In Need of the Most Simple Haskell Program

Question

Can anyone provide me with less than five lines of code that I can save as .hs and run as a haskell program and see the magic happen? The internet is so complicated sometimes.

Answer 1

main = putStrLn "Hello, World!"

From http://www.haskell.org/haskellwiki/Haskell_in_5_steps

The internet isn't so bad!

Answer 2

Someone should have mentioned interact which is simple and actually practical:

main = interact reverse 

$ cat interact.hs | runhaskell interact.hs
esrever tcaretni = niam

and thus with

main = interact (unwords . reverse . words)

$ cat interact.hs | runhaskell interact.hs
words) . reverse . (unwords interact = main

or with an import

import Data.List
 main = interact (intersperse '\n')

$ echo "hello" | runhaskell interact.hs
h
e
l
l
o

or, now compiling:

main = interact showCharcount 
  where showCharcount str = show (length str) ++ "\n"

$ ghc --make -O2 interact.hs -o charcount
$ echo "hello world" | ./charcount
12

In which case it makes sense to start doing a bit of poor man's benchmarking:

$ time cat /usr/share/dict/words | ./charcount
2486813
real 0m0.096s