OK, what did I do wrong here?
module Main () where
import Data.List
top = 10 ^ 8 :: Int
main = do
let numbers = [1 .. top]
print $ foldl' (+) 0 numbers
-- Runtime is 20 seconds.
#include <stdio.h>
int main(void)
{
int total = 0;
int n;
for (n = 1, n < 100000000; n++)
total += n;
printf("%d", n);
}
// Runtime is 0.8 seconds.
module Main () where
import Data.List
top = 10 ^ 8 :: Int
kernel i o = if i < top then o `seq` kernel (i+1) (o+i) else o
main = do
print $ kernel 1 0
-- Runtime is 0.5 seconds.
Clearly these two nearly identical Haskell programs should have exactly
the same runtime. Instead, one is 40x slower. Clearly something has gone
catastrophically wrong here. The whole point of GHC's extensive
optimiser passes are to turn the first example into the second example -
but something somewhere hasn't worked. Any suggestions?
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