[Haskell] GHC Error question

[Norman Ramsey]

 > [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.