#3346: Strange and incorrect type error when using rewrite rules with type
families
-----------------------------+----------------------------------------------
Reporter: dreixel | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.11 | Severity: normal
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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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.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3346>
GHC <http://www.haskell.org/ghc/>
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