(whoops, should reply all)
I have a macro that does for/fold behavior on some arguments and for/lists on
the other. It's fairly handy.
Here's my solution with that macro:
(let-values ([(_ result)
(for/fold/lists ([s 0]) (result) ([i (in-range 1 21)])
(values (+ s i) (+ s i)))])
result)
And here's the macro for your libraries (uses syntax/parse):
(define-syntax (for/fold/lists stx)
(syntax-parse stx
;; lists go last
[(_ ([fold-target:id fold-start:expr] ...) (lst:id ...) (clauses ...)
body1:expr body:expr ...)
(with-syntax ([(lsttmps ...) (generate-temporaries #'(lst ...))])
#`(let-values ([(fold-target ... lst ... )
(for/fold/derived #,stx ([fold-target fold-start] ...
[lst '()] ...) (clauses ...)
(let-values ([(fold-target ... lsttmps ...) (let ()
body1 body ...)])
(values fold-target ... (cons lsttmps lst) ...)))])
(values fold-target ... (reverse lst) ... )))]
;; lists go first.
[(_ (lst:id ...) ([fold-target:id fold-start:expr] ...) (clauses ...)
body1:expr body:expr ...)
(with-syntax ([(lsttmps ...) (generate-temporaries #'(lst ...))])
#`(let-values ([(lst ... fold-target ...)
(for/fold/derived #,stx ([lst '()] ... [fold-target
fold-start] ...) (clauses ...)
(let-values ([(lsttmps ... fold-target ...) (let ()
body1 body ...)])
(values (cons lsttmps lst) ... fold-target ...)))])
(values (reverse lst) ... fold-target ...)))]))
Haven't gotten around to making a for*/fold/lists form yet.
-Ian
----- Original Message -----
From: "Joe Gilray" <jgil...@gmail.com>
To: "Racket mailing list" <users@racket-lang.org>
Sent: Saturday, April 28, 2012 3:54:31 PM GMT -05:00 US/Canada Eastern
Subject: [racket] idiomatic and fast
Hi,
I'm trying to come up with the most idiomatic way to generate triangle numbers
that is also fast:
(for/list ([i (in-range 2 22)]) (for/sum ([j (in-range i)]) j))
works but is slow because it regenerates the sums each time
(define s 0) (for/list ([i (in-range 1 21)]) (set! s (+ s i)) s)
is fast, but ugly.
How do I keep the sum in an idiomatic fashion?
Thanks,
-Joe
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