Manuel Chakravarty wrote:
Ganesh Sittampalam:
On Mon, 7 Apr 2008, Manuel M T Chakravarty wrote:
Ganesh Sittampalam:
The following program doesn't compile in latest GHC HEAD, although it does if I remove the signature on foo'. Is this expected?
Yes, unfortunately, this is expected, although it is very unintuitive. This is for the following reason.
Let's alpha-rename the signatures and use explicit foralls for clarity:
foo :: forall a. Id a -> Id a foo' :: forall b. Id b -> Id b
GHC will try to match (Id a) against (Id b). As Id is a type synonym family, it would *not* be valid to derive (a ~ b) from this. After all, Id could have the same result for different argument types. (That's not the case for your one instance, but maybe in another module, there are additional instances for Id, where that is the case.)
Can't it derive (Id a ~ Id b), though?
That's what it does derive as a proof obligation and finds it can't prove. The error message you are seeing is GHC's way of saying, I cannot assert that (Id a ~ Id b) holds.
No, I meant can't it derive that equality when matching (Id a) against (Id b)? As you say, it can't derive (a ~ b) at that point, but (Id a ~ Id b) is known, surely?
Generally speaking, is there any way to give a signature to foo'?
Sorry, but in the heat of explaining what GHC does, I missed the probably crucial point. Your function foo is useless, as is foo'. Not only can't you rename foo (to foo'), but you generally can't use it. It's signature is ambiguous. Try evaluating (foo (1::Int)). The problem is related to the infamous (show . read).
My real code is somewhat analogous to (foo :: (Id Int -> Id Int)) (1::Int). Isn't that unambiguous in the same way as (show.read) is if I give show or read a signature? Cheers, Ganesh ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================