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Eric Hanchrow 2013-09-28 11:42:41 -04:00
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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 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 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 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 <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, 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: <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 Let's write a transformer function that reverses the syntax it was
given: 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[ @i[
(define-syntax (reverse-me stx) (define-syntax (reverse-me stx)
(datum->syntax stx (reverse (cdr (syntax->datum 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 () (syntax-case stx ()
[(_ a b (args ...) body0 body ...) [(_ a b (args ...) body0 body ...)
(with-syntax ([name (datum->syntax stx (with-syntax ([name (datum->syntax stx
(string->symbol (format "~a-~a" (string->symbol (format "~a-~a"
(syntax->datum #'a) (syntax->datum #'a)
(syntax->datum #'b))))]) (syntax->datum #'b))))])
#'(define (name args ...) #'(define (name args ...)
@ -1180,7 +1183,7 @@ like JavaScript?
[(_ chain default) [(_ chain default)
(let ([xs (map (lambda (x) (let ([xs (map (lambda (x)
(datum->syntax stx (string->symbol x))) (datum->syntax stx (string->symbol x)))
(regexp-split #rx"\\." (regexp-split #rx"\\."
(symbol->string (syntax->datum #'chain))))]) (symbol->string (syntax->datum #'chain))))])
(with-syntax ([h (car xs)] (with-syntax ([h (car xs)]
[ks (cdr xs)]) [ks (cdr xs)])
@ -1216,7 +1219,7 @@ messages when used in error. Let's try to do that here.
stx #'chain)) stx #'chain))
(let ([xs (map (lambda (x) (let ([xs (map (lambda (x)
(datum->syntax stx (string->symbol x))) (datum->syntax stx (string->symbol x)))
(regexp-split #rx"\\." (regexp-split #rx"\\."
(symbol->string (syntax->datum #'chain))))]) (symbol->string (syntax->datum #'chain))))])
;; Check that we have at least hash.key ;; Check that we have at least hash.key
(unless (and (>= (length xs) 2) (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 syntax introduces a variable that accidentally conflicts with one in
the code surrounding our macro. 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 @hyperlink["http://docs.racket-lang.org/reference/syntax-model.html#(part._transformer-model)" "Transformer
Bindings"], has a good explanation and example. Basically, syntax Bindings"], has a good explanation and example. Basically, syntax
has "marks" to preserve lexical scope. This makes your macro behave has "marks" to preserve lexical scope. This makes your macro behave