Newtypes and type families combine to produce inconsistent FC(X) axiom sets
Given:
{-# OPTIONS_GHC -ftype-families #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
data family Z :: * -> *
newtype Moo = Moo Int
newtype instance Z Int = ZI Double
newtype instance Z Moo = ZM (Int,Int)
We generate the axioms:
(from the instances)
Z Int ~ Double
Z Moo ~ (Int,Int)
(from the newtype)
Moo ~ Int
And can prove:
(production REFL in the FC(X) paper)
Z ~ Z
(production COMP)
Z Moo ~ Z Int
(production TRANS)
Z Moo ~ Double
(production SYM)
Double ~ Z Moo
(production TRANS)
Double ~ (Int,Int)
Tickling this seems to require the newtype cheat, but the inconsistant axioms allow code to pass Core Lint and still crash:
newtype Moo = Moo Int deriving(IsInt)
class IsInt t where
isInt :: c Int -> c t
instance IsInt Int where isInt = id
main = case isInt (ZI 4.0) of ZM tu -> print tu
stefan@stefans:/tmp$ ghc -dcore-lint Z.hs
stefan@stefans:/tmp$ ./a.out
Segmentation fault
stefan@stefans:/tmp$ ghc -V
The Glorious Glasgow Haskell Compilation System, version 6.7.20070612
stefan@stefans:/tmp$