Turtle.Tutorial

Use turtle if you want to write light-weight and maintainable shell scripts.

turtle embeds shell scripting directly within Haskell for three main reasons:

• Haskell code is easy to refactor and maintain because the language is statically typed

  • Haskell is syntactically lightweight, thanks to global type inference
  • Haskell programs can be type-checked and interpreted very rapidly (< 1 second)

  These features make Haskell ideal for scripting, particularly for replacing large and unwieldy Bash scripts.

  This tutorial introduces how to use the turtle library to write Haskell scripts. This assumes no prior knowledge of Haskell, but does assume prior knowledge of Bash or a similar shell scripting language.

  If you are already proficient with Haskell, then you can get quickly up to speed by reading the Quick Start guide at the top of Turtle.Prelude.

  To follow along with the examples, install the Haskell Platform:

  http://www.haskell.org/platform/

  ... and then install the turtle library by running:

$ cabal install turtle

Introduction

Let's translate some simple Bash scripts to Haskell and work our way up to more complex scripts. Here is an example "Hello, world!" script written in both languages:

#!/usr/bin/env runhaskell
                                    -- #!/bin/bash
{-# LANGUAGE OverloadedStrings #-}  --
                                    --
import Turtle                       --
                                    --
main = echo "Hello, world!"         -- echo Hello, world!

In Haskell you can use -- to comment out the rest of a line. The above example uses comments to show the equivalent Bash script side-by-side with the Haskell script.

You can execute the above code by saving it to the file example.hs. If you are copying and pasting the code, then remove the leading 1-space indent. After you save the file, make the script executable and run the script:

$ chmod u+x example.hs 
$ ./example.hs
Hello, world!

If you delete the first line of the program, you can also compile the above code to generate a native executable which will have a much faster startup time and improved performance:

$ ghc -O2 example.hs  # -O2 turns on all optimizations
$ ./example
Hello, world!

You can even run Haskell code interactively using ghci, which is an interactive REPL for Haskell. You can either use ghci by itself:

$ ghci
<ghci links in some libraries>
Prelude> :set -XOverloadedStrings
Prelude> import Turtle
Prelude Turtle> echo "Hello, world!"
<ghci links in some libraries>
Hello, world!
Prelude Turtle> :quit
$

... or you can load Haskell code into ghci, which will bring all top-level values from that program into scope:

$ ghci example.hs
<ghci links in some libraries>
[1 of 1] Compiling Main             ( example.hs, interpreted )
Ok, modules loaded: Main.
*Main> main
<ghci links in some libraries>
Hello, world!
*Main> :quit
$

From now on I'll omit ghci's linker output in tutorial examples. You can also silence this linker output by passing the -v0 flag to ghci.

Comparison

You'll already notice a few differences between the Haskell code and Bash code.

First, the Haskell code requires two additional lines of overhead to import the turtle library and enable overloading of string literals. This overhead is mostly unavoidable.

Second, the Haskell echo explicitly quotes its string argument whereas the Bash echo does not. In Bash every token is a string by default and you distinguish variables by prepending a dollar sign to them. In Haskell the the situation is reversed: every token is a variable by default and you distinguish strings by quoting them. The following example highlights the difference:

#!/usr/bin/env runhaskell
                                    -- #!/bin/bash
{-# LANGUAGE OverloadedStrings #-}  --
                                    --
import Turtle                       --
                                    --
str = "Hello!"                      --STR=Hello!
                                    --
main = echo str                     --echo $STR

Third, you have to explicitly assign a subroutine to main to specify which subroutine to run when your program begins. This is because Haskell lets you define things out of order. For example, we could have written our original program this way instead:

#!/usr/bin/env runhaskell

{-# LANGUAGE OverloadedStrings #-}

import Turtle

main = echo str

str = "Hello, world!"

Notice how the above program defines str after main, which is valid. Haskell does not care in what order you define top-level values or functions (using the = sign). However, the top level of a Haskell program only permits definitions. If you were to insert a statement at the top-level:

#!/usr/bin/env runhaskell

{-# LANGUAGE OverloadedStrings #-}

import Turtle

echo "Hello, world!"

... then you would get this error when you tried to run your program:

example.hs:7:1: Parse error: naked expression at top level

Subroutines

You can use do notation to create a subroutine that runs more than one command:

#!/usr/bin/env runhaskell
                                    -- #!/bin/bash
{-# LANGUAGE OverloadedStrings #-}  --
                                    --
import Turtle                       --
                                    --
main = do                           --
    echo "Line 1"                   -- echo Line 1
    echo "Line 2"                   -- echo Line 2

$ ./example.hs
Line 1
Line 2

do blocks can use either use the indentation level to control their duration or they can use curly braces and semicolons. To see the full rules for do syntax, read: http://en.wikibooks.org/wiki/Haskell/Indentation.

Some commands can return a value, and you can store the result of a command using the <- symbol. For example, the following program prints the creation time of the current working directory by storing two intermediate results:

#!/usr/bin/env runhaskell
                           -- #!/bin/bash
import Turtle              --
                           --
main = do                  --
    dir  <- pwd            -- DIR=$(pwd)
    time <- datefile dir   -- TIME=$(date -r $DIR)
    print time             -- echo $TIME

$ ./example.hs
2015-01-24 03:40:31 UTC

The main difference between = and <- is that:

• The <- symbol is overloaded and its meaning is context-dependent; in this context it just means "store the current result"
• The = symbol is not overloaded and always means that the two sides of the equality are interchangeable

do notation lets you combine smaller subroutines into larger subroutines. For example, we could refactor the above code to split the first two commands into their own smaller subroutine and then invoke that smaller subroutine within a larger subroutine:

#!/usr/bin/env runhaskell
                            -- #!/bin/bash
import Turtle               --
                            --
datePwd = do                -- datePwd() {
    dir    <- pwd           --     DIR=$(pwd)
    result <- datefile dir  --     RESULT=$(date -r $DIR)
    return result           --     echo $RESULT
                            -- }
main = do                   --
    time <- datePwd         -- TIME=$(datePwd)
    print time              -- echo $TIME

The refactored program still returns the exact same result:

$ ./example.hs
2015-01-24 03:40:31 UTC

We can also simplify the code a little bit because do notation implicitly returns the value of the last command within a subroutine. We can use this trick to simplify both the Haskell and Bash code:

datePwd = do      -- datePwd() {
    dir <- pwd    --     DIR=$(pwd)
    datefile dir  --     date -r $DIR
                  -- }

However, keep in mind that the return statement is something of a misnomer since it does not break or exit from the surrounding subroutine. All it does is create a trivial subroutine that has no side effects and returns its argument as its result. If you return an expression, you're just giving it a new name:

do x <- return expr  -- X=EXPR
   command x         -- command $X

-- Same as:
command expr         -- command EXPR

In fact, the first line is equivalent to let x = expr, which more closely mirrors the equivalent Bash syntax:

do let x = expr      -- X=EXPR
   command x         -- command $X

-- Same as:
command expr         -- command EXPR

Also, for a subroutine with a single command, you can omit the do:

main = do echo "Hello, world!"

-- Same as:
main =    echo "Hello, world!"

Types

Notice how the above Haskell example used print instead of echo. Run the following script to find out what happens if we choose echo instead:

#!/usr/bin/env runhaskell

import Turtle

main = do
    dir  <- pwd
    time <- datefile dir
    echo time

If we run that we get a type error:

$ ./example.hs

example.hs:8:10:
    Couldn't match expected type `Text' with actual type `UTCTime'
    In the first argument of `echo', namely `time'
    In a stmt of a 'do' block: echo time
    In the expression:
      do { dir <- pwd;
           time <- datefile dir;
           echo time }

The error points to the last line of our program: (example.hs:8:10) means line 8, column 10 of our program. If you study the error message closely you'll see that the echo function expects a Text value, but we passed it 'time', which was a UTCTime value. Although the error is at the end of our script, Haskell catches this error before even running the script. When we "interpret" a Haskell script the Haskell compiler actually compiles the script without any optimizations to generate a temporary executable and then runs the executable, much like Perl does for Perl scripts.

You might wonder: "where are the types?" None of the above programs had any type signatures or type annotations, yet the compiler still detected type errors correctly. This is because Haskell uses "global type inference" to detect errors, meaning that the compiler can infer the types of expressions within the program without any assistance from the programmer.

You can even ask the compiler what the type of an expression is using ghci. Let's open up the REPL and import this library so that we can study the types and deduce why our program failed:

$ ghci
Prelude> import Turtle
Prelude Turtle>

You can interrogate the REPL for an expression's type using the :type command:

Prelude Turtle> :type pwd
pwd :: IO Turtle.FilePath

Whenever you see something of the form (x :: t), that means that 'x' is a value of type 't'. The REPL says that pwd is a subroutine (IO) that returns a FilePath. The Turtle prefix before FilePath is just the module name since the FilePath exported by the turtle library conflicts with the default FilePath exported by Haskell's Prelude. The compiler uses the fully qualified name, Turtle.FilePath, to avoid ambiguity.

We can similarly ask for the type of datefile:

Prelude Turtle> :type datefile
datefile :: Turtle.FilePath -> IO UTCTime

datefile is a function whose argument must be a FilePath and whose result is a subroutine (IO) that returns a UTCTime. Notice how the input argument of datefile (which is a FilePath) is the same type as the return value of pwd (also a FilePath).

Now let's study type of echo to see why we get the type error:

Prelude Turtle> :type echo
echo :: Text -> IO ()

The above type says that echo is a function whose argument is a value of type Text and whose result is a subroutine (IO) with an empty return value (denoted '()').

Now we can understand the type error: echo expects a Text argument but datefile returns a UTCTime, which is not the same thing. Unlike Bash, not everything is Text in Haskell and the compiler will not cast or coerce types for you.

The reason print worked is because print has a more general type than echo:

Prelude Turtle> :type print
print :: Show a => a -> IO ()

This type signature says that print can display any value of type 'a' so long as 'a' implements the Show interface. In this case UTCTime does implement the Show interface, so everything works out when we use print.

This library provides a helper function that lets you convert any type that implements Show into a Text value:

-- This behaves like Python's `repr` function
repr :: Show a => a -> Text

You could therefore implement print in terms of echo and repr:

 print x = echo (repr x)

Shell

You can use ghci for more than just inferring types. ghci is a general-purpose Haskell shell for your system when you extend it with turtle:

$ ghci
Prelude> :set -XOverloadedStrings
Prelude> import Turtle
Prelude Turtle> cd "/tmp"
Prelude Turtle> pwd
FilePath "/tmp"
Prelude Turtle> mkdir "test"
Prelude Turtle> cd "test"
Prelude Turtle> touch "file"
Prelude Turtle> testfile "file"
True
Prelude Turtle> rm "file"
Prelude Turtle> testfile "file"
False

You can also optionally configure ghci to run the first two commands every time you launch ghci. Just create a .ghci within your current directory with these two lines:

:set -XOverloadedStrings
import Turtle

The following ghci examples will all assume that you run these two commands at the beginning of every session, either manually or automatically. You can even enable those two commands permanently by adding the above .ghci file to your home directory.

Within ghci you can run a subroutine and ghci will print the subroutine's value if it is not empty:

Prelude Turtle> shell "true" empty
ExitSuccess
Prelude Turtle> shell "false" empty
ExitFailure 1

You can also type in a pure expression and ghci will evaluate that expression:

Prelude Turtle> 2 + 2
4
Prelude Turtle> "123" <> "456"  -- (<>) concatenates strings
"123456"

This works because ghci automatically wraps anything that's not a subroutine with print. It's as if we had written:

Prelude Turtle> print (2 + 2)
4
Prelude Turtle> print ("123" <> "456")
"123456"

Type signatures

Haskell performs global type inference, meaning that the compiler never requires any type signatures. When you add type signatures, they are purely for the benefit of the programmer and behave like machine-checked documentation.

Let's illustrate this by adding types to our original script:

#!/usr/bin/env runhaskell

import Turtle

datePwd :: IO UTCTime  -- Type signature
datePwd = do
    dir <- pwd
    datefile dir

main :: IO ()          -- Type signature
main = do
    time <- datePwd
    print time

The first type signature says that datePwd is a subroutine that returns a UTCTime:

--         +----- A subroutine ...
--         |
--         |  +-- ... that returns `UTCTime`
--         |  |
--         v  v
datePwd :: IO UTCTime

The second type signature says that main is a subroutine that returns an empty value:

--      +----- A subroutine ...
--      |
--      |  +-- ... that returns an empty value (i.e. `()`)
--      |  |
--      v  v
main :: IO ()

Not every top-level value has to be a subroutine, though. For example, you can define unadorned Text values at the top-level, as we saw previously:

#!/usr/bin/env runhaskell

{-# LANGUAGE OverloadedStrings #-}

import Turtle

str :: Text
str = "Hello!"

main :: IO ()
main = echo str

These type annotations do not assist the compiler. Instead, the compiler independently infers the type and then checks whether it matches the documented type. If there is a mismatch the compiler will raise a type error.

Let's test this out by providing an incorrect type for 'str':

#!/usr/bin/env runhaskell

{-# LANGUAGE OverloadedStrings #-}

import Turtle

str :: Int
str = "Hello!"

main :: IO ()
main = echo str

If you run that script, you will get two error messages:

$ ./example.hs

example.hs:8:7:
    No instance for (IsString Int)
      arising from the literal `"Hello, world!"'
    Possible fix: add an instance declaration for (IsString Int)
    In the expression: "Hello, world!"
    In an equation for `str': str = "Hello, world!"

example.hs:11:13:
    Couldn't match expected type `Text' with actual type `Int'
    In the first argument of `echo', namely `str'
    In the expression: echo str
    In an equation for `main': main = echo str

The first error message relates to the OverloadedStrings extensions. When we enable OverloadedStrings the compiler overloads string literals, interpreting them as any type that implements the IsString interface. The error message says that Int does not implement the IsString interface so the compiler cannot interpret a string literal as an Int. On the other hand the Text and FilePath types do implement IsString, which is why we can interpret string literals as Text or FilePath values.

The second error message says that echo expects a Text value, but we declared str to be an Int, so the compiler aborts compilation, requiring us to either fix or delete our type signature.

Notice that there is nothing wrong with the program other than the type signature we added. If we were to delete the type signature the program would compile and run correctly. The sole purpose of this type signature is for us to communicate our expectations to the compiler so that the compiler can alert us if the code does not match our expectations.

Let's also try reversing the type error, providing a number where we expect a string:

#!/usr/bin/env runhaskell

{-# LANGUAGE OverloadedStrings #-}

import Turtle

str :: Text
str = 4

main :: IO ()
main = echo str

This gives a different error:

$ ./example.hs

example.hs:8:7:
    No instance for (Num Text)
      arising from the literal `4'
    Possible fix: add an instance declaration for (Num Text)
    In the expression: 4
    In an equation for `str': str = 4

Haskell also automatically overloads numeric literals, too. The compiler interprets integer literals as any type that implements the Num interface. The Text type does not implement the Num interface, so we cannot interpret integer literals as Text strings.

System

You can invoke arbitrary shell commands using the shell command. For example, we can write a program that creates an empty directory and then uses a shell command to archive the directory:

#!/usr/bin/env runhaskell
                                             -- #!/bin/bash
{-# LANGUAGE OverloadedStrings #-}           --
                                             --
import Turtle                                --
                                             --
main = do                                    --
    mkdir "test"                             -- mkdir test
    shell "tar czf test.tar.gz test" empty   -- tar czf test.tar.gz test

If you run this program, it will generate the test.tar.gz archive:

$ ./example.hs
ExitSuccess
$ echo $?
0
$ ls test.tar.gz
test.tar.gz

Like ghci, the runhaskell command running our script prints any non-empty result of the main subroutine (ExitSuccess in this case).

The easiest way to learn a new command like shell is to view its documentation. Click on the word shell, which will take you to documentation that looks like this:

shell
    :: Text         -- Command line
    -> Shell Text   -- Standard input (as lines of `Text`)
    -> IO ExitCode  -- Exit code of the shell command

The first argument is a Text representation of the command to run. The second argument lets you feed input to the command, and you can provide empty for now to feed no input.

The final result is an ExitCode, which you can use to detect whether the command completed successfully. For example, we could print a more descriptive error message if an external command fails:

#!/usr/bin/env runhaskell

{-# LANGUAGE OverloadedStrings #-}

import Turtle

main = do
    let cmd = "false"
    x <- shell cmd empty
    case x of
        ExitSuccess   -> return ()
        ExitFailure n -> die (cmd <> " failed with exit code: " <> repr n)

This prints an error message since the false command always fails:

$ ./example.hs
example.hs: user error (false failed with exit code: 1)

Most of the commands in this library do not actually invoke an external shell. Instead, they indirectly wrap other Haskell libraries that bind to C code.

String formatting

This library provides type-safe string formatting utilities, too. For example, instead of writing this:

 cmd <> " failed with exit code: " <> repr n

... you could format the string using printf style instead:

 format (s%" failed with exit code: "%d) cmd n

What's neat is that the compiler will automatically infer the number of arguments and their types from the Format string:

$ ghci
Prelude Turtle> :type format (s%" failed with exit code: "%d)
format (s%" failed with exit code: "%d) :: Text -> Int -> Text

The compiler deduces that the above Format string requires one argument of type Text to satisfy the s at the beginning of the format string and another argument of type Int to satisfy the d at the end of the format string.

If you are interested in this feature, check out the Turtle.Format module for more details.

Streams

The turtle library provides support for streaming computations, just like Bash. The primitive turtle streams are little more verbose than their Bash counterparts, but turtle streams can be built and combined in more ways.

The key type for streams is the Shell type, which represents a stream of values. For example, the ls function has a streaming result:

Prelude Turtle> :type ls
ls :: Turtle.FilePath -> Shell Turtle.FilePath

That type says that ls takes a single FilePath as its argument (the directory to list) and the result is a Shell stream of FilePaths (the immediate children of that directory).

You can't run a Shell stream directly within ghci. You will get a type error like this if you try:

Prelude Turtle> ls "/tmp"

<interactive>:2:1:
    No instance for (Show (Shell Turtle.FilePath))
      arising from a use of `print'
    Possible fix:
      add an instance declaration for (Show (Shell Turtle.FilePath))
    In a stmt of an interactive GHCi command: print it

Instead, you must consume the stream as it is generated and the simplest way to consume a Shell stream is view:

view :: Show a => Shell a -> IO ()

view takes any Shell stream of values and prints them to standard output:

Prelude Turtle> view (ls "/tmp")
FilePath "/tmp/.X11-unix"
FilePath "/tmp/.X0-lock"
FilePath "/tmp/pulse-PKdhtXMmr18n"
FilePath "/tmp/pulse-xHYcZ3zmN3Fv"
FilePath "/tmp/tracker-gabriel"
FilePath "/tmp/pulse-PYi1hSlWgNj2"
FilePath "/tmp/orbit-gabriel"
FilePath "/tmp/ssh-vREYGbWGpiCa"
FilePath "/tmp/.ICE-unix

You can build your own Shell streams using a few primitive operations,

The first primitive is empty, which represents an empty stream of values:

Prelude Turtle> view empty  -- Outputs nothing
Prelude Turtle>

Another way to say that is:

view empty = return ()

The type of empty is:

empty :: Shell a

The lower-case 'a' is "polymorphic", meaning that it will type check as any type. That means that you can produce an empty stream of any type of value.

The next simplest function is return, which lets you take any value and transform it into a singleton Shell that emits just that one value:

Prelude Turtle> view (return 1)
1

Another way to say that is:

view (return x) = print x

The type of return is:

return :: a -> Shell a

Notice that this is the same return function we saw before. This is because return is overloaded and works with both IO and Shell.

You can also take any subroutine (IO) and transform it into a singleton Shell:

Prelude Turtle> view (liftIO readline)
ABC<Enter>
Just "ABC"

Another way to say that is:

view (liftIO io) = do x <- io
                      print x

The type of liftIO is:

liftIO :: IO a -> Shell a

Once you have those primitive Shell streams you can begin to combine them into larger Shell streams. For example, you can concatenate two Shell streams using (<|>):

view (return 1 <|> return 2)
1
2

Another way to say that is:

view (xs <|> ys) = do view xs
                      view ys

The type of (<|>) is:

(<|>) :: Shell a -> Shell a -> Shell a

In other words, you can concatenate two Shell streams of the same element type to get a new Shell stream, also of the same element type.

Let's try using (<|>) on two real streams:

Prelude Turtle> view (ls "/tmp" <|> ls "/usr")
FilePath "/tmp/.X11-unix"
FilePath "/tmp/.X0-lock"
FilePath "/tmp/pulse-PKdhtXMmr18n"
FilePath "/tmp/pulse-xHYcZ3zmN3Fv"
FilePath "/tmp/tracker-gabriel"
FilePath "/tmp/pulse-PYi1hSlWgNj2"
FilePath "/tmp/orbit-gabriel"
FilePath "/tmp/ssh-vREYGbWGpiCa"
FilePath "/tmp/.ICE-unix"
FilePath "/usr/lib"
FilePath "/usr/src"
FilePath "/usr/sbin"
FilePath "/usr/include"
FilePath "/usr/share"
FilePath "/usr/games"
FilePath "/usr/local"
FilePath "/usr/bin"

Finally, note that Shell implements the IsString interface, so a string literal will type-check as a Shell that emits a single Text value:

Prelude Turtle> view "123"
"123"
Prelude Turtle> view (return "123")  -- Same thing
"123"
Prelude Turtle> view ("123" <|> "456")
"123"
"456"
Prelude Turtle> view (return "123" <|> return "456")  -- Same thing
"123"
"456"

Loops

This library also provides the select function for conveniently emitting a list of values:

Prelude Turtle> view (select [1, 2, 3])
1
2
3

We can use select to implement loops within a Shell:

#!/usr/bin/env runhaskell
                                    -- #!/bin/bash
{-# LANGUAGE OverloadedStrings #-}  --
                                    --
import Turtle                       --
                                    --
example = do                        --
    x <- select [1, 2]              -- for x in 1 2; do
    y <- select [3, 4]              --     for y in 3 4; do
    liftIO (print (x, y))           --         echo \(${x},${y}\);
                                    --     done;
main = sh example                   -- done

That will print every permutation of 'x' and 'y':

$ ./example
(1,3)
(1,4)
(2,3)
(2,4)

This uses the sh utility instead of view. The only difference is that sh doesn't print any values (since print is doing that already):

sh :: Shell a -> IO ()

This trick isn't limited to select. You can loop over the output of any Shell by just binding its result. For example, this is how view loops over its argument:

view :: Show a => Shell a -> IO ()
view s = sh (do
    x <- s -- `x` ranges over every output of `s`
    liftIO (print x) )

You can also loop over a stream in a one-liner, still using do notation. Just insert semi-colons between statements:

Prelude Turtle> -- for file in /tmp/*; do echo $file; done
Prelude Turtle> sh (do file <- ls "/tmp"; liftIO (print file))
FilePath "/tmp/.X11-unix"
FilePath "/tmp/.X0-lock"
FilePath "/tmp/pulse-PKdhtXMmr18n"
FilePath "/tmp/pulse-xHYcZ3zmN3Fv"
FilePath "/tmp/tracker-gabriel"
FilePath "/tmp/pulse-PYi1hSlWgNj2"
FilePath "/tmp/orbit-gabriel"
FilePath "/tmp/ssh-vREYGbWGpiCa"
FilePath "/tmp/.ICE-unix"

Folds

There are other ways you can consume a Shell stream. For example, you can fold the stream using predefined Folds from Control.Foldl:

Prelude Turtle> import qualified Control.Foldl as Fold
Prelude Turtle Fold> fold (ls "/tmp") Fold.length
9

Prelude Turtle Fold> fold (ls "/tmp") Fold.head
Just (FilePath "/tmp/.X11-unix")

Prelude Turtle Fold> fold (ls "/tmp") Fold.list
[FilePath "/tmp/.X11-unix",FilePath "/tmp/.X0-lock",FilePath "/tmp/pulse-PKd
htXMmr18n",FilePath "/tmp/pulse-xHYcZ3zmN3Fv",FilePath "/tmp/tracker-gabriel
",FilePath "/tmp/pulse-PYi1hSlWgNj2",FilePath "/tmp/orbit-gabriel",FilePath 
"/tmp/ssh-vREYGbWGpiCa",FilePath "/tmp/.ICE-unix"]

You can also compute multiple things in a single pass over the stream:

Prelude Turtle> fold (select [1..10]) ((,) <$> Fold.minimum <*> Fold.maximum)
(Just 1,Just 10)

If you are interested in this feature, check out the documentation in Control.Foldl.

Input and output

turtle comes with built-in support for the standard text streams.

For example, you can write to standard output using the stdout utility:

stdout :: Shell Text -> IO ()
stdout s = sh (do
    txt <- s
    liftIO (echo txt)

stdout outputs each Text value on its own line:

Prelude Turtle> stdout "Line 1"
Line 1
Prelude Turtle> stdout ("Line 1" <|> "Line 2")
Line 1
Line 2

Another useful stream is stdin, which emits one line of Text per line of standard input:

stdin :: Shell Text

Let's combine stdin and stdout to forward all input from standard input to standard output:

#!/usr/bin/env runhaskell
                                    -- #!/bin/bash
{-# LANGUAGE OverloadedStrings #-}  --
                                    --
import Turtle                       --
                                    --
main = stdout stdin                 -- cat

If you run that it will continue to echo lines until you signal end of input using Ctrl-D:

$ ./example.hs
ABC<Enter>
ABC
Test<Enter>
Test
42<Enter>
42
<Ctrl-D>
$

You can also read and write to files using the input and output utilities:

Prelude Turtle> output "file.txt" ("Test" <|> "ABC" <|> "42")
Prelude Turtle> stdout (input "file.txt")
Test
ABC
42

Patterns

You can transform streams using Unix-like utilities. For example, you can filter a stream using grep.

Prelude Turtle> stdout (input "file.txt")
Test
ABC
42
Prelude Turtle> stdout (grep "ABC" (input "file.txt"))
ABC

Let's look at the type of grep:

grep :: Pattern a -> Shell Text -> Shell Text

The first argument of grep is actually a Pattern, which implements IsString. When we pass a string literal we just create a Pattern that matches the given literal.

Patterns generalize regular expressions and you can use this table to roughly translate several regular expression idioms to Patterns:

Regex      Pattern
=========  =========
"string"   "string"
.          dot
e1 e2      e1 <> e2
e1 | e2    e1 <|> e2
e*         star e
e+         plus e
e*?        selfless (star e)
e+?        selfless (plus e)
e{n}       count n e
e?         optional e
[xyz]      oneOf "xyz"
[^xyz]     noneOf "xyz"

Here are some examples:

Prelude Turtle> -- grep '^[[:digit:]]\+$' file.txt
Prelude Turtle> stdout (grep (plus digit) (input "file.txt"))
42
Prelude Turtle> -- grep '^[[:digit:]]\+\|Test$' file.txt
Prelude Turtle> stdout (grep (plus digit <|> "Test") (input "file.txt"))
Test
42

Note that turtle's grep subtly differs from the traditional grep command. The Pattern you provide must match the entire line. If you want to match the interior of a line, you can use the has utility:

Prelude Turtle> -- grep B file.txt
Prelude Turtle> stdout (grep (has "B") (input "file.txt"))
ABC

You can also use prefix or suffix to match the beginning or end of a string, respectively:

Prelude Turtle> -- grep '^A' file.txt
Prelude Turtle> stdout (grep (prefix "A") (input "file.txt"))
ABC
Prelude Turtle> -- grep 'C$' file.txt
Prelude Turtle> stdout (grep (suffix "C") (input "file.txt"))
ABC

sed also uses Patterns, too, and is more flexible than Unix sed:

Prelude Turtle> -- sed 's/C/D/g' file.txt
Prelude Turtle> stdout (sed ("C" *> return "D") (input "file.txt"))
Test
ABD
42
Prelude Turtle> -- sed 's/[[:digit:]]/!/g' file.txt
Prelude Turtle> stdout (sed (digit *> return "!") (input "file.txt"))
Test
ABC
!!
Prelude Turtle> import qualified Data.Text as Text
Prelude Turtle> -- rev file.txt
Prelude Turtle> stdout (sed (fmap Text.reverse (plus dot)) (input "file.txt"))
tseT
CBA
24
Prelude Turtle>

You can also use Patterns by themselves to parse arbitrary text into more structured values:

Prelude Turtle> let pair = do x <- decimal; " "; y <- decimal; return (x, y)
Prelude Turtle> :type pair
pair :: Pattern (Integer, Integer)
Prelude Turtle> match pair "123 456"
[(123,456)]
Prelude Turtle> data Pet = Cat | Dog deriving (Show)
Prelude Turtle> let pet = ("cat" *> return Cat) <|> ("dog" *> return Dog) :: Pattern Pet
Prelude Turtle> match pet "dog"
[Dog]
Prelude Turtle> match (pet `sepBy` ",") "cat,dog,cat"
[[Cat,Dog,Cat]]

See the Turtle.Pattern module for more details if you are interested in writing more complex Patterns.

Exception Safety

Sometimes you may want to acquire resources and ensure they get released correctly if there are any exceptions. You can use Managed resources to acquire things safely within a Shell.

You can think of a Managed resource as some resource that needs to be acquired and then released afterwards. Example: you want to create a temporary file and then guarantee it's deleted afterwards, even if the program fails with an exception.

Turtle.Prelude provides two Managed utilities for creating temporary directories or files:

mktempdir
    :: FilePath          -- Parent directory
    -> Text              -- Directory name template
    -> Managed FilePath  -- Temporary directory

mktemp
    :: FilePath                    -- Parent directory
    -> Text                        -- File name template
    -> Managed (FilePath, Handle)  -- Temporary file

You can acquire a Managed resource within a Shell with using:

using :: Managed a -> Shell a

... and here is an example of creating a temporary directory and file within a Shell:

#!/usr/bin/env runhaskell

{-# LANGUAGE OverloadedStrings #-}

import Turtle

main = sh (do
    dir       <- using (mktempdir "/tmp" "turtle")
    (file, _) <- using (mktemp dir "turtle")
    liftIO (print file) )

When you run the above script it will print out the name of the temporary directory and file:

$ ./example.hs
FilePath "/tmp/turtle15976/turtle15976"

... and you can verify that they were deleted afterwards:

Turtle Prelude> view (find (has "turtle") "/tmp")
Turtle Prelude> -- No results

As an exercise, try inserting an exception and verifying that the temporary: file and directory are still cleaned up correctly:

#!/usr/bin/env runhaskell

{-# LANGUAGE OverloadedStrings #-}

import Turtle

main = sh (do
    dir       <- using (mktempdir "/tmp" "turtle")
    (file, _) <- using (mktemp dir "turtle")
    liftIO (print file)
    liftIO (die "Urk!") )

To learn more about Managed resources, read the documentation in Control.Monad.Managed.

Conclusion

By this point you should be able to write basic shell scripts in Haskell. If you would like to learn more advanced tricks, take the time to read the documentation in these modules:

• Turtle.Prelude
• Turtle.Format
• Turtle.Pattern
• Turtle.Shell
• Control.Foldl
• Control.Monad.Managed

If you have more questions or need help learning the library, ask a question on Stack Overflow under the haskell-turtle tag. For bugs or feature requests, create an issue on Github at https://github.com/Gabriel439/Haskell-Turtle-Library/issues

This library provides an extended suite of Unix-like utilities, but would still benefit from adding more utilities for better parity with the Unix ecosystem. Pull requests to add new utilities are highly welcome!

The turtle library does not yet provide support for command line argument parsing, but I highly recommend the optparse-applicative library for this purpose. A future release of this library might include a simplified interface to optparse-applicative.