Evaluation, bang patterns, and deepseq

ensure.hs

{-# LANGUAGE CPP, BangPatterns #-}

module Main where

import Control.DeepSeq
import Data.List

ensure :: [Int] -> IO [Int]
ensure xs = xs `deepseq` return xs

testList :: [Int]
testList = [1..100] ++ [error "if we get here, the whole list is evaluated"]

main :: IO ()
main = do
    !r <- ensure testList
    putStrLn "we survived without evaluating r"
    let x = foldl' (+) 0 $ take 50 r
    putStrLn $ "x was " ++ show x