On Jul 5, 2014, at 2:25 PM, Alexander D. Knauth <alexan...@knauth.org> wrote:
> > On Jul 5, 2014, at 12:10 PM, Brian Adkins <racketus...@lojic.com> wrote: > >> Is match typically used to destructure lists? > > Match can be used for a lot of things, but (looking ahead) you might want to > look at one of the match-lambda s: > > http://docs.racket-lang.org/reference/match.html?q=match-lambda#%28form._%28%28lib._racket%2Fmatch..rkt%29._match-lambda%29%29 > > Probably the one you want is either match-lambda* or maybe match-lambda**. Also you might want to look at match-define. >> I used it a few times in the program, but in the following example, a couple >> defines w/ second & third seemed better given the complexity of the nested >> folds. > > I think nested folds are discouraged in favor of for loops: > http://docs.racket-lang.org/style/Choosing_the_Right_Construct.html?q=match-lambda#%28part._.Traversals%29 > > By the way (to the rest of the list) is there something like a > for/fold/contract that allows contracts in similar places as for > for/fold in typed racket? > > Because for/fold made no sense to me until I tried for/lists and for/fold in > typed racket, and types/contracts make it > easier for me to wrap my head around what it’s doing (at least for this > case). I haven’t tested it, but would this work for what you want?: (define (summed-history history) (for/fold ([hsh (hash)]) ([3tuple (in-list history)]) (match-define (list _ rest users) 3tuple) (for/fold ([hsh hsh]) ([user (in-list users)]) (hash-set hsh (list user rest) (+ 1 (hash-ref hsh (list user rest) 0)))))) By the way (to the rest of the list) is there anything like a for/match that would allow something like this?: (for/fold/match ([hsh (hash)]) ([(list _ rest users) (in-list history)]) ...) And if there was, would this work?: (for*/fold/match ([hsh (hash)]) ([(list _ rest users) (in-list history)] [user (in-list users)]) (hash-set hsh (list user rest) (+ 1 (hash-ref hsh (list user rest) 0))) > >> (define (summed-history history) >> (foldl (λ (3tuple hsh) >> (define rest (second 3tuple)) >> (define users (third 3tuple)) >> (foldl (λ (user hsh) >> (hash-set hsh (list user rest) (+ 1 (hash-ref hsh (list >> user rest) 0)))) >> hsh >> users)) >> (hash) >> history)) >> >> vs. >> >> (define (summed-history history) >> (foldl (λ (3tuple hsh) >> (match 3tuple >> [(list _ rest users) >> (foldl (λ (user hsh) >> (hash-set hsh >> (list user rest) >> (+ 1 (hash-ref hsh (list user rest) 0)))) >> hsh >> users)])) >> (hash) >> history)) >> >> What I'd really like is to replace the 3tuple arg with a pattern: >> >> (define (summed-history history) >> (foldl (λ ((_ rest users) hsh) >> (foldl (λ (user hsh) >> (hash-set hsh (list user rest) (+ 1 (hash-ref hsh (list >> user rest) 0)))) >> hsh >> users)) >> (hash) >> history)) > > I think one of the match-lambda s would be a good thing for that. > > http://docs.racket-lang.org/reference/match.html?q=match-lambda#%28form._%28%28lib._racket%2Fmatch..rkt%29._match-lambda%29%29 > > Probably the one you want is either match-lambda* or maybe match-lambda**. > >> >> >> Thanks, >> Brian >> >> >> ____________________ >> Racket Users list: >> http://lists.racket-lang.org/users > > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users
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