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Greg Hendershott 2013-09-28 10:55:47 -07:00
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@ -36,7 +36,7 @@ and so on recursively&#8212;<wbr></wbr>which is what <span class="RktSym"><a hre
not get side-tracked now.)</p><p>When we want to transform syntax, we&rsquo;ll generally take the pieces we
were given, maybe rearrange their order, perhaps change some of the
pieces, and often introduce brand-new pieces.</p><h4>3.3<tt>&nbsp;</tt><a name="(part._.Actually_transforming_the_input)"></a>Actually transforming the input</h4><p>Let&rsquo;s write a transformer function that reverses the syntax it was
given:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">&gt; </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym">reverse-me</span><span class="hspace">&nbsp;</span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace">&nbsp;&nbsp;</span></span><span class="hspace">&nbsp;&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum-&gt;syntax</a></span><span class="hspace">&nbsp;</span><span class="RktSym">stx</span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28lib._racket%2Fprivate%2Flist..rkt%29._reverse%29%29" class="RktValLink" data-pltdoc="x">reverse</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._cdr%29%29" class="RktValLink" data-pltdoc="x">cdr</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3edatum%29%29" class="RktValLink" data-pltdoc="x">syntax-&gt;datum</a></span><span class="hspace">&nbsp;</span><span class="RktSym">stx</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">&gt; </span><span class="RktPn">(</span><span class="RktSym">reverse-me</span><span class="hspace">&nbsp;</span><span class="RktVal">"backwards"</span><span class="hspace">&nbsp;</span><span class="RktVal">"am"</span><span class="hspace">&nbsp;</span><span class="RktVal">"i"</span><span class="hspace">&nbsp;</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/values.html#%28def._%28%28quote._~23~25kernel%29._values%29%29" class="RktValLink" data-pltdoc="x">values</a></span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"i"</span></p></td></tr><tr><td><p><span class="RktRes">"am"</span></p></td></tr><tr><td><p><span class="RktRes">"backwards"</span></p></td></tr></table></blockquote><p>Understand Yoda, can we. Great, but how does this work?</p><p>First we take the input syntax, and give it to
given:</p><p><div class="SIntrapara"><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>the <span class="RktSym"><a href="http://docs.racket-lang.org/reference/values.html#%28def._%28%28quote._~23~25kernel%29._values%29%29" class="RktValLink" data-pltdoc="x">values</a></span> at the end allows the result to evaluate nicely. Try <span class="RktPn">(</span><span class="RktSym">reverse-me</span><span class="stt"> </span><span class="RktVal">"backwards"</span><span class="stt"> </span><span class="RktVal">"am"</span><span class="stt"> </span><span class="RktVal">"i"</span><span class="RktPn">)</span> to see why it&rsquo;s handy.</p></blockquote></blockquote></blockquote></div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">&gt; </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym">reverse-me</span><span class="hspace">&nbsp;</span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace">&nbsp;&nbsp;</span></span><span class="hspace">&nbsp;&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum-&gt;syntax</a></span><span class="hspace">&nbsp;</span><span class="RktSym">stx</span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28lib._racket%2Fprivate%2Flist..rkt%29._reverse%29%29" class="RktValLink" data-pltdoc="x">reverse</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._cdr%29%29" class="RktValLink" data-pltdoc="x">cdr</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3edatum%29%29" class="RktValLink" data-pltdoc="x">syntax-&gt;datum</a></span><span class="hspace">&nbsp;</span><span class="RktSym">stx</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">&gt; </span><span class="RktPn">(</span><span class="RktSym">reverse-me</span><span class="hspace">&nbsp;</span><span class="RktVal">"backwards"</span><span class="hspace">&nbsp;</span><span class="RktVal">"am"</span><span class="hspace">&nbsp;</span><span class="RktVal">"i"</span><span class="hspace">&nbsp;</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/values.html#%28def._%28%28quote._~23~25kernel%29._values%29%29" class="RktValLink" data-pltdoc="x">values</a></span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"i"</span></p></td></tr><tr><td><p><span class="RktRes">"am"</span></p></td></tr><tr><td><p><span class="RktRes">"backwards"</span></p></td></tr></table></blockquote></div></p><p>Understand Yoda, can we. Great, but how does this work?</p><p>First we take the input syntax, and give it to
<span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3edatum%29%29" class="RktValLink" data-pltdoc="x">syntax-&gt;datum</a></span>. This converts the syntax into a plain old
list:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">&gt; </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3edatum%29%29" class="RktValLink" data-pltdoc="x">syntax-&gt;datum</a></span><span class="hspace">&nbsp;</span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym">reverse-me</span><span class="hspace">&nbsp;</span><span class="RktVal">"backwards"</span><span class="hspace">&nbsp;</span><span class="RktVal">"am"</span><span class="hspace">&nbsp;</span><span class="RktVal">"i"</span><span class="hspace">&nbsp;</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/values.html#%28def._%28%28quote._~23~25kernel%29._values%29%29" class="RktValLink" data-pltdoc="x">values</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'(reverse-me<span class="stt"> </span>"backwards"<span class="stt"> </span>"am"<span class="stt"> </span>"i"<span class="stt"> </span>values)</span></p></td></tr></table></blockquote><p>Using <span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._cdr%29%29" class="RktValLink" data-pltdoc="x">cdr</a></span> slices off the first item of the list,
<span class="RktSym">reverse-me</span>, leaving the remainder:

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@ -318,6 +318,9 @@ pieces, and often introduce brand-new pieces.
Let's write a transformer function that reverses the syntax it was
given:
@margin-note{The @racket[values] at the end of the example allows the
result to evaluate nicely. Try
@racket[(reverse-me "backwards" "am" "i")] to see why it's handy.}
@i[
(define-syntax (reverse-me stx)
(datum->syntax stx (reverse (cdr (syntax->datum stx)))))
@ -855,7 +858,7 @@ if we need to define more than one pattern variable.
(syntax-case stx ()
[(_ a b (args ...) body0 body ...)
(with-syntax ([name (datum->syntax stx
(string->symbol (format "~a-~a"
(string->symbol (format "~a-~a"
(syntax->datum #'a)
(syntax->datum #'b))))])
#'(define (name args ...)
@ -1180,7 +1183,7 @@ like JavaScript?
[(_ chain default)
(let ([xs (map (lambda (x)
(datum->syntax stx (string->symbol x)))
(regexp-split #rx"\\."
(regexp-split #rx"\\."
(symbol->string (syntax->datum #'chain))))])
(with-syntax ([h (car xs)]
[ks (cdr xs)])
@ -1216,7 +1219,7 @@ messages when used in error. Let's try to do that here.
stx #'chain))
(let ([xs (map (lambda (x)
(datum->syntax stx (string->symbol x)))
(regexp-split #rx"\\."
(regexp-split #rx"\\."
(symbol->string (syntax->datum #'chain))))])
;; Check that we have at least hash.key
(unless (and (>= (length xs) 2)
@ -1286,7 +1289,7 @@ certain kind of mistake in our macros. The mistake is if our new
syntax introduces a variable that accidentally conflicts with one in
the code surrounding our macro.
The Racket @italic{Reference} section,
The Racket @italic{Reference} section,
@hyperlink["http://docs.racket-lang.org/reference/syntax-model.html#(part._transformer-model)" "Transformer
Bindings"], has a good explanation and example. Basically, syntax
has "marks" to preserve lexical scope. This makes your macro behave