#2722: <<loop> when compiling with -O option with ghc-6.10.0.20081019
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Reporter: uwe | Owner:
Type: bug | Status: new
Priority: high | Milestone: 6.10.1
Component: Compiler | Version: 6.10.1
Severity: normal | Resolution:
Keywords: Optimization Loop | Difficulty: Unknown
Testcase: | Architecture: x86
Os: Linux |
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Comment (by simonpj):
OK I know what is happening. Sigh.
I boiled it down still more:
{{{
module Foo (main, arid1, arid2) where
import Prelude hiding (id, (.))
import qualified Prelude
import Control.Category
import Control.Arrow
import System.IO
import Debug.Trace
main = runXIOState arid1 -- Loop with arid1
-- Works with arid2
arid1 :: Arrow m => m a a
arid1 = arr id
arid2 :: Arrow m => m a a
arid2 = arr Prelude.id
runXIOState :: IOSLA () c -> IO [c]
runXIOState f = runIOSLA f ()
newtype IOSLA a b = IOSLA { runIOSLA :: a -> IO [b] }
instance Arrow IOSLA where
arr f = IOSLA $ \ x -> return [f x]
instance Category IOSLA where
id = arr id
-- arr :: Arrow m => (b->c) -> m b c
-- id :: Category m => m b b
-- (arr id) :: Arrow m => m a a
}}}
What is happening is this.
* In `Control.Arrow` we find the following rewrite rule
{{{
{-# RULES "identity" arr id = id
}}}
* But, as you see above, `id` is defined to be `arr id`.
So the result is (unsurprisingly) a loop.
I'm not sure whether the fault lies with the person who wrote the RULE in
`Control.Arrow`, or the person who wrote the instance of IOSLA above.
(The instance does look suspicious, because `Category` is a superclass of
`Arrow`, and yet uses the arrow `arr` method to define `id`.)
Very tricky to track down. At all events I say that GHC is not at fault.
I'm leaving this open because someone should
* Decide whether the rules in `Control.Arrow` are valid in general.
* If so, write some documentation somewhere about the constraints that
these rules impose on instance declarations.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2722#comment:5>
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