#7393: GHC cannot solve functional dependency
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Reporter: Khudyakov | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.6.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: GHC rejects valid program | Testcase:
Blockedby: | Blocking:
Related: |
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GHC fails to compile following program:
{{{
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
-- | All unordered pairs of elements of lists. If list is set result
-- will be set as well.
class TyPairs (xs :: [*]) (ys :: [*]) | xs -> ys
instance TyPairs '[] '[]
instance TyAddPairs x (x ': xs) xs ys => TyPairs (x ': xs) ys
-- Add all pairs for /x/ and /xs/ and that build all pairs for the
-- remaining part of the list.
class TyAddPairs x (xs :: [*]) (rest :: [*]) (pairs :: [*]) | x xs rest ->
pairs
instance TyPairs ys rs => TyAddPairs x '[] ys rs
instance TyAddPairs x xs ys rs => TyAddPairs x (a ': xs) ys ((x,a) ': rs)
data K = K
data B = B
type List = '[K,B]
type Pairs = '[(K,K), (K,B), (B,B)]
data Row (vars :: [*]) = Row
failure :: TyPairs List r => Row r
failure = Row :: Row Pairs
fine :: TyPairs List Pairs => ()
fine = ()
}}}
with following error message:
{{{
OKA/Statistics/fail.hs:36:11:
Could not deduce (r
~ (':) * (K, K) ((':) * (K, B) ((':) * (B, B) ('[]
*))))
from the context (TyPairs List r)
bound by the type signature for failure :: TyPairs List r => Row r
at OKA/Statistics/fail.hs:35:12-34
`r' is a rigid type variable bound by
the type signature for failure :: TyPairs List r => Row r
at OKA/Statistics/fail.hs:35:12
Expected type: Row r
Actual type: Row Pairs
In the expression: Row :: Row Pairs
In an equation for `failure': failure = Row :: Row Pairs
Failed, modules loaded: none.
}}}
r is determined by functional dependency so it looks like bug to me.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7393>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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