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Latest revision as of 21:31, 12 January 2014
 1 Input and output
|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"
 2 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..]
 3 The environment
Many programs need access to computer environment variables. On POSIX systems, access to all variables is by
NOTE: example uses to be added