Duplicate type family instances are permitted
This threw me for a loop recently. To my surprise, GHC is quite happy to allow duplicate type family instances, provided that their RHSes are the same:
{-# LANGUAGE TypeFamilies #-}
module Lib where
type family Foo b
{-# LANGUAGE TypeFamilies #-}
module A where
import Lib
type instance Foo Bool = Bool
{-# LANGUAGE TypeFamilies #-}
module B where
import Lib
type instance Foo Bool = Bool
module C where
import A
import B
import Lib
f :: Bool -> Foo Bool
f x = not x
$ /opt/ghc/8.2.1/bin/ghci C.hs
GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 4] Compiling Lib ( Lib.hs, interpreted )
[2 of 4] Compiling B ( B.hs, interpreted )
[3 of 4] Compiling A ( A.hs, interpreted )
[4 of 4] Compiling C ( C.hs, interpreted )
Ok, 4 modules loaded.
λ> :i Foo
type family Foo b :: * -- Defined at Lib.hs:4:1
type instance Foo Bool = Bool -- Defined at A.hs:6:15
type instance Foo Bool = Bool -- Defined at B.hs:6:15
Is this intended? My intuition screams "no", since if we offer //class// instance coherence, it seems like one ought to offer //type family// instance coherence as well. At the same time, I can't think of any threat to type soundness imposed by this (although it's quite strange to see two duplicate type family instances in the output of :i
with two completely different provenances).
Trac metadata
Trac field | Value |
---|---|
Version | 8.2.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |