#2600: Bind type variables in RULES
--------------------------------+-------------------------------------------
Reporter: simonpj | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.8.3
Severity: normal | Keywords:
Difficulty: Unknown | Testcase:
Architecture: Unknown | Os: Unknown
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Roman writes: here's an example I came across while working on the
recycling paper
and which I subsequently forgot about. Suppose we have:
{{{
{-# LANGUAGE Rank2Types #-}
module T where
class T t where
to :: [a] -> t a
from :: t a -> [a]
tmap :: (a -> a) -> t a -> t a
{-# RULES
"myrule" forall f x.
from (tmap f (to x)) = map f (from (to x))
#-}
}}}
Alas, this fails with:
{{{
Ambiguous type variable `t' in the constraint:
`T t' arising from a use of `to' at T.hs:12:40-43
Probable fix: add a type signature that fixes these type variable(s)
}}}
Of course, I'd like the t on the rhs to be the same as on the lhs but
I don't see how to tell this to GHC. Is it actually possible? The only
solution I've found was to add a dummy argument to 'to':
{{{
to' :: t a -> [a] -> t a
to = to' undefined
{-# RULES
"myrule" forall f t x.
from (tmap f (to' t x)) = map f (from (to' t x))
#-}
}}}
That's ugly, of course. Am I missing something or is this just
impossible to do with the current system?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2600>
GHC <http://www.haskell.org/ghc/>
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