Difference between revisions of "Cookbook/Interactivity"
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usage: tac [-vh] [file..] |
usage: tac [-vh] [file..] |
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</pre> |
</pre> |
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+ | == The environment == |
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+ | Many programs need access to computer environment variables. On POSIX systems, access to all variables is by |
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+ | <pre> |
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+ | import System.Environment |
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+ | </pre> |
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+ | |||
+ | NOTE: example uses to be added |
Revision as of 21:31, 12 January 2014
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