What's wrong with my code?

[曹朝]

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))))

[Matthias Felleisen]

With some for/loops in TR you’re out of luck. The expansion are too complex to type-check easily. 

<截屏2019-07-0800.15.13.png>

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[曹朝]

Alright, thank you very much.

在 2019年7月8日星期一 UTC+8上午3:50:18，Matthias Felleisen写道：

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[Alex Knauth]

On Jul 7, 2019, at 10:24 AM, 曹朝 <cz1...@gmail.com> wrote:

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

[曹朝]

🤩 Wow! I get it, thanks.

在 2019年7月9日星期二 UTC+8下午10:04:59，Alex Knauth写道：