Consider (define/memoized (a b c d) form0 form1 form2)
. body allows the body to consist of more than one form.
Without the dot, syntax define/memoized would accept bodies of one form
only,
that is (define/memoized (a b c d) form0) would match, but
(define/memoized (a b c d) form0 form1 form2) would not match.
With the dot (define/memoized (a b c d) form0 form1 form2) matches too.
Another way to achieve the same is:
(define-syntax define/memoized
(syntax-rules ()
((_ (name . args) form ... expr)
(define name (memoize (lambda args form ... expr))))))
or
(define-syntax define/memoized
(syntax-rules ()
((_ (name arg ...) form ... expr)
(define name (memoize (lambda (arg ...) form ... expr))))))
Hope this helps.
Jos
-----Original Message-----
From: racket-users@googlegroups.com [mailto:racket-users@googlegroups.com]
On Behalf Of Pedro Caldeira
Sent: domingo, 13 de marzo de 2016 16:05
To: Racket Users
Subject: [racket-users] Pattern Matching in Macros | Meaning of dot
Hello everyone,
Since I've discovered the concept of metaprogramming I've been quite
interested in Racket and its syntax extension capabilities.
While searching for a memoization syntax extension I found a macro whose
pattern extension remained unclear.
(define-syntax define/memoized
(syntax-rules ()
((_ (name . args) . body)
(define name (memoize (lambda args . body))))))
With the memoization function being defined as such:
(define (memoize f)
(local ([define table (make-hash)])
(lambda args
(dict-ref! table args (lambda () (apply f args))))))
What does the '.' mean in '(name . args)' and '. body'?
In a call like (define/memoized (foo x y z) (displayln "bar"))) I would
imagine that name would be matched to foo; (x y z) to args and body to
(display ln "bar") but why do we use the '. body' in the lambda expression
at the end?
Thank you for your attention.
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