Matt Morrow:
On Mon, Apr 6, 2009 at 7:39 PM, Manuel M T Chakravarty <chak@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

Manuel

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.

Manuel

PS: I do realise that ambiguous signatures are the single most confusing issue concerning type families (at least judging from the amount of mailing list traffic generated).  We'll do our best to improve the situation before 6.12 comes out.