How to force evaluation in Haskell?

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.