2015-01-02 13:23 GMT+01:00 Daniel Prager <daniel.a.pra...@gmail.com>: > Slightly off-topic: > > Norvig's code is both concise and -- to my eye -- highly readable. E.g. > > def edits1(word): > splits = [(word[:i], word[i:]) for i in range(len(word) + 1)] > deletes = [a + b[1:] for a, b in splits if b] > transposes = [a + b[1] + b[0] + b[2:] for a, b in splits if len(b)>1] > replaces = [a + c + b[1:] for a, b in splits for c in alphabet if b] > inserts = [a + c + b for a, b in splits for c in alphabet] > return set(deletes + transposes + replaces + inserts) > > Python's concise list comprehensions, destructuring, string slice operators, > and operator overloading slot together nicely to make this kind of thing > easy to knock out. > > I was able to achieve the following in Racket, but it's a fair bit of extra > for a lesser result: > >> (define (edits1 word) >> (define sub substring) >> (define nth string-ref) >> (define splits (for/list ([i (range (add1 (string-length word)))]) >> (cons (sub word 0 i) (sub word i)))) >> >> (define (map-splits min-b f [chars '(dummy)]) >> (for*/list ([split splits] >> [a (in-value (car split))] >> [b (in-value (cdr split))] >> #:when (>= (string-length b) min-b) >> [ch chars]) >> (f a b ch))) >> >> (let ([deletes (map-splits 1 (λ (a b _) (~a a (sub b 1))))] >> [transposes (map-splits 2 (λ (a b _) (~a a (nth b 1) (nth b 0) >> (sub b 2))))] >> [replaces (map-splits 1 (λ (a b ch) (~a a ch (sub b 1))) >> alphabet)] >> [inserts (map-splits 0 (λ (a b ch) (~a a ch b)) alphabet)]) >> (append deletes transposes replaces inserts))) > > > Comments? Suggestions?
A DSL to make string manipulations more concise would be nice to have. An ad hoc attempt can be seen here (the concat macro): http://scheme.dk/blog/2007/04/writing-spelling-corrector-in-plt.html The blog post was written before for and friends arrived in Racket, so the code uses srfi 42 instead. -- Jens Axel Søgaard ____________________ Racket Users list: http://lists.racket-lang.org/users
