content: {-# LANGUAGE NoCPP #-}
  {-# OPTIONS_GHC -fno-warn-unused-imports #-}
  
  {-| 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".
  
      If you are on Windows, the easiest way to follow along is to install
      <https://git-scm.com/download/win Git for Windows> and use the Git Bash
      program that it installs to get a fully featured Unix-like environment.
  
      For all operating systems, the recommended way to compile and run the
      following examples is to download the `stack` package management tool by
      following the instructions here:
  
      <https://github.com/commercialhaskell/stack>
  
      ... and then run the following instruction anywhere outside of a Haskell
      project:
  
  > $ stack install turtle
  
      This tutorial will mostly focus on using Haskell as a scripting language.
      The first two lines of each script below contain boilerplate instructions
      so that `stack` will load and run the script.  This helps ensure that a
      script will run on any computer that has a `stack` executable, as `stack`
      can install a Haskell compiler if one is not already present.
      If you are curious about how these two lines work, they are described here:
  
      <https://github.com/commercialhaskell/stack/blob/master/doc/GUIDE.md#script-interpreter>
  
      If you want to make a Windows script independently executable outside of a
      Git Bash environment, you can either (A) compile the script into an
      executable or (B) run these two commands from a @cmd@ shell with
      administrator privileges to make all @*.hs@ scripts executable:
  
  > assoc .hs=Haskell
  > ftype Haskell="C:\path\to\stack.exe" "%1" %*
  -}
  
  module Turtle.Tutorial (
      -- * Introduction
      -- $introduction
  
      -- * Comparison
      -- $compare
  
      -- * Subroutines
      -- $do
  
      -- * Types
      -- $types
  
      -- * Shell
      -- $shell
  
      -- * Type signatures
      -- $signatures
  
      -- * System
      -- $system
  
      -- * String formatting
      -- $format
  
      -- * Streams
      -- $streams
  
      -- * Loops
      -- $loops
  
      -- * Folds
      -- $folds
  
      -- * Input and output
      -- $io
  
      -- * External commands
      -- $external
  
      -- * Patterns
      -- $patterns
  
      -- * Exception Safety
      -- $exceptions
  
      -- * MonadIO
      -- $monadio
  
      -- * MonadManaged
      -- $monadmanaged
  
      -- * Command line options
      -- $cmdline
  
      -- * Conclusion
      -- $conclusion
  
      -- * Nix scripts
      -- $nix
  
      -- * FAQ
      -- $faq
      ) where
  
  import 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 stack
  -- \-\- stack \-\-resolver lts-10.2 script
  -- \ 
  --                                     -- #!\/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 two lines 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:
  --
  -- > $ # `-O2` turns on all optimizations
  -- > $ # `-threaded` helps with piping shell output in and out of Haskell
  -- > $ stack ghc -- -O2 -threaded example.hs
  -- > $ ./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:
  --
  -- > $ stack 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
  -- > $
  --
  -- From now on I'll omit @ghci@'s linker output in tutorial examples.  You can
  -- also silence this linker output by passing @--ghc-options -v0@ to
  -- @stack ghci@.
  
  -- $compare
  -- 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 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 stack
  -- > -- stack --resolver lts-10.2 script
  -- >
  -- >                                     -- #!/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 stack
  -- > -- stack --resolver lts-10.2 script
  -- > 
  -- > {-# 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 stack
  -- > -- stack --resolver lts-10.2 script
  -- > 
  -- > {-# 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
  
  -- $do
  -- You can use @do@ notation to create a subroutine that runs more than one
  -- command:
  --
  -- > #!/usr/bin/env stack
  -- > -- stack --resolver lts-10.2 script
  -- >
  -- >                                     -- #!/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 stack
  -- \-\- stack \-\-resolver lts-10.2 script
  -- \ 
  --                            -- #!\/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 stack
  -- > -- stack --resolver lts-10.2 script
  -- >
  -- >                             -- #!/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 stack
  -- > -- stack --resolver lts-10.2 script
  -- > 
  -- > 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 `Line' 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 `Line` value (a piece of text
  -- without newlines), 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:
  --
  -- > $ stack 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 :: `MonadIO` io => io Turtle.`Turtle.FilePath`
  -- @
  --
  -- For right now, ignore all occurrences of `MonadIO` and just read the type
  -- as:
  --
  -- @
  -- Prelude Turtle> :type pwd
  -- pwd :: `IO` Turtle.`Turtle.FilePath`
  -- @
  --
  -- We will cover `MonadIO` later on.
  --
  -- 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 `Turtle.FilePath`.  The "Turtle" prefix before
  -- `Turtle.FilePath` is just the module name since the `Turtle.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.`Turtle.FilePath` -> `IO` `UTCTime`
  -- @
  --
  -- `datefile` is a function whose argument must be a `Turtle.FilePath` and whose
  -- result is a subroutine (`IO`) that returns a `UTCTime`.  Notice how the
  -- input argument of `datefile` (which is a `Turtle.FilePath`) is the same type
  -- as the return value of `pwd` (also a `Turtle.FilePath`).
  --
  -- Now let's study type of `echo` to see why we get the type error:
  --
  -- @
  -- Prelude Turtle> :type echo
  -- echo :: `Line` -> `IO` ()
  -- @
  --
  -- The above type says that `echo` is a function whose argument is a value of
  -- type `Line` and whose result is a subroutine (`IO`) with an empty return
  -- value (denoted @\'()\'@).
  --
  -- `Line` is a wrapper around `Text` and represents a `Text` value with no
  -- internal newlines:
  --
  -- @
  -- newtype `Line` = `Line` `Text`
  -- @
  --
  -- Now we can understand the type error: `echo` expects a `Line` 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 any other type that implements `IsString`:
  -- 
  -- @
  -- \-\- This behaves like Python's \`repr\` function
  -- `repr` :: (`Show` a, `IsString` b) => a -> b
  -- @
  --
  -- You could therefore implement `print` in terms of `echo` and `repr`:
  --
  -- >  print x = echo (repr x)
  --
  -- ... which works because `Line` implements `IsString`
  
  -- $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@:
  --
  -- @
  -- \$ stack 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"
  
  -- $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 stack
  -- > -- stack --resolver lts-10.2 script
  -- > 
  -- > 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 `Line` values at the top-level, as we saw previously:
  --
  -- > #!/usr/bin/env stack
  -- > -- stack --resolver lts-10.2 script
  -- > 
  -- > {-# LANGUAGE OverloadedStrings #-}
  -- > 
  -- > import Turtle
  -- > 
  -- > str :: Line
  -- > 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 stack
  -- > -- stack --resolver lts-10.2 script
  -- > 
  -- > {-# 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 `Line' 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`, `Line` and `Turtle.FilePath` types do implement `IsString`,
  -- which is why we can interpret string literals as `Text`, `Line` or
  -- `Turtle.FilePath` values.
  --
  -- The second error message says that `echo` expects a `Line` 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 stack
  -- > -- stack --resolver lts-10.2 script
  -- > 
  -- > {-# LANGUAGE OverloadedStrings #-}
  -- > 
  -- > import Turtle
  -- > 
  -- > str :: Line
  -- > str = 4
  -- > 
  -- > main :: IO ()
  -- > main = echo str
  --
  -- This gives a different error:
  --
  -- > $ ./example.hs
  -- > 
  -- > example.hs:8:7:
  -- >     No instance for (Num Line)
  -- >       arising from the literal `4'
  -- >     Possible fix: add an instance declaration for (Num Line)
  -- >     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 `Line` type does not implement the `Num` interface, so we cannot
  -- interpret integer literals as `Line` 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 stack
  -- \-\- stack \-\-resolver lts-10.2 script
  -- \ 
  --                                              -- #!\/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 Line   -- 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 stack
  -- \-\- stack \-\-resolver lts-10.2 script
  -- 
  -- {-\# 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)
  --
  -- You should also check out the `proc` command, which is less powerful but
  -- safer since it decreases the likelihood of code injection or malformed
  -- commands:
  --
  -- @
  -- `proc`
  --     :: Text         -- Program
  --     -> [Text]       -- Arguments
  --     -> Shell Line   -- Standard input (as lines of \`Text\`)
  --     -> IO ExitCode  -- Exit code of the shell command
  -- @
  --
  -- Most of the commands in this library do not actually invoke an external
  -- shell or program.  Instead, they indirectly wrap other Haskell libraries that
  -- bind to C code.
  --
  -- Also, some people prefer that subprocess runners throw exceptions instead of
  -- returning an `ExitCode`.  `procs` and `shells` are the exception-throwing
  -- variations on `proc` and `shell`.
  
  -- $format
  --
  -- 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:
  --
  -- > $ stack 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.
  --
  -- Note that this is also the idiomatic way to convert a `FilePath` to `Text`:
  --
  -- > format fp :: FilePath -> Text
  --
  -- For the very common case where you `echo` a formatted value, you can use
  -- `printf`:
  --
  -- >>> printf ("Hello, "%s%"!\n") "world"
  -- Hello, world!
  --
  -- If you are interested in this feature, check out the "Turtle.Format" module
  -- for more details.  For more complex string formatting needs, check out the
  -- @text-format@ library.
  
  -- $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 `Turtle.FilePath` as its argument
  -- (the directory to list) and the result is a `Shell` stream of
  -- `Turtle.FilePath`s (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 `print`s 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-gabriella"
  -- > FilePath "/tmp/pulse-PYi1hSlWgNj2"
  -- > FilePath "/tmp/orbit-gabriella"
  -- > 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 (Line "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-gabriella"
  -- > FilePath "/tmp/pulse-PYi1hSlWgNj2"
  -- > FilePath "/tmp/orbit-gabriella"
  -- > 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 stack
  -- > -- stack --resolver lts-10.2 script
  -- > 
  -- >                                     -- #!/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 is because `Shell` behaves like a list comprehension, running each
  -- following command once for each element in the stream.  This implies that
  -- an `Shell` stream that produces 0 elements will short-circuit and prevent
  -- subsequent commands from being run.
  --
  -- > -- This stream emits 0 elements but still has side effects
  -- > inner :: Shell a
  -- > inner = do
  -- >     x <- select [1, 2]
  -- >     y <- select [3, 4]
  -- >     liftIO (print (x, y))
  -- >     empty
  -- > 
  -- > outer :: Shell ()
  -- > outer = do
  -- >     inner
  -- >     liftIO (echo "This step will never run")
  --
  -- If you want to run a `Shell` stream just for its side effects, wrap the
  -- `Shell` with `sh`.  This ensures that you don't alter the surrounding
  -- `Shell`'s control flow by unintentionally running subsequent commands zero
  -- times or multiple times:
  --
  -- > outer :: Shell ()
  -- > outer = do
  -- >     sh inner
  -- >     liftIO (echo "Now this step will exactly once")
  --
  -- 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-gabriella"
  -- > FilePath "/tmp/pulse-PYi1hSlWgNj2"
  -- > FilePath "/tmp/orbit-gabriella"
  -- > FilePath "/tmp/ssh-vREYGbWGpiCa"
  -- > FilePath "/tmp/.ICE-unix"
  --
  -- You can filter streams using @"Control.Monad".`Control.Monad.mfilter`@, like
  -- this:
  --
  -- >>> view (select [1..10])
  -- 1
  -- 2
  -- 3
  -- 4
  -- 5
  -- 6
  -- 7
  -- 8
  -- 9
  -- 10
  -- >>> view (mfilter even (select [1..10]))
  -- 2
  -- 4
  -- 6
  -- 8
  -- 10
  --
  -- This works because `Control.Monad.mfilter`'s implementation is equivalent to:
  --
  -- > mfilter predicate stream = do
  -- >     element <- stream
  -- >     if predicate element then return element else empty
  --
  -- In other words, `Control.Monad.mfilter` loops over each @element@ of the
  -- @stream@ and only `return`s the element if the @predicate@ is `True`
  
  -- $folds
  --
  -- There are other ways you can consume a `Shell` stream.  For example, you can
  -- `fold` the stream using predefined `Fold`s from "Control.Foldl":
  --
  -- @
  -- Prelude Turtle> import qualified "Control.Foldl" as Fold
  -- Prelude Turtle Fold> `fold` (ls \"/tmp\") Fold.length
  -- 9
  -- @
  --
  -- @
  -- Prelude Turtle Fold> `fold` (ls \"/tmp\") Fold.head
  -- Just (FilePath \"\/tmp\/.X11-unix\")
  -- @
  --
  -- @
  -- Prelude Turtle Fold> `fold` (ls \"\/tmp\") Fold.list
  -- [FilePath \"\/tmp\/.X11-unix\",FilePath \"\/tmp\/.X0-lock\",FilePath \"\/tmp\/pulse-PKd
  -- htXMmr18n\",FilePath \"\/tmp\/pulse-xHYcZ3zmN3Fv\",FilePath \"\/tmp\/tracker-gabriella
  -- \",FilePath \"\/tmp\/pulse-PYi1hSlWgNj2\",FilePath \"\/tmp\/orbit-gabriella\",FilePath 
  -- \"\/tmp\/ssh-vREYGbWGpiCa\",FilePath \"\/tmp\/.ICE-unix\"]
  -- @
  --
  -- You can also compute multiple things in a single pass over the stream:
  --
  -- > Prelude Turtle> fold (select [1..10]) ((,) <$> Fold.minimum <*> Fold.maximum)
  -- > (Just 1,Just 10)
  --
  -- If you are interested in this feature, check out the documentation in
  -- "Control.Foldl".
  
  -- $io
  --
  -- @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 Line -> IO ()
  -- `stdout` s = sh (do
  --     txt <- s
  --     liftIO (echo txt) )
  -- @
  --
  -- `stdout` outputs each `Line` 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` value per line of
  -- standard input:
  --
  -- @
  -- `stdin` :: Shell Line
  -- @
  --
  -- Let's combine `stdin` and `stdout` to forward all input from standard input
  -- to standard output:
  --
  -- > #!/usr/bin/env stack
  -- > -- stack --resolver lts-10.2 script
  -- > 
  -- >                                     -- #!/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
  -- @
  
  -- $external
  --
  -- You can embed external shell commands as streams within your Haskell program.
  --
  -- For example, suppose that we want to use the system's built in @ls@ command.
  -- We can just run:
  --
  -- > Prelude Turtle> stdout (inshell "ls" empty)
  -- > .X11-unix
  -- > .X0-lock
  -- > pulse-PKdhtXMmr18n
  -- > pulse-xHYcZ3zmN3Fv
  -- > tracker-gabriella
  -- > pulse-PYi1hSlWgNj2
  -- > orbit-gabriella
  -- > ssh-vREYGbWGpiCa
  -- > .ICE-unix
  --
  -- This works because type of `inshell` is:
  --
  -- @
  -- `inshell`
  --     :: Text    -- Command line
  --     -> Shell Line  -- Standard input to feed to program
  --     -> Shell Line  -- Standard output produced by program
  -- @
  --
  -- This means you can use `inshell` to embed arbitrary external utilities as
  -- first class streams within your Haskell program:
  --
  -- > Turtle Prelude> stdout (inshell "awk '{ print $1 }'" "123 456")
  -- > 123
  --
  -- You should also check out the `inproc` command, which is less powerful but
  -- safer since it decreases the likelihood of code injection or malformed
  -- commands:
  --
  -- @
  -- `inproc`
  --     :: Text        -- Program
  --     -> [Text]      -- Arguments
  --     -> Shell Line  -- Standard input to feed to program
  --     -> Shell Line  -- Standard output produced by program
  -- @
  --
  -- Using `inproc`, you would write:
  --
  -- > Turtle Prelude> stdout (inproc "awk" ["{ print $1 }"] "123 456")
  -- > 123
  
  -- $patterns
  --
  -- You can transform streams using Unix-like utilities.  For example, you can
  -- filter a stream using `grep`.
  --
  -- @
  -- Prelude Turtle> stdout (input \"file.txt\")
  -- Test
  -- ABC
  -- 42
  -- Prelude Turtle> stdout (`grep` \"ABC\" (input \"file.txt\"))
  -- ABC
  -- @
  --
  -- Let's look at the type of `grep`:
  --
  -- @
  -- `grep` :: Pattern a -> Shell Line -> Shell Line
  -- @
  --
  -- The first argument of `grep` is actually a `Pattern`, which implements
  -- `IsString`.  When we pass a string literal we just create a `Pattern` that
  -- matches the given literal.
  --
  -- `Pattern`s generalize regular expressions and you can use this table to
  -- roughly translate several regular expression idioms to `Pattern`s:
  --
  -- @
  -- Regex      Pattern
  -- =========  =========
  -- \"string\"   \"string\"
  -- .          `dot`
  -- e1 e2      e1 `<>` e2
  -- e1 | e2    e1 `<|>` e2
  -- e*         `star` e
  -- e+         `plus` e
  -- e*?        `selfless` (`star` e)
  -- e+?        `selfless` (`plus` e)
  -- e{n}       `count` n e
  -- e{m,n}     `bounded` m n e
  -- e{0,n}     `upperBounded` n e
  -- e?         `optional` e
  -- [xyz]      `oneOf` \"xyz\"
  -- [^xyz]     `noneOf` \"xyz\"
  -- @
  --
  -- Here are some examples:
  --
  -- > Prelude Turtle> -- grep '^[[:digit:]]\+$' 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 `Pattern`s, 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 `Pattern`s 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 `Pattern`s.
  
  -- $exceptions
  --
  -- 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 stack
  -- > -- stack --resolver lts-10.2 script
  -- > 
  -- > {-# 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 stack
  -- > -- stack --resolver lts-10.2 script
  -- > 
  -- > {-# 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".
  
  -- $monadio
  --
  -- If you are sick of having to type `liftIO` everywhere, you can omit it.
  -- This is because all subroutines in @turtle@ are overloaded using the
  -- `MonadIO` type class, like our original `pwd` command where we first
  -- encountered the the `MonadIO` type:
  --
  -- @
  -- Prelude Turtle> :type pwd
  -- pwd :: `MonadIO` io => io Turtle.`Turtle.FilePath`
  -- @
  --
  -- This means this command is overloaded to run in any context that
  -- implements the `MonadIO` interface, including:
  --
  -- * `IO` (obviously)
  --
  -- * `Shell`
  --
  -- * `Managed`
  --
  -- You can tell if a type constructor like `Shell` implements `MonadIO` by
  -- clicking the link to the type constructor and looking for the instance list.
  -- There you will see a list of instances like:
  --
  -- > Monad Shell
  -- > Functor Shell
  -- > MonadPlus Shell
  -- > Applicative Shell
  -- > Alternative Shell
  -- > MonadIO Shell
  -- > MonadManaged Shell
  -- > ...
  -- 
  -- These instances represent the overloaded functions associated with `Shell`
  -- and we can see from the list that `Shell` implements `MonadIO` so we can
  -- use `pwd` (or any other subroutine in this library) within a `Shell`.
  --
  -- However, not all subroutines in the Haskell ecosystem are overloaded in this
  -- way (such as `print`), so you will still occasionally need to wrap
  -- subroutines in `liftIO`.
  
  -- $monadmanaged
  --
  -- All `Managed` operations are also overloaded in @turtle@, meaning that you
  -- can omit the `using` command.  For example, we could change our last example
  -- to:
  --
  -- > #!/usr/bin/env stack
  -- > -- stack --resolver lts-10.2 script
  -- > 
  -- > {-# LANGUAGE OverloadedStrings #-}
  -- > 
  -- > import Turtle
  -- > 
  -- > main = sh (do
  -- >     dir       <- mktempdir "/tmp" "turtle"
  -- >     (file, _) <- mktemp dir "turtle"
  -- >     liftIO (print file)
  -- >     die "Urk!" )
  --
  -- Any command that is generalized over the `MonadManaged` interface can run
  -- in the following contexts:
  --
  -- * `Managed` (obviously)
  -- * `Shell`
  
  -- $cmdline
  --
  -- The "Turtle.Options" module lets you easily parse command line arguments,
  -- using either flags or positional arguments.
  --
  -- For example, if you want to write a @cp@-like script that takes two
  -- positional arguments for the source and destination file, you can write:
  --
  -- > #!/usr/bin/env stack
  -- > -- stack --resolver lts-10.2 script
  -- >
  -- > -- cp.hs
  -- >
  -- > {-# LANGUAGE OverloadedStrings #-}
  -- >
  -- > import Turtle
  -- > import Prelude hiding (FilePath)
  -- >
  -- > parser :: Parser (FilePath, FilePath)
  -- > parser = (,) <$> argPath "src"  "The source file"
  -- >              <*> argPath "dest" "The destination file"
  -- >
  -- > main = do
  -- >     (src, dest) <- options "A simple `cp` utility" parser
  -- >     cp src dest
  --
  -- If you run the script without any arguments, you will get an auto-generated
  -- usage output:
  --
  -- > $ ./cp.hs
  -- > Usage: cp.hs SRC DEST
  --
  -- ... and you can get a more descriptive output if you supply the @--help@
  -- flag:
  --
  -- > $ ./cp.hs --help
  -- > A simple `cp` utility
  -- > 
  -- > Usage: cp.hs SRC DEST
  -- > 
  -- > Available options:
  -- >   -h,--help                Show this help text
  -- >   SRC                      The source file
  -- >   DEST                     The destination file
  --
  -- ... and the script works as expected if you provide both arguments:
  --
  -- > echo "Test" > file1.txt
  -- > $ ./cp.hs file1.txt file2.txt
  -- > cat file2.txt
  -- > Test
  --
  -- This works because `argPath` produces a `Parser`:
  --
  -- > argPath :: ArgName -> Optional HelpMessage -> Parser FilePath
  --
  -- ... and multiple `Parser`s can be combined into a single `Parser` using
  -- operations from the `Applicative` type class since the `Parser` type
  -- implements the `Applicative` interface:
  --
  -- > instance Applicative Parser
  --
  -- You can also make any argument optional using the `optional` utility
  -- provided by `Control.Applicative`:
  --
  -- @
  -- `optional` :: `Alternative` f => f a -> f (Maybe a)
  -- @
  --
  -- For example, we can change our program to make the destination argument
  -- optional, defaulting to `stdout` if the user does not provide a destination:
  --
  --
  -- > #!/usr/bin/env stack
  -- > -- stack --resolver lts-10.2 script
  -- > 
  -- > {-# LANGUAGE OverloadedStrings #-}
  -- > 
  -- > import Turtle
  -- > import Prelude hiding (FilePath)
  -- > 
  -- > parser :: Parser (FilePath, Maybe FilePath)
  -- > parser = (,) <$>           argPath "src"  "The source file"
  -- >              <*> optional (argPath "dest" "The destination file")
  -- > 
  -- > main = do
  -- >     (src, mDest) <- options "A simple `cp` utility" parser
  -- >     case mDest of
  -- >         Nothing   -> input src & stdout
  -- >         Just dest -> cp src dest
  --
  -- Now the auto-generated usage information correctly indicates that the second
  -- argument is optional:
  --
  -- > $ ./cp.hs
  -- > Usage: cp.hs SRC [DEST]
  -- > 
  -- > $ ./cp.hs --help
  -- > A simple `cp` utility
  -- > 
  -- > Usage: cp.hs SRC [DEST]
  -- > 
  -- > Available options:
  -- >   -h,--help                Show this help text
  -- >   SRC                      The source file
  -- >   DEST                     The destination file
  --
  -- ... and if we omit the argument the result goes to standard output:
  --
  -- > $ ./cp.hs file1.txt
  -- > Test
  --
  -- We can use the `optional` utility because the `Parser` type also implements
  -- the `Alternative` interface:
  --
  -- > instance Alternative Parser
  --
  -- We can also specify arguments on the command lines using flags instead of
  -- specifying them positionally.  Let's change our example to specify the
  -- input and output using the @--src@ and @--dest@ flags, using @-s@ and @-d@
  -- as short-hands for the flags:
  --
  -- > #!/usr/bin/env stack
  -- > -- stack --resolver lts-10.2 script
  -- > 
  -- > {-# LANGUAGE OverloadedStrings #-}
  -- > 
  -- > import Turtle
  -- > import Prelude hiding (FilePath)
  -- > 
  -- > parser :: Parser (FilePath, FilePath)
  -- > parser = (,) <$> optPath "src"  's' "The source file"
  -- >              <*> optPath "dest" 'd' "The destination file"
  -- > 
  -- > main = do
  -- >     (src, dest) <- options "A simple `cp` utility" parser
  -- >     cp src dest
  --
  -- This now lets us specify the arguments in terms of flags:
  --
  -- > $ ./cp
  -- > Usage: cp.hs (-s|--src SRC) (-d|--dest DEST)
  -- > 
  -- > $ ./cp --help
  -- > A simple `cp` utility
  -- > 
  -- > Usage: cp.hs (-s|--src SRC) (-d|--dest DEST)
  -- > 
  -- > Available options:
  -- >   -h,--help                Show this help text
  -- >   -s,--src SRC             The source file
  -- >   -d,--dest DEST           The destination file
  -- > 
  -- > $ ./cp --src file1.txt --dest file3.txt
  -- > $ cat file3.txt
  -- > Test
  --
  -- You can also provide `subcommand` functionality such as the following
  -- example which pretends to increase or decrease the system volume:
  --
  -- > {-# LANGUAGE OverloadedStrings #-}
  -- > 
  -- > import Turtle
  -- > 
  -- > data Command = IncreaseVolume Int | DecreaseVolume Int deriving (Show)
  -- > 
  -- > parser :: Parser Command
  -- > parser
  -- >     =   fmap IncreaseVolume
  -- >             (subcommand "up" "Turn the volume up"
  -- >                 (argInt "amount" "How much to increase the volume") )
  -- >     <|> fmap DecreaseVolume
  -- >             (subcommand "down" "Turn the volume down"
  -- >                 (argInt "amount" "How much to decrease the volume") )
  -- > 
  -- > main = do
  -- >     x <- options "Volume adjuster" parser
  -- >     case x of
  -- >         IncreaseVolume n -> printf ("Increasing the volume by "%d%"\n") n
  -- >         DecreaseVolume n -> printf ("Decreasing the volume by "%d%"\n") n
  --
  -- This will provide @--help@ output at both the top level and for each
  -- subcommand:
  --
  -- > $ ./options --help
  -- > Volume adjuster
  -- > 
  -- > Usage: options (up | down)
  -- > 
  -- > Available options:
  -- >   -h,--help                Show this help text
  -- > 
  -- >   Available commands:
  -- >     up
  -- >     down
  -- > 
  -- > $ ./options up --help
  -- > Turn the volume up
  -- > 
  -- > Usage: options up AMOUNT
  -- > 
  -- > Available options:
  -- >   -h,--help                Show this help text
  -- >   AMOUNT                   How much to increase the volume
  -- >
  -- > $ ./options up 10
  -- > Increasing the volume by 10
  -- > 
  --
  -- See the "Turtle.Options" module for more details and utilities related to
  -- parsing command line options.  This module is built on top of the
  -- @optparse-applicative@ library, which provides even more extensive
  -- functionality.
  
  -- $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/Gabriella439/turtle/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!
  
  -- $nix
  --
  -- You can also turn `turtle` scripts into runnable scripts using Nix, like
  -- this:
  --
  -- > #! /usr/bin/env nix-shell
  -- > #! nix-shell -i runghc --packages "ghc.withPackages (x: [ x.turtle ])"
  -- > 
  -- > {-# LANGUAGE OverloadedStrings #-}
  -- > 
  -- > import Turtle
  -- > 
  -- > main = echo "Hello, world!"
  --
  -- ... or create an auto-reloading script like this:
  --
  -- > #! /usr/bin/env nix-shell
  -- > #! nix-shell -i "ghcid -T main" --packages ghcid "ghc.withPackages (x: [ x.turtle ])"
  -- > 
  -- > {-# LANGUAGE OverloadedStrings #-}
  -- > 
  -- > import Turtle
  -- > 
  -- > main = echo "Hello, world!"
  
  -- $faq
  --
  -- These are the most frequently asked questions from new users:
  --
  -- /Question:/ My program hangs when I run a subprocess that reads from standard
  -- input.  What do I do?
  --
  -- /Answer:/ Make sure you compile your program with the @-threaded@ flag
  --
  -- /Question:/ How do I convert `FilePath` to `Text`?
  --
  -- /Answer:/ Use @(`format` `fp`)@
  --
  -- /Question:/ My program prints some extra output every time it starts.  How do
  -- I remove it?
  --
  -- /Answer:/ Compile your program and run the executable instead of interpreting
  -- the program.
  --
  -- /Question:/ How do I transform a @(`Pattern` a)@ into a @(`Pattern` [a])@?
  --
  -- /Answer:/ Use `many` or `some` (both are from "Control.Applicative" and
  -- re-exported by "Turtle")
  --
  -- /Question:/ Why are `star` \/ `plus` not the same as `many` \/ `some`?
  --
  -- /Answer:/ Because @[Char]@ is a `String`, which is not the same thing as
  -- `Text`.  `String` is deprecated in favor of `Text` in modern Haskell code,
  -- primarily for performance reasons and also because `Text` provides better
  -- support for Unicode.
  --
  -- /Question:/ Some Haskell libraries still use `String`.  How do I convert
  -- back and forth between `String` and `Text`?
  --
  -- /Answer:/ Use @"Data.Text".`Data.Text.pack`@ and
  -- @"Data.Text".`Data.Text.unpack`@
  --
  -- /Question:/ What's the easiest way to fail with a descriptive error message
  -- if a subprocess command like `proc`/`shell` returns a non-zero exit code?
  --
  -- /Answer:/ Use @(`procs` cmd args input)@ or
  -- @(`proc` cmd args input `.||.` `die` "Descriptive error message")@ (or
  -- `shell` / `shells`, respectively)
  --
  -- /Question:/ How do I close a resource that I acquired?
  --
  -- /Answer:/ Use `runManaged`, `sh`, or (`<|>`) (all resources acquired in the
  -- left stream will close before beginning the right stream).  Alternatively,
  -- use `with` to acquire a resource for a limited scope.
  --
  -- /Question:/ How do I use @turtle@ to run another shell as a subprocess?
  --
  -- /Answer:/ Use `Turtle.system` in conjunction with the `process` library,
  -- like this:
  --
  -- > Turtle.system (System.Process.proc "/bin/sh" []) empty
  
  