You can use Template Haskell to perform arbitrary computation at
compile-time (even if it requires IO!), and then `lift` the result into a
Haskell literal. This works for any type with a `Lift` instance (or with a
bit of trick, any serializable type).
Coming back to your use case, you may try avoid using raw lists and switch
to unboxed vectors, turn on -O2 and rely on stream fusion of the vector
package. That will result in a considerable speedup.
On Tue, Feb 27, 2018, 11:09 PM Dušan Kolář
Dear Café,
I'm trying to do very small, but impressive example about optimizations possible during Haskell compilation. So far, I can demonstrate that the following two programs (if compiled) perform computation in the same time:
1)
main =
putStrLn $ show $ sum $ map (*(2::Int)) [(1::Int)..(100000000::Int)]
2)
main =
putStrLn $! show $! sumup 1 0
sumup :: Int -> Int -> Int
sumup n total =
if n<=(100000000::Int) then sumup (n+1) $! total+(2*n)
else total
Nevertheless, I expect a question on comparison with C:
3)
#include
int main(void) {
long sum, i;
sum = 0;
for (i=1; i <= 100000000L; ++i) {
sum += 2*i;
}
printf("%ld\n",sum);
return 0;
}
Unfortunately, in this case the C is much more faster (it prints the result immediately), at least on my machine. Is it due to a fact that C compiler does a brutal optimization leading to compile-time evaluation, while ghc is not able to do that?
I'm using -O2 -dynamic --make ghc compiler flags. For gcc for C compilation just -O2, running Arch Linux.
Is there any option, how to force compile time evaluation? The reason, why I think it works this way is the fact, that when running out of long type values in C a code is generated that computes the values regularly (providing misleading value as a result) taking its time.
Best regards,
Dušan
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