On Mon, Apr 6, 2009 at 7:39 PM, Manuel M T Chakravarty <c...@cse.unsw.edu.au> wrote: > Peter Berry: > >> 3) we apply appl to x, so Memo d1 a = Memo d a. unify d = d1 >> >> But for some reason, step 3 fails. > > Step 3 is invalid - cf, > <http://www.haskell.org/pipermail/haskell-cafe/2009-April/059196.html>. > > More generally, the signature of memo_fmap is ambiguous, and hence, > correctly rejected. We need to improve the error message, though. > Here is a previous discussion of the subject: > > http://www.mail-archive.com/haskell-cafe@haskell.org/msg39673.html
Aha! Very informative, thanks. On 07/04/2009, Manuel M T Chakravarty <c...@cse.unsw.edu.au> wrote: > Matt Morrow: >> The thing that confuses me about this case is how, if the type sig >> on memo_fmap is omitted, ghci has no problem with it, and even gives >> it the type that it rejected: > > Basically, type checking proceeds in one of two modes: inferring or > checking. The former is when there is no signature is given; the > latter, if there is a user-supplied signature. GHC can infer > ambiguous signatures, but it cannot check them. This is of course > very confusing and we need to fix this (by preventing GHC from > inferring ambiguous signatures). The issue is also discussed in the > mailing list thread I cited in my previous reply. I see. So GHC is wrong to accept memo_fmap? -- Peter Berry <pwbe...@gmail.com> Please avoid sending me Word or PowerPoint attachments. See http://www.gnu.org/philosophy/no-word-attachments.html _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
