Cookbook/Interactivity

Input and output

Problem	Solution	Examples
printing a string	putStr	Prelude> putStr "Foo" FooPrelude>
printing a string with a newline character	putStrLn	Prelude> putStrLn "Foo" Foo Prelude>
reading a string	getLine	Prelude> getLine Foo bar baz --> "Foo bar baz"

Parsing command line arguments

Command line argument processing is provided by System.Environment library.

The following small program (tac) concatenates and prints the contents of files in reverse (or reads from stdin with no arguments).

-- 
import System.Environment
import System.Exit

main = getArgs >>= parse >>= putStrLn . tac

tac = unlines . reverse . lines

parse ["-h"] = usage >> exit
parse ["-v"] = version >> exit
parse []     = getContents
parse fs     = concat `fmap` mapM readFile fs

usage   = putStrLn "usage: tac [-vh] [file..]"
version = putStrLn "Haskell tac 0.1"
exit    = exitWith ExitSuccess
die     = exitWith (ExitFailure 1)

Some example uses:

$ ./tac -h
usage: tac [-vh] [file..]

The environment

Many programs need access to computer environment variables. On POSIX systems, access to all variables is by

import System.Environment

NOTE: example uses to be added