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. In python using generators would be very
straightforward:
def gen(input):
for score, letters in input.items():
for l in letters: yield l, score
dict(gen(input))
I am stumbling on the racket solution using generators and I was wondering if I
could get some pointers from the list. This could actually be a good candidate
for the "from python to racket" idioms that is being discussed in a separate
thread.
I find that the racket docs are lacking in describing generator use cases that
yield more than one value. I tried passing `(void)` as a stop value to the
`in-producer` call but says it needs a predicate when the producer yields more
than one value.
Also I would assume if the generator is iterating over a finite sequence we
would not need to define a stop value, but this doesn't seem to work either.
Thanks in advance for any help!
#lang racket
(require racket/generator)
(provide etl
etl-gen)
(define mixed-case-input (hash 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")))
(define (etl input)
(define result (make-hash))
(define/contract (transpose score letters)
(-> positive? list? any)
(for-each (lambda (letter)
(hash-set! result (string-downcase letter) score)) letters))
(hash-for-each input transpose)
result)
(define (etl-gen input)
(define unpack (generator ()
(for* ([(score letters) input]
[l letters])
(yield (string-downcase l) score))))
(for/hash ([(letter score) (in-producer unpack)]) (values letter score)))
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