[racket] Performance help

Although it can't hurt, I don't think `train` comes close to
dominating the time?

Through the sophisticated profiling technique of commenting-out `or`
branches in `correct`, I got the sense that the third one is the
issue:


(define (correct m s)
  (let ([best-known (λ (xs) (best-known m xs))])
    (or (best-known (list s))
        (best-known (edits1 s))
        (best-known (append-map edits1 (edits1 s))) <-- if reached, very slow
        s)))
real	0m16.527s
user	0m14.554s
sys	0m1.859s


(define (correct m s)
  (let ([best-known (λ (xs) (best-known m xs))])
    (or (best-known (list s))
        (best-known (edits1 s))
        (best-known (edits2 m s)) ;; what I posted yesterday
        s)))
real	0m9.807s
user	0m9.463s
sys	0m0.266s


(define (correct m s)
  (let ([best-known (λ (xs) (best-known m xs))])
    (or (best-known (list s))
        (best-known (edits1 s))
        ;; (best-known (edits2 m s)) ;; skip it entirely
        s)))
real	0m2.074s
user	0m1.708s
sys	0m0.184s


p.s. `best-known` = combining `best` and `known` into a single pass for/fold:

;; Given a hash map and a list of words, returns
;; one of the words with the highest frequency.
;; Given an empty list returns #f
(define (best-known ht xs)
  (car (for*/fold ([best (cons #f 0)])
                  ([x (in-list xs)]
                   [v (in-value (hash-ref ht x #f))] ;; should default be 1?
                   #:when v)
         (if (> v (cdr best)) (cons x v) best))))

... which I thought might be faster but wasn't significant.