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172 lines
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>SRFI-10 (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="SRFI_002d10">
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<p>
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Next: <a href="SRFI_002d11.html" accesskey="n" rel="next">SRFI-11 - let-values</a>, Previous: <a href="SRFI_002d9.html" accesskey="p" rel="prev">SRFI-9 - define-record-type</a>, Up: <a href="SRFI-Support.html" accesskey="u" rel="up">SRFI Support Modules</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
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<h4 class="subsection" id="SRFI_002d10-_002d-Hash_002dComma-Reader-Extension"><span>7.5.9 SRFI-10 - Hash-Comma Reader Extension<a class="copiable-link" href="#SRFI_002d10-_002d-Hash_002dComma-Reader-Extension"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-SRFI_002d10"></a>
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<a class="index-entry-id" id="index-hash_002dcomma"></a>
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<a class="index-entry-id" id="index-_0023_002c_0028_0029"></a>
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<p>This SRFI implements a reader extension <code class="code">#,()</code> called hash-comma.
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It allows the reader to give new kinds of objects, for use both in data
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and as constants or literals in source code. This feature is available
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with
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</p>
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<div class="example">
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<pre class="example-preformatted">(use-modules (srfi srfi-10))
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</pre></div>
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<p>The new read syntax is of the form
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</p>
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<div class="example">
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<pre class="example-preformatted">#,(<var class="var">tag</var> <var class="var">arg</var>...)
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</pre></div>
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<p>where <var class="var">tag</var> is a symbol and the <var class="var">arg</var>s are objects taken as
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parameters. <var class="var">tag</var>s are registered with the following procedure.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-define_002dreader_002dctor"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">define-reader-ctor</strong> <var class="def-var-arguments">tag proc</var><a class="copiable-link" href="#index-define_002dreader_002dctor"> ¶</a></span></dt>
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<dd><p>Register <var class="var">proc</var> as the constructor for a hash-comma read syntax
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starting with symbol <var class="var">tag</var>, i.e. <code class="code">#,(<var class="var">tag</var> arg…)</code>.
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<var class="var">proc</var> is called with the given arguments <code class="code">(<var class="var">proc</var>
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arg…)</code> and the object it returns is the result of the read.
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</p></dd></dl>
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<p>For example, a syntax giving a list of <var class="var">N</var> copies of an object.
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</p>
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<div class="example">
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<pre class="example-preformatted">(define-reader-ctor 'repeat
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(lambda (obj reps)
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(make-list reps obj)))
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(display '#,(repeat 99 3))
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-| (99 99 99)
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</pre></div>
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<p>Notice the quote <code class="code">'</code> when the <code class="code">#,( )</code> is used. The
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<code class="code">repeat</code> handler returns a list and the program must quote to use
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it literally, the same as any other list. Ie.
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</p>
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<div class="example">
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<pre class="example-preformatted">(display '#,(repeat 99 3))
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⇒
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(display '(99 99 99))
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</pre></div>
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<p>When a handler returns an object which is self-evaluating, like a
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number or a string, then there’s no need for quoting, just as there’s
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no need when giving those directly as literals. For example an
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addition,
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</p>
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<div class="example">
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<pre class="example-preformatted">(define-reader-ctor 'sum
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(lambda (x y)
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(+ x y)))
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(display #,(sum 123 456)) -| 579
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</pre></div>
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<p>Once <code class="code">(srfi srfi-10)</code> has loaded, <code class="code">#,()</code> is available
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globally, there’s no need to use <code class="code">(srfi srfi-10)</code> in later
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modules. Similarly the tags registered are global and can be used
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anywhere once registered.
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</p>
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<p>We do not recommend <code class="code">#,()</code> reader extensions, however, and for
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three reasons.
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</p>
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<p>First of all, this SRFI is not modular: the tag is matched by name, not
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as an identifier within a scope. Defining a reader extension in one
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part of a program can thus affect unrelated parts of a program because
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the tag is not scoped.
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</p>
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<p>Secondly, reader extensions can be hard to manage from a time
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perspective: when does the reader extension take effect? See <a class="xref" href="Eval-When.html">Eval-when</a>, for more discussion.
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</p>
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<p>Finally, reader extensions can easily produce objects that can’t be
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reified to an object file by the compiler. For example if you define a
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reader extension that makes a hash table (see <a class="pxref" href="Hash-Tables.html">Hash Tables</a>), then it
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will work fine when run with the interpreter, and you think you have a
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neat hack. But then if you try to compile your program, after wrangling
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with the <code class="code">eval-when</code> concerns mentioned above, the compiler will
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carp that it doesn’t know how to serialize a hash table to disk.
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</p>
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<p>In the specific case of hash tables, it would be possible for Guile to
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know how to pack hash tables into compiled files, but this doesn’t work
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in general. What if the object you produce is an instance of a record
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type? Guile would then have to serialize the record type to disk too,
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and then what happens if the program independently loads the code that
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defines the record type? Does it define the same type or a different
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type? Guile’s record types are nominal, not structural, so the answer
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is not clear at all.
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</p>
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<p>For all of these reasons we recommend macros over reader extensions.
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Macros fulfill many of the same needs while preserving modular
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composition, and their interaction with <code class="code">eval-when</code> is well-known.
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If you need brevity, instead use <code class="code">read-hash-extend</code> and make your
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reader extension expand to a macro invocation. In that way we preserve
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scoping as much as possible. See <a class="xref" href="Reader-Extensions.html">Reader Extensions</a>.
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</p>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="SRFI_002d11.html">SRFI-11 - let-values</a>, Previous: <a href="SRFI_002d9.html">SRFI-9 - define-record-type</a>, Up: <a href="SRFI-Support.html">SRFI Support Modules</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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