1. You had a } mistake.
2. You are using a polymorphic map and TR needs an additional annotation to
figure this out:
(: encode : (All (a) (Listof a) -> (Listof (List Nonnegative-Integer a))))
(define (encode xs)
({inst map (List Nonnegative-Integer a) (Listof a)}
(λ ([ys : (Listof a)])
(list (length ys) (first ys)))
(pack xs)))
3. I provided a pack of type (: pack (All (a) (Listof a) -> (Listof (Listof
a))))
4. When you run this now, (first ys) may raise a run-time exception (just like
in ML).
> On May 25, 2016, at 5:31 AM, Daniel Prager <daniel.a.pra...@gmail.com> wrote:
>
> Thanks Sam
>
> I still get a type-checking error with
>
> (: encode : (All (a) (Listof a) -> (Listof (List Nonnegative-Integer a))))
>
> (define (encode xs)
> (map (λ ([ys : (Listof a)])
> (list (length ys) (first ys))
> (pack xs))))
>
> which I think is what you suggested. Adding a return type to the λ as well
> yields more type errors.
>
>
>
> Dan
>
>
>
> On Wed, May 25, 2016 at 8:14 AM, Sam Tobin-Hochstadt <sa...@cs.indiana.edu
> <mailto:sa...@cs.indiana.edu>> wrote:
> What's happening here is that Typed Racket can't infer what the type
> of `ys` is in the lambda that's an argument to `map`, since you didn't
> provide an annotation. In the `for/list`, the call to `(pack xs)` is
> right there, so inference works.
>
> To make the use of `map` work, you just annotate `ys`: (λ ([ys :
> (Listof a)]) (list (length ys) (first ys)))
>
> Sam
>
> On Tue, May 24, 2016 at 5:50 PM, Daniel Prager
> <daniel.a.pra...@gmail.com <mailto:daniel.a.pra...@gmail.com>> wrote:
> > The following program works in Typed Racket, but if you comment out the
> > for/list in (encode ...) and uncomment the seemingly equivalent map it fails
> > to type check.
> >
> > Why the discrepancy and how do the experienced Typed Racketeers diagnose and
> > fix these sorts of issues?
> >
> >
> > Thanks
> >
> > Dan
> >
> >
> > P.S. These are taken from Nintey-nine Lisp Problems:
> > http://www.ic.unicamp.br/~meidanis/courses/mc336/2006s2/funcional/L-99_Ninety-Nine_Lisp_Problems.html
> >
> > <http://www.ic.unicamp.br/~meidanis/courses/mc336/2006s2/funcional/L-99_Ninety-Nine_Lisp_Problems.html>
> >
> >
> > #lang typed/racket
> >
> > (require typed/test-engine/racket-tests)
> >
> > ;P09 (**) Pack consecutive duplicates of list elements into sublists.
> > (: pack : (All (a) (Listof a) -> (Listof (Listof a))))
> > (define (pack xs)
> > (cond [(null? xs) null]
> > [(null? (rest xs)) (list xs)]
> > [else (let ([packed (pack (rest xs))]
> > [head (first xs)])
> > (if (equal? (first xs) (second xs))
> > (cons (cons head (first packed))
> > (rest packed))
> > (cons (list head) packed)))]))
> >
> > (check-expect (pack '(a a a a b c c a a d e e e e))
> > '((a a a a) (b) (c c) (a a) (d) (e e e e)))
> >
> >
> > ;P10 (*) Run-length encoding of a list.
> > (: encode : (All (a) (Listof a) ->
> > (Listof (List Nonnegative-Integer a))))
> > (define (encode xs)
> > #;(map (λ (ys) (list (length ys) (first ys))) (pack xs))
> > (for/list ([ys (pack xs)])
> > (list (length ys) (first ys))))
> >
> > (check-expect (pack '(a a a a b c c a a d e e e e))
> > '((a a a a) (b) (c c) (a a) (d) (e e e e)))
> >
> >
> > (test)
> >
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