#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: normal | Milestone: Not GHC
Component: libraries (other) | Version: 6.10.1
Severity: normal | Resolution:
Keywords: arrows | Difficulty: Unknown
Testcase: | Architecture: x86
Os: Linux |
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Comment (by simonpj):
Well the `Arrow` class in `Control.Arrow` is immediately followed by a
whole bunch of rules:
{{{
{-# RULES
"identity"
arr id = id
"compose/arr" forall f g .
(arr f) . (arr g) = arr (f . g)
"first/arr" forall f .
first (arr f) = arr (first f)
"second/arr" forall f .
second (arr f) = arr (second f)
"product/arr" forall f g .
arr f *** arr g = arr (f *** g)
"fanout/arr" forall f g .
arr f &&& arr g = arr (f &&& g)
"compose/first" forall f g .
(first f) . (first g) = first (f . g)
"compose/second" forall f g .
(second f) . (second g) = second (f . g)
#-}
}}}
If you are saying that they are not valid in general (eg you say the first
is valid only for `(->)`), then these rules had better be speicialised for
the types they work for.
Can you and Ross discuss this, and figure out what to do? It's not a GHC
issue any more: rather a question of the Arrow/Category library design.
Simon
PS: Yes if you define `id` explicitly that ought to be ok. But remember
that the RULE `arr id ---> id` is still going to apply (perhaps bogusly?)
at all arrows. So you want to do more than work around the problem.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2722#comment:8>
GHC <http://www.haskell.org/ghc/>
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