[Haskell-cafe] Foreign function performance: monadic vs pure

[Serguei Son]

Consider two versions of sin wrapped:
foreign import ccall "math.h sin"
    c_sin_m :: CDouble -> IO CDouble
and
foreign import ccall "math.h sin"
    c_sin :: CDouble -> CDouble

One can invoke them so:

mapM c_sin_m [1..n]
mapM (return . c_sin) [1..n]

On my computer with n = 10^7 the first
version never finishes, whereas the second
one calculates the result within seconds.

To give you my context, I need to call
a random variable generator multiple times,
so that it must return IO a.

Any explanation for this behavior?