[GHC] #10839: Consistent pretty-printing of type families

3 Sep 2015

      #10839: Consistent pretty-printing of type families
-------------------------------------+-------------------------------------
              Reporter:  jstolarek   |             Owner:
                  Type:  task        |            Status:  new
              Priority:  lowest      |         Milestone:
             Component:  Compiler    |           Version:  7.11
              Keywords:  newcomer    |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 Say I have this silly open type family declaration:

 {{{#!hs
 type family F a
 type instance F a = a
 type instance F a = Int
 }}}
 GHC complains:
 {{{
 Foo.hs:8:15: error:
     Conflicting family instance declarations:
       F a -- Defined at Foo.hs:8:15
       F a -- Defined at Foo.hs:9:15
 }}}
 Now let's do some silliness with a closed type family:
 {{{#!hs
 type family F a where
     F a = a
     F a = Int
 }}}
 GHC warns:
 {{{
 T6018.hs:9:5: warning:
     Type family instance equation is overlapped:
       F a = Int
 }}}
 Finally, some silliness with a kind-polymorphic type family:
 {{{#!hs
 type family Gc (a :: k) (b :: k) = r | r -> k where
     Gc a b = Int
 }}}
 GHC complains:
 {{{
 T6018.hs:9:5: error:
     Type family equation violates injectivity annotation.
     Injective kind variable ‘k’ is not inferable from the RHS type
 variables.
     In the RHS of type family equation:
     forall (k :: BOX) (a :: k) (b :: k). Gc a b = Int
     In the equations for closed type family ‘Gc’
     In the type family declaration for ‘Gc’
 }}}

 There are some inconsistencies in pprinting of type family equations:

   * for open type families we print the location where the equation was
 defined but we do not print the RHS
   * for closed type families we print the RHS but we do not print the
 location in the source file
   * kind-polymorphic equations are not indented

 I would like to have a consistent pretty-printing of type family
 equations, one that prints the RHS, source code location and indents kind-
 polymorphic equations. The difference in pretty printing between open and
 closed type families comes from the difference between pprinting
 `CoAxBranch` and `FamInst`.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10839
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