#2962: Reduce space usage of genericLength for common Num instances
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Reporter: thorkilnaur | Owner: thorkilnaur
Type: bug | Status: new
Priority: normal | Milestone: 6.12 branch
Component: libraries/base | Version: 6.11
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
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Comment (by thorkilnaur):
I attach a patch that adds rules for genericLength::[a]->Int and
genericLength::[a]->Integer. Adding rules for Float and Double seems
excessive, but it is of course easily done.
That the result is as intended is witnessed by:
{{{
$ ghc-6.11.20090211 --version
The Glorious Glasgow Haskell Compilation System, version 6.11.20090211
$ cat R.hs
import List
import IO
main = do hSetBuffering stdout NoBuffering
print $ map ($ [1..4000000])
[ genericLength :: [a] -> Int
, fromIntegral . (genericLength :: [a] -> Integer)
, round . (genericLength :: [a] -> Double)
]
$ ghc-6.11.20090211 -fforce-recomp --make R -O
[1 of 1] Compiling Main ( R.hs, R.o )
Linking R ...
$ ./R
[4000000,4000000,Stack space overflow: current size 8388608 bytes.
Use `+RTS -Ksize' to increase it.
$
}}}
Interestingly, this patch only solves the problem for compiled code. I
don't know if it is possible to solve it for interpreted code also.
Best regards
Thorkil
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2962#comment:4>
GHC <http://www.haskell.org/ghc/>
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