[GHC] #10978: Anonymous type instances

[GHC]

#10978: Anonymous type instances
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           Reporter:                 |             Owner:
  benjamin.hodgson                   |
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
  (Type checker)                     |
           Keywords:                 |  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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 I find that commonly I write a `type family` (because it doesn't ''need''
 to be a `data family`, and `type` families are more flexible), but I find
 myself writing `data` types for some of the instances anyway.

 Presently you're forced to use type synonyms for all the instances of a
 `type family`, even if you don't intend to refer to the type by any name
 other than that of the type family. I find this tiresome:

 {{{#!hs
 type family F a

 type instance F Char = Int

 newtype FBool = FBool Int
 type instance F Bool = FBool

 data FInt = A | B | ...
 type instance F Int = FInt
 }}}

 '''I want to write `data instance`s and `newtype instance`s for `type`
 families'''. These proposed constructs would introduce a new type without
 a name of its own (the only way to refer to it would be through the `type
 family`), just like the current design of `data` families. The above code
 would be roughly equivalent to:

 {{{#!hs
 type family F a

 type instance F Char = Int  -- already legal
 newtype instance F Bool = FBool Int
 data instance F Int = A | B | ...
 }}}

 I call this idea ''anonymous type instances'' but I'm open to suggestions
 for a snappier name.


 Going the other way (writing a `type instance` for a `data family`) would
 remain illegal under this proposal.

 {{{#!hs
 data family G a

 type instance G Char = Int  -- still illegal
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10978>
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