[Haskell] GHC Error question
Lennart Augustsson
lennart at augustsson.net
Thu Dec 7 06:33:23 EST 2006
And why isn't C a b equivalent to C a b1?
forall a b . C a b => a -> a
and
forall a b1 . C a b1 => a -> a
look alpha convertible to me. Or is the inferred type
forall a . C a b1 => a -> a
Btw, this reminds me again that I'd like to be able to use _ in type
signatures.
With the meaning of _ being, "there's a type here, but I can't be
bothered to write it out in full."
-- Lennart
On Dec 6, 2006, at 02:46 , Simon Peyton-Jones wrote:
> I agree that this is confusing. Here is a cut-down example:
>
> class C a b where
> op :: a -> a
>
> -- f :: C a b => a -> a
> f x = op x
>
> 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
>
> Simon
>
> | -----Original Message-----
> | From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-
> haskell-users-
> | bounces at haskell.org] On Behalf Of Norman Ramsey
> | Sent: 06 December 2006 01:41
> | To: Simon Peyton-Jones
> | Cc: GHC users
> | Subject: Re: [Haskell] GHC Error question
> |
> | > [redirecting to ghc users]
> | >
> | > It looks like a splendid error to me.
> |
> | I'm not sure if you meant the error or the message was splendid :-)
> | I yelled for help because my usual strategy failed. That
> strategy is
> |
> | 1. Remove the type annotation.
> | 2. Get ghci to tell me what the 'right type' is.
> | 3. Put the 'right type' in the type annotation.
> |
> | I find it a bit depressing that the most general type inferred by
> ghci
> | does not work as a type signature.
> |
> | > I can't say more without seeing the code. can you give a
> small repo case?
> |
> | Yes, here's a case that fits in one screen of emacs :-)
> |
> |
> | {-# OPTIONS -fglasgow-exts #-}
> | module Ccomp where
> |
> | type Name = String
> |
> | data Inface = N | W
> | data Outface = S | E
> |
> | data Sink box = Boxin Inface box | Result
> | data Source box = Boxout Outface box | Arg Inface
> |
> | data Command = Send0
> |
> | class (Monad b) => Builder b box where
> | box :: Command -> b box
> | wire :: Source box -> Sink box -> b ()
> |
> | type Env box = Name -> Sink box
> |
> | empty = \x -> error (x ++ " not connected or multiply connected
> in circuit")
> |
> | -- either of these explicit signatures causes the compiler to fail
> | -- although the inferred signature is the second.
> | --compile1 :: (Builder b box) => Name -> Name -> ANF -> b Name
> | compile1 :: (Builder t box) => t1 -> Name -> ANF -> t t1 --
> generated by ghci
> | compile1 f x body = do env <- compile body empty
> | wire (Arg W) (env x)
> | return f
> |
> | data ANF = ANF ()
> |
> | compile :: (Builder b box) => ANF -> Env box -> b (Env box)
> | compile (ANF m) out = undefined
> |
> | > | This program is rejected by GHC with the following message:
> | > |
> | > | Ccomp.hs:54:23:
> | > | Could not deduce (Builder b box1) from the context
> (Builder b box)
> | > | arising from use of `wire' at Ccomp.hs:54:23-42
> | > | Possible fix:
> | > | add (Builder b box1) to the type signature(s) for
> `compile1'
> | > | In the expression: wire (Arg W) (env x)
> | > | In a 'do' expression: wire (Arg W) (env x)
> | > | In the expression:
> | > | do env <- compile body empty
> | > | wire (Arg W) (env x)
> | > | return f
> | > |
> | > | Note that compile1 has an explicit type signature much along
> the lines
> | > | suggested by GHC. If I *remove* this type signature, the
> function
> | > | compiles successfully, and ghci reporets this type for
> compile1:
> | > |
> | > | compile1 :: (Builder t box) => t1 -> Name -> Ir.ANF -> t t1
> | > |
> | > | I believe this signature is isomorphic to the explicit
> signature I had
> | > | attempted to use.
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