Question

I have 2 versions of the same program with only little changes between the two. Instead of having separate files, I use #if defined (PAR) – #else – #endif and then compile with or without -cpp -DPAR to switch between the 2 versions. I like this way as you only have to work on a single hs file. However, since my aim is to write parallel/optimised version of the original program, I wonder if using #if-#else#-endif has any performance implication? Basically I would like an explanation of how this works under the hood. Thanks

#if defined(PAR)
import Control.Parallel
import Control.Parallel.Strategies
import Control.DeepSeq
#endif

#if defined(PAR)
test = sum ( map expensiveFunc myList `using` strat )
    where strat = parListChunk 100 rseq
#else 
test = sum ( map expensiveFunc myList )
#endif

Note:

Instead of the -cpp flag, you could use the language options in your source file:

e.g. {-# LANGUAGE CPP #-}

But you still need to provide (or not) -Dxxx when compiling in order to choose which part of the program the compiler should ignore ( where xxx is the defined variable in the hs file).

Answer 1

C preprocessor directives are in effect only during compilation. The compiler simply cuts out the lines within the #ifdef block and then compiles the program as usual, so there is no runtime performance penalty.