GADT + Type family doesn't unify like I expect
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
Trac metadata
| Trac field | Value |
|---|---|
| Version | 6.9 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | Unknown |
| Architecture | Unknown |