktVal">"golden delicious"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#<apple></span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">apple</span><span class="hspace"> </span><span class="RktVal">5
|
||
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<span class="RktVar">super-struct-id</span> is different, and a <span class="RktVar">constructor-id</span> that
|
||
|
|
has a <span class="RktSym">make-</span> prefix on <span class="RktVar">struct-id</span> is implicitly
|
||
|
|
supplied. For the definitions of
|
||
|
|
<span class="RktVar">field</span> and <span class="RktVar">struct-option</span>, see <span class="RktSym"><a href="define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-struct%29%29" class="RktStxLink" data-pltdoc="x">define-struct</a></span>.
|
||
|
|
Like <span class="RktSym"><a href="define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span> versus <span class="RktSym"><a href="define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-struct%29%29" class="RktStxLink" data-pltdoc="x">define-struct</a></span>,
|
||
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|
<span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._struct%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">struct/contract</a></span> is normally preferred to <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._define-struct%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">define-struct/contract</a></span>.</div></p><p>The <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._define-struct%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">define-struct/contract</a></span> form only allows a subset of the
|
||
|
|
<span class="RktVar">struct-option</span> keywords: <span class="RktPn">#:mutable</span>, <span class="RktPn">#:transparent</span>,
|
||
|
|
<span class="RktPn">#:auto-value</span>, <span class="RktPn">#:omit-define-syntaxes</span>, and <span class="RktPn">#:property</span>.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._define-struct%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">define-struct/contract</a></span><span class="hspace"> </span><span class="RktSym">fish</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">color</span><span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._number~3f%29%29" class="RktValLink" data-pltdoc="x">number?</a></span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">make-fish</span><span class="hspace"> </span><span class="RktVal">5</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#<fish></span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">make-fish</span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">make-fish: contract violation</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">expected: number?</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">given: #f</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: the 1st argument of</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">(-> number? symbol? any)</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">contract from: (struct fish)</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">blaming: top-level</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">(assuming the contract is correct)</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._define-struct%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">define-struct/contract</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">salmon</span><span class="hspace"> </span><span class="RktSym">fish</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">ocean</span><span class="hspace"> </span><span class="RktSym"><a href="symbols.html#%28def._%28%28quote._~23~25kernel%29._symbol~3f%29%29" class="RktValLink" data-pltdoc="x">symbol?</a></span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">make-salmon</span><span class="hspace"> </span><span class="RktVal">5</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">atlantic</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#<salmon></span></p></td></tr><tr><td><span class="stt">> </span
|
||
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<span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fbase..rkt%29._invariant-assertion%29%29" class="RktStxLink" data-pltdoc="x">invariant-assertion</a></span> does not establish a boundary
|
||
|
|
between two parties. Instead, it simply attaches a logical assertion
|
||
|
|
to the value. Because the form uses contract machinery to check the
|
||
|
|
assertion, the surrounding module is treated as the party to be blamed
|
||
|
|
for any violations of the assertion.</p><p>This means, for example, that the assertion is checked on
|
||
|
|
recursive calls, when an invariant is used on the right-hand
|
||
|
|
side of a definition:</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">furlongss->feets</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fbase..rkt%29._invariant-assertion%29%29" class="RktStxLink" data-pltdoc="x">invariant-assertion</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._listof%29%29" class="RktValLink" data-pltdoc="x">listof</a></span><span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._real~3f%29%29" class="RktValLink" data-pltdoc="x">real?</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._listof%29%29" class="RktValLink" data-pltdoc="x">listof</a></span><span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._real~3f%29%29" class="RktValLink" data-pltdoc="x">real?</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._~ce~bb%29%29" class="RktStxLink" data-pltdoc="x">λ</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">l</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._cond%29%29" class="RktStxLink" data-pltdoc="x">cond</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28lib._racket%2Flist..rkt%29._empty~3f%29%29" class="RktValLink" data-pltdoc="x">empty?</a></span><span class="hspace"> </span><span class="RktSym">l</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="pairs.html#%28def._%28%28lib._racket%2Flist..rkt%29._empty%29%29" class="RktValLink" data-pltdoc="x">empty</a></span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._else%29%29" class="RktStxLink" data-pltdoc="x">else</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="if.html#%28form._%28%28quote._~23~25kernel%29._if%29%29" class="RktStx
|
||
|
|
information about the current contract region, used by
|
||
|
|
the above forms to determine the candidates for blame
|
||
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assignment.</div></p><h5 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Low-level_Contract_Boundaries"">8.6.2<tt> </tt><a name="(part._.Low-level_.Contract_.Boundaries)"></a>Low-level Contract Boundaries</h5><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><table cellspacing="0" cellpadding="0" class="RktBlk RForeground"><tr><td><span class="RktPn">(</span><a name="(form._((lib._racket/contract/private/provide..rkt)._define-module-boundary-contract))"></a><span title="Provided from: racket/contract/base, racket/contract, racket | Package: base"><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fprovide..rkt%29._define-module-boundary-contract%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">define-module-boundary-contract</a></span></span><span class="hspace"> </span><span class="RktVar">id</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">orig-id</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">contract-expr</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">pos-blame-party</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">source-loc</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">name-for-blame</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">context-limit</span><span class="RktPn">)</span></td></tr></table></blockquote></td></tr><tr><td><span class="stt"> </span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="specgrammar"><tr><td align="right" valign="baseline"><span class="RktVar">pos-blame-party</span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">=</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td></td></tr></table></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:pos-source</span><span class="hspace"> </span><span class="RktVar">pos-source-expr</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td></tr><tr><td align="right" valign="baseline"><span class="RktVar">source-loc</span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">=</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td></td></tr></table></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:srcloc</span><span class="hspace"> </span><span class="RktVar">srcloc-expr</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="cen
|
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<span class="RktVar">contract-expr</span>.</div></p><p>The identifier <span class="RktVar">id</span> is defined as a macro transformer that
|
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consults the context of its use to determine the name for negative
|
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blame assignment (using the entire module where a reference appears
|
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as the negative party).</p><p>The positive party defaults to the module containing the use of
|
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<span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fprovide..rkt%29._define-module-boundary-contract%29%29" class="RktStxLink" data-pltdoc="x">define-module-boundary-contract</a></span>, but can be specified explicitly
|
||
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via the <span class="RktPn">#:pos-source</span> keyword.</p><p>The source location used in the blame error messages for the location
|
||
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of the place where the contract was put on the value defaults to the
|
||
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source location of the use of <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fprovide..rkt%29._define-module-boundary-contract%29%29" class="RktStxLink" data-pltdoc="x">define-module-boundary-contract</a></span>,
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but can be specified via the <span class="RktPn">#:srcloc</span> argument, in which case
|
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it can be any of the things that the third argument to <span class="RktSym"><a href="stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum->syntax</a></span>
|
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can be.</p><p>The name used in the error messages will be <span class="RktVar">orig-id</span>, unless
|
||
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<span class="RktPn">#:name-for-blame</span> is supplied, in which case the identifier
|
||
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following it is used as the name in the error messages.</p><p>If <span class="RktPn">#:context-limit</span> is supplied, it behaves the same as
|
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it does when supplied to <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fbase..rkt%29._contract%29%29" class="RktStxLink" data-pltdoc="x">contract</a></span>.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="module.html#%28form._%28%28quote._~23~25kernel%29._module%29%29" class="RktStxLink" data-pltdoc="x">module</a></span><span class="hspace"> </span><span class="RktSym">server</span><span class="hspace"> </span><span class="RktSym">racket/base</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktSym">racket/contract/base</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">f</span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fprovide..rkt%29._define-module-boundary-contract%29%29" class="RktStxLink" data-pltdoc="x">define-module-boundary-contract</a></span><span class="hspace"> </span><span class="RktSym">g</span><span class="hspace"> </span><span class="RktSym">f</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._integer~3f%29%29" class="RktValLink" data-pltdoc="x">integer?</a></span><span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._integer~3f%29%29" class="RktValLink" data-pltdoc="x">integer?</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._provide%29%29" class="RktStxLink" data-pltdoc="x">provide</a></span><span class="hspace"> </span><span class="RktSym">g</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="module.html#%28form._%28%28quote._~23~25kernel%29._module%29%29" class="RktStxLink" data-pltdoc="x">module</a></span><span class="hspace"> </span><span class="RktSym">client</span><span class="hspace"> </span><span class="RktSym">racket/base</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a><
|
||
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purpose of <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fbase..rkt%29._contract%29%29" class="RktStxLink" data-pltdoc="x">contract</a></span> is as a target for the expansion of some
|
||
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higher-level contract specifying form.</div></p><p>The <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fbase..rkt%29._contract%29%29" class="RktStxLink" data-pltdoc="x">contract</a></span> expression adds the contract specified by
|
||
|
|
<span class="RktVar">contract-expr</span> to the value produced by
|
||
|
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<span class="RktVar">to-protect-expr</span>. The result of a <span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fbase..rkt%29._contract%29%29" class="RktStxLink" data-pltdoc="x">contract</a></span> expression
|
||
|
|
is the result of the <span class="RktVar">to-protect-expr</span> expression, but with the
|
||
|
|
contract specified by <span class="RktVar">contract-expr</span> enforced on
|
||
|
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<span class="RktVar">to-protect-expr</span>.</p><p>The values of <span class="RktVar">positive-blame-expr</span> and <span class="RktVar">negative-blame-expr</span>
|
||
|
|
indicate how to assign blame for positive and negative positions of the contract
|
||
|
|
specified by <span class="RktVar">contract-expr</span>. They may be any value, and are formatted
|
||
|
|
as by <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._display%29%29" class="RktValLink" data-pltdoc="x">display</a></span> for purposes of contract violation error messages.</p><p><div class="SIntrapara">If specified, <span class="RktVar">value-name-expr</span> indicates a name for the protected value
|
||
|
|
to be used in error messages. If not supplied, or if <span class="RktVar">value-name-expr</span>
|
||
|
|
produces <span class="RktVal">#f</span>, no name is printed. Otherwise, it is also formatted as by
|
||
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|
<span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._display%29%29" class="RktValLink" data-pltdoc="x">display</a></span>. More precisely, the <span class="RktVar">value-name-expr</span> ends up in the
|
||
|
|
<span class="RktSym">blame-name</span> field of the blame record, which is used as the first portion
|
||
|
|
of the error message.
|
||
|
|
</div><div class="SIntrapara"><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fbase..rkt%29._contract%29%29" class="RktStxLink" data-pltdoc="x">contract</a></span><span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._integer~3f%29%29" class="RktValLink" data-pltdoc="x">integer?</a></span><span class="hspace"> </span><span class="RktVal">#f</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">pos</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">neg</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">timothy</span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">timothy: broke its own contract</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">promised: integer?</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">produced: #f</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: integer?</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">contract from: pos</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">blaming: pos</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">(assuming the contract is correct)</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fbase..rkt%29._contract%29%29" class="RktStxLink" data-pltdoc="x">contract</a></span><span class="hspace"> </span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._integer~3f%29%29" class="RktValLink" data-pltdoc="x">integer?</a></span><span class="hspace"> </span><span class="RktVal">#f</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">pos</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">neg</span><span class="hspace"> </span><span class="RktVal">#f</span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">broke its own contract</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">promised: integer?</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">produced: #f</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: integer?</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">contract from: pos</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">blaming: pos</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">(assuming the contract is correct)</span></p></td></tr></table></blockquote></div></p></div></p><p>If specified, <span class="RktVar">source-location-expr</span> indicates the source location
|
||
|
|
reported by contract violations. The expression must produce a <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._srcloc%29%29" class="RktValLink" data-pltdoc="x">srcloc</a></span>
|
||
|
|
structure, <a href="syntax-model.html#%28tech._syntax._object%29" class="techoutside" data-pltdoc="x"><span class="techinside">syntax object</span></a>, <span class="RktVal">#f</span>, or a list or vector in the format
|
||
|
|
accepted by the third argument to <span class="RktSym"><a href="stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum->syntax</a></span>.</p><p>If <span class="RktPn">#:context-limit</span> is supplied, the following expression
|
||
|
|
must evaluate to either <span class="RktVal">#f</span> or a natural number. If
|
||
|
|
the expression evaluates to an natural number, the number of
|
||
|
|
layers of context information is limited to at most that
|
||
|
|
many. For example, if the number is <span class="RktVal">0</span>, no context
|
||
|
|
information is recorded and the error messages do not contain
|
||
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the section that starts with <span class="RktInBG"><span class="hspace"></span><span class="RktIn">in:</span><span class="hspace"></span></span>.</p><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" id="searchbox" type="text" tabindex="1" placeholder="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "8.6", "../");"/></form> <a href="https://docs.racket-lang.org/index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("8.6");">top</a><span class="tocsettoggle"> <a href="javascript:void(0);" title="show/hide table of contents" onclick="TocsetToggle();">contents</a></span></span><span class="navright"> <a href="Structure_Type_Property_Contracts.html" title="backward to "8.5 Structure Type Property Contracts"" data-pltdoc="x">← prev</a> <a href="contracts.html" title="up to "8 Contracts"" data-pltdoc="x">up</a> <a href="Building_New_Contract_Combinators.html" title="forward to "8.7 Building New Contract Combinators"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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