#2101: Allow some form of type-level lemma
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Reporter: guest | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.8.2
Severity: normal | Resolution:
Keywords: | Testcase:
Architecture: Unknown | Os: Unknown
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Comment (by ryani):
Replying to [comment:1 Isaac Dupree]:
> can `cat_nil` already be optimized away by `ghc -O`? Or might it return
`_|_`? (is it possible for its recursion not to terminate?)
I am pretty sure that cat_nil cannot be optimized away. Consider the
following additions to the program:
{{{
type instance Cat [a] () = [(a,a)]
unsoundCoerce :: Exp (Cat [a] ()) -> Exp [a]
unsoundCoerce = coerce undefined
}}}
If cat_nil is optimized away then unsoundCoerce exp does an invalid
conversion from Exp [(a,a)] to Exp [a].
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2101#comment:2>
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