[Haskell] GHC Error question

[Simon Peyton-Jones]

Hmm.  GHC currently uses the signature to drive typechecking the expression; it does not infer a type and compare. (Much better error messages that way.)

So (a) it's very undesirable that using the inferred type as a signature can ever not work, but (b) it affects only very few programs and ones that are almost certainly ambiguous anyway, and (c) I can't see an easy way to fix it.  So my current plan is: let it lie.

I'll open a low-priority bug report for it though.

Simon

| -----Original Message-----
| From: Lennart Augustsson [mailto:lennart at augustsson.net]
| Sent: 13 December 2006 13:42
| To: Simon Peyton-Jones
| Cc: GHC users
| Subject: Re: [Haskell] GHC Error question
|
| If the type checker really deduces the type 'forall a b . C a b => a -
|  > a' then an inference algorithm that works seems easy.  Do type
| inference for f, then check that the signature the user has given is
| alpha-convertible with the deduced type (well, in general it's more
| complicated than that, of course).
| If the type checker doesn't really deduce 'forall a b . C a b => a ->
| a' then it shouldn't print what it does.
| So I'm curious, what is the exact deduced type?
|
|         -- Lennart
|
| On Dec 11, 2006, at 07:16 , Simon Peyton-Jones wrote:
|
| > | Tell me how this make sense:
| > |     1. I enter the definition for f.
| > |     2. I ask ghc for the type of f and get an answer.
| > |     3. I take the answer and tell ghc this is the type of f, and
| ghc
| > | tells me I'm wrong.
| > | Somewhere in this sequence something is going wrong.
| >
| > I agree!  Indeed I wrote:
| >
| > | It doesn't get much simpler than that!  With the type sig, GHC
| > can't see that the (C a b) provided can
| > | satisfy the (C a b1) which arises from the call to op.   However,
| > without the constraint, GHC simply
| > | abstracts over the constrains arising in the RHS, namely (C a
| > b1), and hence infers the type
| > |         f :: C a b1 => a -> a
| > | It is extremely undesirable that the inferred type does not work
| > as a type signature, but I don't see
| > | how to fix it
| >
| > If you have an idea for an inference algorithm that would typecheck
| > this program, I'd be glad to hear it.  Just to summarise, the
| > difficulty is this:
| >         I have a dictionary of type (C a b1)
| >         I need a dictionary of type (C a b2)
| >         There is no functional dependency between C's parameters
| >
| > Simon
| >
| > PS: the complete program is this:
| >         class C a b where
| >                 op :: a -> a
| >
| >         f :: C a b => a -> a
| >         f x = op x
| >