{-# 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".

    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
-}

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

    -- * Conclusion
    -- $conclusion
    ) 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 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:
--
-- > $ # `-O2` turns on all optimizations
-- > $ # `-threaded` helps with piping shell output in and out of Haskell
-- > $ 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:
--
-- > $ 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@.

-- $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 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

-- $do
-- 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.`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 `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 :: `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"

-- $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 `Turtle.FilePath` types do implement `IsString`, which
-- is why we can interpret string literals as `Text` or `Turtle.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)
--
-- 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 Text   -- 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.

-- $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:
--
-- > $ 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 `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-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"

-- $folds
--
-- There are other ways you can consume a `Shell` stream.  For example, you can
-- `fold` the stream using predefined `Fold`s from "Control.Foldl":
--
-- @
-- Prelude Turtle> import qualified "Control.Foldl" as Fold
-- Prelude Turtle Fold> `fold` (ls \"/tmp\") Fold.length
-- 9
-- @
--
-- @
-- Prelude Turtle Fold> `fold` (ls \"/tmp\") Fold.head
-- Just (FilePath \"\/tmp\/.X11-unix\")
-- @
--
-- @
-- Prelude Turtle Fold> `fold` (ls \"\/tmp\") Fold.list
-- [FilePath \"\/tmp\/.X11-unix\",FilePath \"\/tmp\/.X0-lock\",FilePath \"\/tmp\/pulse-PKd
-- htXMmr18n\",FilePath \"\/tmp\/pulse-xHYcZ3zmN3Fv\",FilePath \"\/tmp\/tracker-gabriel
-- \",FilePath \"\/tmp\/pulse-PYi1hSlWgNj2\",FilePath \"\/tmp\/orbit-gabriel\",FilePath 
-- \"\/tmp\/ssh-vREYGbWGpiCa\",FilePath \"\/tmp\/.ICE-unix\"]
-- @
--
-- You can also compute multiple things in a single pass over the stream:
--
-- > Prelude Turtle> fold (select [1..10]) ((,) <$> Fold.minimum <*> Fold.maximum)
-- > (Just 1,Just 10)
--
-- If you are interested in this feature, check out the documentation in
-- "Control.Foldl".

-- $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 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
-- @

-- $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-gabriel
-- > pulse-PYi1hSlWgNj2
-- > orbit-gabriel
-- > ssh-vREYGbWGpiCa
-- > .ICE-unix
--
-- This works because type of `inshell` is:
--
-- @
-- `inshell`
--     :: Text    -- Command line
--     -> Shell Text  -- Standard input to feed to program
--     -> Shell Text  -- 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 Text  -- Standard input to feed to program
--     -> Shell Text  -- Standard output produced by program
-- @
--
-- Using `inproc`, you would write:
--
-- > Turtle Prelude> stdout (inproc "awk" ["{ print $1 }"] "123 456")
-- > 123

-- $patterns
--
-- You can transform streams using Unix-like utilities.  For example, you can
-- filter a stream using `grep`.
--
-- @
-- Prelude Turtle> stdout (input \"file.txt\")
-- Test
-- ABC
-- 42
-- Prelude Turtle> stdout (`grep` \"ABC\" (input \"file.txt\"))
-- ABC
-- @
--
-- Let's look at the type of `grep`:
--
-- @
-- `grep` :: Pattern a -> Shell Text -> Shell Text
-- @
--
-- The first argument of `grep` is actually a `Pattern`, which implements
-- `IsString`.  When we pass a string literal we just create a `Pattern` that
-- matches the given literal.
--
-- `Pattern`s generalize regular expressions and you can use this table to
-- roughly translate several regular expression idioms to `Pattern`s:
--
-- @
-- Regex      Pattern
-- =========  =========
-- \"string\"   \"string\"
-- .          `dot`
-- e1 e2      e1 `<>` e2
-- e1 | e2    e1 `<|>` e2
-- e*         `star` e
-- e+         `plus` e
-- e*?        `selfless` (`star` e)
-- e+?        `selfless` (`plus` e)
-- e{n}       `count` n e
-- e?         `optional` e
-- [xyz]      `oneOf` \"xyz\"
-- [^xyz]     `noneOf` \"xyz\"
-- @
--
-- Here are some examples:
--
-- > Prelude Turtle> -- grep '^[[:digit:]]\+$' 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 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@.
