#5633: TypeFamilies don't seem to play with LIberalTypeSynonyms
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Reporter: ocharles | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler | Version: 7.0.4
Resolution: invalid | Keywords: TypeFamilies
LiberalTypeSynonyms
Testcase: | Blockedby:
Difficulty: | Os: Linux
Blocking: | Architecture: x86
Failure: GHC rejects valid program |
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Changes (by simonpj):
* status: new => closed
* resolution: => invalid
Comment:
The first program works because of a non-Haskell98 feature of GHC.
Fundamentally,
type synonyms, and type families, must be fully applied. However with
LiberalTypeSynonyms, GHC fully expands "outer" type synonyms before
checking for the full application of "inner" ones. In this example, we
start
with
{{{
foo : Generic Int Id
}}}
where `Id` is not fully applied. However, if `Generic` is a type synonym
we expand it before making the check, so it's now like
{{{
foo :: Id Int
}}}
and lo, `Id` is fully applies.
Now, type synonyms are easy to apply on the fly; it's another thing
entirely with type families, which can "get stuck" (ie not reduce)
if their arguments are no sufficiently informative Moreover, you can
pattern match on their arguments. For example, if you wrote
{{{
type instance Generic Int Maybe = Bool
}}}
would you expect that to work, with `Maybe` not matching against `Id`?
Presumably not.
The pure type synonyms can always reduce, and never do pattern
matching, so these issue do not arise. I can't see an easy way to
lift the restriction.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5633#comment:1>
GHC <http://www.haskell.org/ghc/>
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