id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,testcase,blockedby,blocking,related,differential,wikipage
2627,GADT + Type family doesn't unify like I expect,ryani,chak,"I'd expect this program to compile; the ""given"" equations in ""conn"" should add additional constraints to the types determined by the GADT. Instead it looks like (Dual a) is failing to unify with (R _ _), and the same for (Dual b) and (W _ _).
{{{
{-# LANGUAGE GADTs, TypeFamilies, EmptyDataDecls #-}
module Dual where
data R a b
data W a b
data Z
type family Dual a
type instance Dual Z = Z
type instance Dual (R a b) = W a (Dual b)
type instance Dual (W a b) = R a (Dual b)
data Comm a where
Rd :: (a -> Comm b) -> Comm (R a b)
Wr :: a -> Comm b -> Comm (W a b)
Fin :: Int -> Comm Z
conn :: (Dual a ~ b, Dual b ~ a) => Comm a -> Comm b -> (Int, Int)
conn (Fin x) (Fin y) = (x,y)
conn (Rd k) (Wr a r) = conn (k a) r
conn (Wr a r) (Rd k) = conn r (k a)
}}}
Running off of GHC HEAD:
{{{The Glorious Glasgow Haskell Compilation System, version 6.11.20080925}}}
",bug,closed,normal,,Compiler (Type checker),6.9,fixed,,,Unknown/Multiple,Unknown/Multiple,,T2627,,,,,