Without going too deep into the details of my type classes, I have
written the following code (focusing on compile1):
{-# OPTIONS -fglasgow-exts #-}
compile1 :: (Builder b box) => t -> Name -> Ir.ANF -> b t
compile1 f x body = do env <- compile body empty
wire (Arg W) (env x)
return f
compile :: (Builder b box) => Ir.ANF -> Env box -> b (Env box)
class (Monad b) => Builder b box where
wire :: Source box -> Sink box -> b ()
...
type Env box = Name -> Sink box
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.
Am I misusing the type-class system in some way, or should I be
reporting a bug in GHC?
Norman
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