Prelude package:turtle

This module provides a large suite of utilities that resemble Unix utilities. Many of these commands are just existing Haskell commands renamed to match their Unix counterparts:

>>> :set -XOverloadedStrings

>>> cd "/tmp"

>>> pwd
FilePath "/tmp"

Some commands are Shells that emit streams of values. view prints all values in a Shell stream:

>>> view (ls "/usr")
FilePath "/usr/lib"
FilePath "/usr/src"
FilePath "/usr/sbin"
FilePath "/usr/include"
FilePath "/usr/share"
FilePath "/usr/games"
FilePath "/usr/local"
FilePath "/usr/bin"

>>> view (find (suffix "Browser.py") "/usr/lib")
FilePath "/usr/lib/python3.4/idlelib/ClassBrowser.py"
FilePath "/usr/lib/python3.4/idlelib/RemoteObjectBrowser.py"
FilePath "/usr/lib/python3.4/idlelib/PathBrowser.py"
FilePath "/usr/lib/python3.4/idlelib/ObjectBrowser.py"

Use fold to reduce the output of a Shell stream:

>>> import qualified Control.Foldl as Fold

>>> fold (ls "/usr") Fold.length
8

>>> fold (find (suffix "Browser.py") "/usr/lib") Fold.head
Just (FilePath "/usr/lib/python3.4/idlelib/ClassBrowser.py")

Create files using output:

>>> output "foo.txt" ("123" <|> "456" <|> "ABC")

>>> realpath "foo.txt"
FilePath "/tmp/foo.txt"

Read in files using input:

>>> stdout (input "foo.txt")
123
456
ABC

Format strings in a type safe way using format:

>>> dir <- pwd

>>> format ("I am in the "%fp%" directory") dir
"I am in the /tmp directory"

Commands like grep, sed and find accept arbitrary Patterns

>>> stdout (grep ("123" <|> "ABC") (input "foo.txt"))
123
ABC

>>> let exclaim = fmap (<> "!") (plus digit)

>>> stdout (sed exclaim (input "foo.txt"))
123!
456!
ABC

Note that grep and find differ from their Unix counterparts by requiring that the Pattern matches the entire line or file name by default. However, you can optionally match the prefix, suffix, or interior of a line:

>>> stdout (grep (has    "2") (input "foo.txt"))
123

>>> stdout (grep (prefix "1") (input "foo.txt"))
123

>>> stdout (grep (suffix "3") (input "foo.txt"))
123

You can also build up more sophisticated Shell programs using sh in conjunction with do notation:

{-# LANGUAGE OverloadedStrings #-}

import Turtle

main = sh example

example = do
-- Read in file names from "files1.txt" and "files2.txt"
file <- fmap fromText (input "files1.txt" <|> input "files2.txt")

-- Stream each file to standard output only if the file exists
True <- liftIO (testfile file)
line <- input file
liftIO (echo line)

See Turtle.Tutorial for an extended tutorial explaining how to use this library in greater detail.