[GHC] #14440: Duplicate type family instances are permitted

7 Nov 2017

      #14440: Duplicate type family instances are permitted
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           Keywords:  TypeFamilies   |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 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:

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 module Lib where

 type family Foo b
 }}}
 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 module A where

 import Lib

 type instance Foo Bool = Bool
 }}}
 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 module B where

 import Lib

 type instance Foo Bool = Bool
 }}}
 {{{#!hs
 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).

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14440
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