{-# 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
{- -- Uncommenting rule1 above gives the error:
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"
-}
-- However, this is fine
{-# 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
main = print ""