How to force evaluation in Haskell?

Question

I am relatively new to Haskell and I am trying to learn how different actions can be executed in sequence using the do notation. In particular, I am writing a program to ben

Answer 1

Answer originally given by user ysdx on programmers:

Indeed you version will not benchmark your algorithm. As r is not used it will not be evaluated at all.

You should be able to do it with DeepSeq:

benchmark :: [String] -> IO Integer
benchmark inputList = do
                     start <- getCPUTime
                     let r = foo inputList
                     end <- r `deepseq` getCPUTime
                     return (end - start)

(a `deepseq` b) is some "magic" expression which forces the complete/recursive evaluation of a before returning b.

Answer 2

I would use the language extension -XBangPatterns, I find that quite expressive in such situations. So you would have to say "let !r = foo inputList" as in:

{-# LANGUAGE BangPatterns #-}
import System.CPUTime

benchmark :: [String] -> IO Integer
benchmark inputList = do
                         start <- getCPUTime
                         let !r = foo inputList
                         end <- getCPUTime
                         return (end - start)