> On Jul 7, 2019, at 10:24 AM, 曹朝 <[email protected]> wrote:
>
> This is a simple algorithm for compute the shortest edit distance, it can
> work with `#lang racket/base`.
> But in Typed Racket, I just got the error message: "insufficient type
> information to typecheck". I don't know why this code can't pass the type
> checker.
> Someone can help? Thank you.
>
> #lang typed/racket/base
> (require math/array)
>
> (: shortest-edit-distance (-> String String Integer))
> (define (shortest-edit-distance str0 str1)
> (let* ([l0 : Integer (string-length str0)]
> [l1 : Integer (string-length str1)]
> [table : (Mutable-Array Integer) (array->mutable-array (make-array
> (vector l0 l1) 0))])
> (for*/last : Integer ([i0 : Integer (in-range l0)]
> [i1 : Integer (in-range l1)])
> (let* ([c0 : Char (string-ref str0 i0)]
> [c1 : Char (string-ref str1 i1)]
> [base : Integer (cond
> [(and (= i0 0) (= i1 0)) 0]
> [(= i0 0) (array-ref table (vector i0 (sub1
> i1)))]
> [(= i1 0) (array-ref table (vector (sub1 i0)
> i1))]
> [else (min (array-ref table (vector i0 (sub1
> i1)))
> (array-ref table (vector (sub1 i0)
> i1))
> (array-ref table (vector (sub1 i0)
> (sub1 i1))))])]
> [answer : Integer (if (char=? c0 c1) base (add1 base))])
>
> (array-set! table (vector i0 i1) answer)
> answer))))
As Matthias Felleisen said, the `for*/last` form is not yet supported by Typed
Racket. The 6 forms for/and, for/first, for/last, for*/and, for*/first, and
for/last are documented as "Like the above, except they are not yet supported
by the typechecker."
However, many uses of `for/first` can be replaced with `for/or`, and many uses
of `for/last` can be replaced with `for/fold`. If you see
(for*/last : type (clause ...)
body)
you can replace it with
(for*/fold ([_unused : type #f])
(clause ...)
body)
and get the same result.
There is also another problem with your code that prevents it from
type-checking. The `for*/last` form or the `for*/fold` replacement can return
#false, and that means you have to replace the result type `Integer` with `(U
Integer #f)` for it to type-check. However, that's probably incorrect since an
edit-distance should always be defined, so either you need to change how you
iterate or handle the #false differently... I'm not sure.
Alex Knauth
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