#3765: Rules should "look through" case binders too
---------------------------------+------------------------------------------
Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone: _|_
Component: Compiler | Version: 6.12.1
Keywords: | Difficulty:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Here's a program suggested by Roman
{{{
inc :: Int -> Int
{-# INLINE CONLIKE [1] inc #-}
inc n = n+1
dec :: Int -> Int
{-# INLINE [1] dec #-}
dec n = n-1
{-# RULES "dec/inc" forall n. dec (inc n) = n #-}
data T = T !Int -- The bang here prevents the rule firing
foo :: T -> Int
{-# INLINE foo #-}
foo (T n) = dec n + n
bar :: Int -> Int
bar n = foo (T (inc n))
}}}
The rule doesn't fire with the bang in the definition of T. If I
remove the bang, it fires. It should fire in both cases.
The trouble is that, with the bang, we see something like this (in the
output of phase 2 of the simplifier):
{{{
Roman.bar =
\ (n_aat :: GHC.Types.Int) ->
case Roman.inc n_aat of tpl_X3 { GHC.Types.I# ipv_skx ->
case Roman.dec tpl_X3 of _ { GHC.Types.I# x_ak2 ->
GHC.Types.I# (GHC.Prim.+# x_ak2 ipv_skx)
}
}}}
but `tpl_X3` is bound to `(I# ipv_skx)`, not to `(inc n_aat)`. Somehow we
need ''both'' unfoldings for `tpl_X3`. That seems like a big step, so
I'm just capturing the ticket but not actually doing anything about it
yet.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3765>
GHC <http://www.haskell.org/ghc/>
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