#2203: TFs in class instances heads
-------------------------------------+--------------------------------------
Reporter: chak | Owner: chak
Type: bug | Status: new
Priority: normal | Milestone: 6.10 branch
Component: Compiler (Type checker) | Version: 6.9
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: T2203a | Architecture: Multiple
Os: Multiple |
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Comment (by claus):
Replying to [comment:3 chak]:
> I fixed the first half of ticket (ie, disallow family synonym
applications in instance heads).
This is an interesting case, considering this type family application
desugaring:
{{{
type family F a
forall a . .. F a .. === forall a . F a ~ b => .. fa ..
}}}
At first glance, your fix seems to deny this equivalence (right hand side
valid, left hand side invalid) but on further consideration, that isn't
actually the case.
The missing piece is the location of the constraint resulting from the
desugaring.
{{{
type family F a
instance Context => Class a (F a) -- 1
}}}
should desugar to
{{{
type family F a
instance Context => Class a (F a~fa => fa) -- 2
}}}
but there is no valid place to put that constraint in the instance head,
so ruling out the sugared form (1) makes sense. Moreover,
{{{
type family F a
instance (Context,F a~fa) => Class a fa -- 3
}}}
is not a desugaring of (1): (1), if it were valid, would restrict instance
selection (other instance heads might match), whereas (3) restricts
instance validity (this instance head always matches, but its context
might not be satisfiable).
(just a note to self, and other interested parties who might find this
ticket resolution confusing at first)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2203#comment:4>
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