167 lines
7.5 KiB
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167 lines
7.5 KiB
HTML
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Hygiene and the Top-Level (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Hygiene-and-the-Top_002dLevel">
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<p>
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Next: <a href="Internal-Macros.html" accesskey="n" rel="next">Internal Macros</a>, Previous: <a href="Macro-Expansion.html" accesskey="p" rel="prev">Macro Expansion</a>, Up: <a href="Macros.html" accesskey="u" rel="up">Macros</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="Hygiene-and-the-Top_002dLevel-1"><span>6.8.10 Hygiene and the Top-Level<a class="copiable-link" href="#Hygiene-and-the-Top_002dLevel-1"> ¶</a></span></h4>
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<p>Consider the following macro.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define-syntax-rule (defconst name val)
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(begin
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(define t val)
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(define-syntax-rule (name) t)))
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</pre></div>
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<p>If we use it to make a couple of bindings:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(defconst foo 42)
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(defconst bar 37)
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</pre></div>
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<p>The expansion would look something like this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(begin
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(define t 42)
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(define-syntax-rule (foo) t))
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(begin
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(define t 37)
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(define-syntax-rule (bar) t))
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</pre></div>
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<p>As the two <code class="code">t</code> bindings were introduced by the macro, they should
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be introduced hygienically – and indeed they are, inside a lexical
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contour (a <code class="code">let</code> or some other lexical scope). The <code class="code">t</code>
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reference in <code class="code">foo</code> is distinct to the reference in <code class="code">bar</code>.
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</p>
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<p>At the top-level things are more complicated. Before Guile 2.2, a use
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of <code class="code">defconst</code> at the top-level would not introduce a fresh binding
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for <code class="code">t</code>. This was consistent with a weaselly interpretation of the
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Scheme standard, in which all possible bindings may be assumed to exist,
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at the top-level, and in which we merely take advantage of toplevel
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<code class="code">define</code> of an existing binding being equivalent to <code class="code">set!</code>.
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But it’s not a good reason.
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</p>
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<p>The solution is to create fresh names for all bindings introduced by
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macros – not just bindings in lexical contours, but also bindings
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introduced at the top-level.
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</p>
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<p>However, the obvious strategy of just giving random names to introduced
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toplevel identifiers poses a problem for separate compilation. Consider
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without loss of generality a <code class="code">defconst</code> of <code class="code">foo</code> in module
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<code class="code">a</code> that introduces the fresh top-level name <code class="code">t-1</code>. If we
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then compile a module <code class="code">b</code> that uses <code class="code">foo</code>, there is now a
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reference to <code class="code">t-1</code> in module <code class="code">b</code>. If module <code class="code">a</code> is then
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expanded again, for whatever reason, for example in a simple
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recompilation, the introduced <code class="code">t</code> gets a fresh name; say,
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<code class="code">t-2</code>. Now module <code class="code">b</code> has broken because module <code class="code">a</code> no
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longer has a binding for <code class="code">t-1</code>.
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</p>
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<p>If introduced top-level identifiers “escape” a module, in whatever
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way, they then form part of the binary interface (ABI) of a module. It
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is unacceptable from an engineering point of view to allow the ABI to
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change randomly. (It also poses practical problems in meeting the
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recompilation conditions of the Lesser GPL license, for such modules.)
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For this reason many people prefer to never use identifier-introducing
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macros at the top-level, instead making those macros receive the names
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for their introduced identifiers as part of their arguments, or to
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construct them programmatically and use <code class="code">datum->syntax</code>. But this
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approach requires omniscience as to the implementation of all macros one
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might use, and also limits the expressive power of Scheme macros.
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</p>
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<p>There is no perfect solution to this issue. Guile does a terrible thing
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here. When it goes to introduce a top-level identifier, Guile gives the
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identifier a pseudo-fresh name: a name that depends on the hash of the
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source expression in which the name occurs. The result in this case is
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that the introduced definitions expand as:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(begin
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(define t-1dc5e42de7c1050c 42)
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(define-syntax-rule (foo) t-1dc5e42de7c1050c))
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(begin
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(define t-10cb8ce9fdddd6e9 37)
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(define-syntax-rule (bar) t-10cb8ce9fdddd6e9))
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</pre></div>
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<p>However, note that as the hash depends solely on the expression
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introducing the definition, we also have:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(defconst baz 42)
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⇒ (begin
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(define t-1dc5e42de7c1050c 42)
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(define-syntax-rule (baz) t-1dc5e42de7c1050c))
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</pre></div>
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<p>Note that the introduced binding has the same name! This is because the
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source expression, <code class="code">(define t 42)</code>, was the same. Probably you
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will never see an error in this area, but it is important to understand
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the components of the interface of a module, and that interface may
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include macro-introduced identifiers.
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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="Internal-Macros.html">Internal Macros</a>, Previous: <a href="Macro-Expansion.html">Macro Expansion</a>, Up: <a href="Macros.html">Macros</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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</div>
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