#7171: erroneous overlapping instances reported with FunDeps
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Reporter: jwlato | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type
checker)
Version: 7.6.1-rc1 | Keywords: type classes, fundeps
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: GHC rejects valid program | Testcase:
Blockedby: | Blocking:
Related: |
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When a superclass constraint has functional dependencies, in certain cases
GHC-7.6 erroneously reports that duplicate instances are found.
file Foo.hs
{{{
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
module Foo where
import Data.ByteString as B
import Data.Word
class Foo a b | a -> b
class (Foo a b) => Bar a b | a -> b
instance Foo [a] a
instance Bar [a] a
instance Foo ByteString Word8
instance Bar ByteString Word8
test :: Bar full item => full -> full
test inp = inp
-- this works
-- _test_x :: ByteString -> ByteString
-- _test_x = test
}}}
file Main.hs
{{{
module Main where
import Foo
import Data.ByteString
-- this works
-- test1 :: [Int] -> [Int]
-- test1 = test
-- this fails
test2 :: ByteString -> ByteString
test2 = test
}}}
ghc reports
{{{
Main.hs:12:9:
Overlapping instances for Foo ByteString GHC.Word.Word8
arising from a use of `test'
Matching instances:
instance Foo ByteString GHC.Word.Word8 -- Defined at Foo.hs:20:10
There exists a (perhaps superclass) match:
(The choice depends on the instantiation of `'
To pick the first instance above, use -XIncoherentInstances
when compiling the other instance declarations)
In the expression: test
In an equation for `test2': test2 = test
}}}
For this to manifest, I think that at least the following must be true:
- a function with class constraints must be called from a separate
module
- the type class instance must be monomorphic in the fundep-to parameter
This example works in ghc-7.4.2.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7171>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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