Ah, you are of course right. Somehow I thought about `for/list` and returning lists and then doing something on the returned list, instead of simply doing it _inside_ the `for`. Apologies!
On 2/21/19 8:46 PM, Robby Findler wrote: > For the record, `for` iterators go element-by-element. For example, > this program does not construct anything that has all of the natural > numbers in it: > > #lang racket > > (define my-stuffs-value > (hash 'telephone 3 > 'bluejeans 24 > 'house 100000 > 'computer 3000)) > > (define my-estates-value > (for/sum ([i (in-naturals)] > [(k v) (in-hash my-stuffs-value)]) > (printf "item ~a is worth ~a\n" i v) > v)) > > my-estates-value > > On Thu, Feb 21, 2019 at 1:40 PM Zelphir Kaltstahl > <zelphirkaltst...@gmail.com> wrote: >> I don't think one can see `for` in Racket as equivalent to generators in >> Python. >> >> Generators in Python are used when you want to save memory by not producing >> all values at once, but want to go value by value (which to my knowledge >> Racket's `for` variants do not do, but I'd like to be corrected, if I am >> wrong about this, as it would mean, that those `for` are even more awesome >> than I knew.) They are more like lazy possibly infinite lists. I think lazy >> Racket has that and that there was some advice about not calling `length` on >> an infinite list. >> >> Python, I believe, has some kind of `Iterable` interface, which a generator >> satisfies and which specifies the method to call to get the next value. In >> Racket maybe one would have to do with a convention of how to get a next >> value from a lazy list. >> >> I believe something like the following could be an example. I did not yet >> think about macros, nor am I experienced with that, so maybe one can create >> something more elegant than what I am posting: >> >> ~~~ >> #lang racket >> >> ;; ================ >> ;; DATA ABSTRACTION >> ;; ================ >> >> ;; The value is always the first value of the generator. >> (define get-value car) >> >> ;; A generator's producing procedure needs to be applied to get the >> ;; next thing. >> (define (get-next a-gen) >> ((cdr a-gen))) >> >> ;; A generator is only a tuple containing the current value and the >> ;; procedure to create the next value >> (define (make-gen proc start) >> (cons start >> (lambda () >> (make-gen proc (proc start))))) >> >> (define STOP-SYMBOL 'none) >> >> ;; ======== >> ;; EXAMPLES >> ;; ======== >> >> ;; An infinite list of numbers. >> (define natural-numbers >> (make-gen (lambda (x) (+ x 1)) >> 0)) >> >> ;; A limited list of natural numbers. >> (define (limited-natural-numbers limit) >> (make-gen (lambda (x) >> (cond [(< x limit) (+ x 1)] >> [else STOP-SYMBOL])) >> 0)) >> >> ;; Print a certain amount of natural numbers. >> (define (print-natural-numbers natural-numbers limit) >> (let ([current-number (get-value natural-numbers)]) >> (cond >> [(and (symbol? current-number) (symbol=? current-number STOP-SYMBOL)) >> (displayln "No more numbers available!")] >> [(< current-number limit) >> (displayln current-number) >> (print-natural-numbers (get-next natural-numbers) limit)] >> [else >> (displayln "Done")]))) >> ~~~ >> >> Here the conventions are the structure of the generator and the stop symbol. >> In Python those are already taken care of by the standard library's >> generator interface. >> >> >> On Wednesday, February 20, 2019 at 10:10:16 PM UTC, Jens Axel Søgaard wrote: >>> Den ons. 20. feb. 2019 kl. 22.25 skrev Dave McDaniel <mcdani...@gmail.com>: >>>> Hello, >>>> >>>> I have interest in picking up racket and have done some koans and also >>>> have been doing the racket track on exercism. >>>> >>>> There is a fairly simple exercise called `etl` on exercism related to >>>> taking a hash for scoring scrabble letters and unpacking it into a >>>> flatter, more efficient structure for lookups. >>>> >>>> The input hash has score as the key and a list of letters of that score as >>>> the value. The output is a hash of letter -> score. One pair per letter >>>> instead of one pair per score. >>>> >>>> It was easy enough to solve with `for-each` using side-effects to update a >>>> mutable hash, however I think using a generator is a cleaner approach and >>>> doesn't require mutability. >>> >>> FWIW here is a solution using immutable hashes. I see `for` and friends as >>> Racket "generators". >>> >>> /Jens Axel >>> >>> >>> #lang racket >>> (require racket/hash) >>> >>> ; In Scrabble each letter gives a certain number of points. >>> ; In the old data we have for each point a list of numbers, >>> ; that awarded the given number of points. >>> >>> (define old-data >>> ; list of associations from point to list of strings (letters) >>> '((1 "A" "E" "I" "O" "U" "L" "N" "R" "S" "T") >>> (2 "D" "G") >>> (3 "B" "C" "M" "P") >>> (4 "F" "H" "V" "W" "Y") >>> (5 "K") >>> (8 "J" "X") >>> (10 "Q" "Z"))) >>> >>> >>> ; Each point list needs to be transformed to a >>> ; hash table from letters to points. >>> >>> (define (transform association) >>> (match association >>> [(list point letters ...) >>> (for/hash ([letter letters]) >>> (values (string-downcase letter) point))])) >>> >>> ; Now we only need to combine the hash lists. >>> (define ht >>> (apply hash-union >>> (hash) >>> (for/list ([association old-data]) >>> (transform association)))) >>> >>> (define (lookup letter) >>> (hash-ref ht letter #f)) >>> >>> (lookup "e") >>> (lookup "x") >>> >>> >>> >> -- >> You received this message because you are subscribed to the Google Groups >> "Racket Users" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to racket-users+unsubscr...@googlegroups.com. >> For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. 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