I write this kind of code all the darn time:
;; take (listof (list a b)) into (hashof a (listof b))
(define (gather l)
(for/fold ([ht (make-hash)])
([pr l])
(hash-set ht (first pr) (cons (second pr)
(hash-ref ht (first pr) empty)))))
;; gather the responses into a table:
(define rating-table (gather responses2))
;; compute the mean rating for each student
(for/list ([(student ratings) (in-hash rating-table)])
(list student (mean ratings)))
… that is: given an association list, gather them together, then compute (say)
the mean of the values. In this case, I’m trying to compute the mean rating for
a bunch of students. Very simple.
So… why does it take so much code? I feel like these should be built-in
abstractions. There are (at least) two possibilities:
1) There’s an obvious, built-in way to do this, and I’m just missing it.
2) There’s an abstraction that we’re missing, and I feel like Clojure might
have it. Or am I just making this up? Getting clojure running is such a pain….
Thanks for any advice!
John
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