Question

Consider this block of code:

isPrime primes' n = foldr (\p r -> p * p > n || (n `rem` p /= 0 && r)) True primes'

primes = 2 : filter (isPrime primes) [3..]

main = putStrLn $ show $ sum $ takeWhile (< 1000000) primes

which calculates the sum of all primes below one million. It takes 0.468 seconds to print the result on my machine. But if the definitions of isPrime and primes are extracted into another module, the time cost is 1.23 sec, it’s almost 3x slower.

Of course I can copy/paste the difinitions everywhere it’s required, but I’m also curious about why this is happening, and how to solve it.

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[Edit]
I’m using GHC 7.0.3 (Windows 7 + MinGW). The code is written in EclipseFP (It uses Scion as IDE back-end), and built into an executable file with -O2 flags.

I also tried building the package outside the IDE:

executable test
  hs-source-dirs:  src
  main-is:         Main.hs
  build-depends:   base >= 4
  ghc-options:     -O2
  other-modules:   Primes

executable test2
  hs-source-dirs:  src2
  main-is:         Main.hs
  build-depends:   base >= 4
  ghc-options:     -O2

Here’s the result:

$ time test/test
37550402023

real    0m1.296s
user    0m0.000s
sys     0m0.031s

$ time test2/test2
37550402023

real    0m0.520s
user    0m0.015s
sys     0m0.015s

Answer 1

I can reproduce this if I put isPrime and primes in different modules. (If they are in the same module, but still separate from main, I see no difference).

Adding {-# INLINE isPrime #-} gives back the same performance as having all three in one module, so it would appear that GHC needed a nudge to do cross-module inlining in this case.

_{This is on GHC 7.0.2, Ubuntu 11.04, 64-bit}