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 ============================================================================== _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
