haskell - Is getLine lazy? -

is getline lazy?

say have long line on input. it's sequence of numbers. need sum 3 first numbers. getline efficient , read first part of line, or have create own function lazy line reading, read characters 1 one?

will implementation efficient if sum whole line? (will there overhead due reading characters 1 one?)

import control.applicative  main =     line <- getline'     print $ sum $ map read $ take 3 $ words line  getline' :: io string getline' =     c <- getchar     if c == '\n' return [] else (c:) <$> getline' 

while getline isn't lazy, getcontents is, , can combined functions lines , words. therefore, following program read enough of stdin (up to) 3 integers first line , print sum:

main :: io () main = contents <- getcontents           let lns = lines contents               result = sum $ map read $ take 3 $ words $ head lns           print (result :: integer) 

note that, if modify program access subsequent lines -- example, if added:

putstrln $ take 80 $ lns !! 1 

to bottom of program print first 80 characters of second line, program have finish reading first line (and hang bit between last 2 lines of program) before processing first 80 characters of second. in other words, lazy line reading useful if only need read first bit of first line, if wasn't obvious -- haskell doesn't have magic way skip rest of first line second.

finally, note that, above program, if there fewer 3 integers on first line, it'll sum numbers , won't try read past first line (which think wanted). if don't care line endings , want sum first 3 numbers in file, regardless of how they're divided lines, can break contents directly words so:

main = contents <- getcontents           let result = sum $ map read $ take 3 $ words contents           print (result :: integer)