[GHC] #4175: GHCi support for type/data families should match that of related features

[GHC]

#4175: GHCi support for type/data families should match that of related features
---------------------------------+------------------------------------------
    Reporter:  claus             |       Owner:                  
        Type:  bug               |      Status:  patch           
    Priority:  low               |   Milestone:  7.6.2           
   Component:  GHCi              |     Version:  6.12.3          
    Keywords:  FD TF             |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:                    |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
---------------------------------+------------------------------------------
Changes (by parcs):

  * status:  new => patch


Comment:

 I've attached a patch that implements support for showing type family
 instances through `:info`.

 Examples:

 {{{
 {-# LANGUAGE TypeFamilies #-}

 type family A a b
 type instance A Int Int = ()
 type instance A (Maybe a) a = a

 data family B a
 data instance B () = MkB

 class C a where
     type D a b

 instance C Int where
     type D Int () = String

 instance C () where
     type D () () = Bool

 type family E a

 type instance where
     E ()  = Bool
     E Int = String
 }}}

 {{{
 Main> :i A
 type family A a b :: *  -- Defined at tf.hs:3:13
 type instance A (Maybe a) a -- Defined at tf.hs:5:15
 type instance A Int Int -- Defined at tf.hs:4:15
 }}}
 {{{
 *Main> :i B
 data family B a         -- Defined at tf.hs:7:13
 data instance B () -- Defined at tf.hs:8:15
 }}}
 {{{
 *Main> :i D
 class C a where
   type family D a b :: *
         -- Defined at tf.hs:11:10
 type D () () -- Defined at tf.hs:17:10
 type D Int () -- Defined at tf.hs:14:10
 }}}
 {{{
 Main> :i E
 type family E a :: *    -- Defined at tf.hs:23:13
 type instance where
   E () -- Defined at tf.hs:26:5
   E Int -- Defined at tf.hs:27:5
 }}}
 {{{
 *Main> :i ()
 data () = ()    -- Defined in ‛GHC.Tuple’
 instance C () -- Defined at tf.hs:16:10
 ...
 data instance B () -- Defined at tf.hs:8:15
 type D () () -- Defined at tf.hs:17:10
 type D Int () -- Defined at tf.hs:14:10
 type instance where
   E () -- Defined at tf.hs:26:5
   E Int -- Defined at tf.hs:27:5
 }}}
 {{{
 *Main> :i Maybe
 data Maybe a = Nothing | Just a         -- Defined in ‛Data.Maybe’
 ...
 type instance A (Maybe a) a -- Defined at tf.hs:5:15
 }}}
 {{{
 *Main> :i Int
 data Int = GHC.Types.I# GHC.Prim.Int#   -- Defined in ‛GHC.Types’
 instance C Int -- Defined at tf.hs:13:10
 ...
 type instance A Int Int -- Defined at tf.hs:4:15
 type D Int () -- Defined at tf.hs:14:10
 type instance where
   E () -- Defined at tf.hs:26:5
   E Int -- Defined at tf.hs:27:5
 }}}

 The patch causes a trivial testsuite failure:

 {{{
 Unexpected failures:
    ghci/scripts   T5417 [bad stdout] (ghci)
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4175#comment:6>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler