[Haskell] GHC Error question

[Norman Ramsey]

 > 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!  

Indeed not.  I salaam in your general direction.

 > 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

Pity.  Do you see a way for the error message to avoid suggesting the
'possible fix', and indeed instead suggesting 'possible fix: remove
the type signature from f'?


 > 
 > 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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