#4259: Relax restrictions on type family instance overlap
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Reporter: lilac | Owner:
Type: feature request | Status: new
Priority: normal | Milestone: 7.8.1
Component: Compiler (Type checker) | Version: 6.12.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by carter):
Replying to [comment:23 goldfire]:
> It has also become clear that the ideas leading to this thread have a
more general setting than just allowing for relaxed overlap restrictions.
For example, given a type family for {{{And}}}, we may want GHC to be able
to simplify {{{And x y ~ True}}} to {{{(x ~ True, y ~ True)}}}... but this
is certainly not straightforward type family simplification. I'm still not
quite sure what the solution is, but my thought is that simply relaxed
restrictions on overlap will be 1) hard to do right and 2) not quite
enough power anyway.
being able to "prove" (And x y) ~ True ---> (x~ True,y~True), and other
things like it, that seems like it'd be a generalization of whats needed
for Injective type families, right?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4259#comment:26>
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