id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
3346	Strange and incorrect type error when using rewrite rules with type families	dreixel		"The following code

{{{
{-# OPTIONS_GHC -fglasgow-exts -O1 #-}

module Main where

{-# RULES ""rule1""   forall x. to (from x) = x #-}
{-# RULES ""rule2""   forall x. from (to x) = x #-}

class EP a where
  type Result a
  from :: a -> Result a
  to   :: Result a -> a
}}}

gives the compilation error with GHC 6.11.20090703:
{{{
    Couldn't match expected type `Result a'
           against inferred type `Result a'
      NB: `Result' is a type function, and may not be injective
    In the first argument of `to', namely `(from x)'
    In the expression: to (from x)
    When checking the transformation rule ""rule1""
}}}

I don't understand this error. rule2 seems unproblematic, though.

The similar code with functional dependencies

{{{
{-# RULES ""rule3""   forall x. to' (from' x) = x #-}
{-# RULES ""rule4""   forall x. from' (to' x) = x #-}

class EP' a b | a -> b where
  from' :: a -> b
  to'   :: b -> a
}}}

raises no errors.
"	bug	closed	normal	6.12.1	Compiler	6.11	fixed			Unknown/Multiple	Unknown/Multiple		Unknown	typecheck/should_compile/T3346