#1968: data family + GADT: not implemented yet
---------------------------------------------------+------------------------
Reporter: Remi | Owner: chak
Type: bug | Status: new
Priority: normal | Milestone: 6.10 branch
Component: Compiler (Type checker) | Version: 6.9
Severity: normal | Resolution:
Keywords: data type family GADT choose_univs | Difficulty: Moderate (1
day)
Testcase: GADT13 | Os:
Unknown/Multiple
Architecture: Unknown/Multiple |
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Changes (by jeltsch):
* cc: [EMAIL PROTECTED] (added)
Comment:
Replying to [comment:4 chak]:
> What do you mean by "varying non-instance type"? Whether you write
> {{{data family Blah x y :: *}}} or {{{data family Blah x :: * -> *}}}
> makes no difference whatsoever for data families.
I think it ''does'' make a difference if you use associated types.
Consider this:
{{{
{-# LANGUAGE TypeFamilies, GADTs #-}
class C t where
data D t :: * -> *
instance C Bool where
data D Bool Int = A
}}}
This doesn’t work, since the compiler expects a type variable at the place
of the {{{Int}}}. Even more, you cannot introduce another constructor
which constructs a {{{D Bool Char}}}, for example. With GADT syntax
allowed in data family instance declarations, you could write the
following:
{{{
{-# LANGUAGE TypeFamilies, GADTs #-}
class C t where
data D t :: * -> *
instance C Bool where
data D Bool where
A :: D Bool Int
B :: D Bool Char
}}}
This code would also be different to declaring two different data family
instances ({{{D Bool Int}}} and {{{D Bool Char}}}) in that it makes it
impossible to introduce, say, a {{{D Bool Double}}} instance later. This
might be exactly what a library designer wants
I’d like to be able to use GADTs as data family instances for both of the
above reasons. I think, I’d have good use for them.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1968#comment:11>
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