How to find the number of cores at runtime in Haskell

Question

Does Haskell have a method for determining the number of CPU cores present on a machine at runtime?

Answer 1

Since version 6.12, GHC RTS includes a function getNumberOfProcessors, which is used to implement +RTS -N. You can access it in much the same manner as ordinary foreign functions. Warning: GHC-only and only works if the program was built with -threaded:

{-# LANGUAGE ForeignFunctionInterface #-}
import Foreign.C.Types (CInt)

foreign import ccall "getNumberOfProcessors" c_getNumberOfProcessors :: IO CInt

main :: IO ()
main = c_getNumberOfProcessors >>= print

Testing:

$ ghc --make -threaded Main.hs
[1 of 1] Compiling Main             ( Main.hs, Main.o )
Linking Main ...
$ ./Main                      
1