“Ambiguous type variable in the constraint” error in rewrite rule

10 Jul 2012

      Hello,

Why does GHC 7.4.1 reject the rewrite rule in the following code?
...
module Test where
import Data.Monoid
import Control.Monad.Writer.Strict
f :: Monad m => a -> m a
f = return
g :: Monoid w => a -> Writer w a
g = return
{-# RULES
"f->g" f = g
  #-}
On the line containing the rewrite rule, GHC shows the following error message:

Test.hs:13:12:
    Ambiguous type variable `w0' in the constraint:
      (Monoid w0) arising from a use of `g'
    Probable fix: add a type signature that fixes these type variable(s)
    In the expression: g
    When checking the transformation rule "f->g"

Interestingly, the code compiles if the rewrite rule is replaced with
the following SPECIALIZE pragma:
...
{-# SPECIALIZE f :: Monoid w => a -> Writer w a #-}
I find this strange because if I am not mistaken, this specialization
is handled by using a rewrite rule of the same type as the one which
GHC rejects.

The following ticket might be related, but I am not sure:
    Subclass Specialization in Rewrite Rules
    http://hackage.haskell.org/trac/ghc/ticket/6102

Best regards,
  Tsuyoshi

“Ambiguous type variable in the constraint” error in rewrite rule

Tsuyoshi Ito