# optparse-applicative

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optparse-applicative is a haskell library for parsing options on

the command line, providing a powerful [applicative] interface

for composing these options.

optparse-applicative takes care of reading and validating the

arguments passed to the command line, handling and reporting errors,

generating a usage line, a comprehensive help screen, and enabling

context-sensitive bash completions.

**Table of Contents**

- [Introduction](#introduction)

- [Quick Start](#quick-start)

- [Basics](#basics)

    - [Parsers](#parsers)

    - [Applicative](#applicative)

    - [Alternative](#alternative)

    - [Running parsers](#running-parsers)

- [Builders](#builders)

    - [Regular options](#regular-options)

    - [Flags](#flags)

    - [Arguments](#arguments)

    - [Commands](#commands)

    - [Modifiers](#modifiers)

- [Custom parsing and error handling](#custom-parsing-and-error-handling)

    - [Parser runners](#parser-runners)

    - [Option readers](#option-readers)

    - [Preferences](#preferences)

    - [Disambiguation](#disambiguation)

    - [Customising the help screen](#customising-the-help-screen)

    - [Command Groups](#command-groups)

- [Bash completion](#bash-completion)

    - [Actions and completers](#actions-and-completers)

    - [Internals](#internals)

- [Arrow interface](#arrow-interface)

- [Applicative Do](#applicative-do)

- [FAQ](#faq)

- [How it works](#how-it-works)

## Introduction

The core type in optparse-applicative is a `Parser`

```haskell

data Parser a

instance Functor Parser

instance Applicative Parser

instance Alternative Parser

```

A value of type `Parser a` represents a specification for a set of

options, which will yield a value of type `a` when the command line

arguments are successfully parsed.

If you are familiar with parser combinator libraries like [parsec],

[attoparsec], or the json parser [aeson] you will feel right at

home with optparse-applicative.

If not, don't worry! All you really need to learn are a few basic

parsers, and how to compose them as instances of `Applicative` and

`Alternative`.

## Quick Start

Here's a simple example of a parser.

```haskell

import Options.Applicative

import Data.Semigroup ((<>))

data Sample = Sample

  { hello      :: String

  , quiet      :: Bool

  , enthusiasm :: Int }

sample :: Parser Sample

sample = Sample

      <$> strOption

          ( long "hello"

         <> metavar "TARGET"

         <> help "Target for the greeting" )

      <*> switch

          ( long "quiet"

         <> short 'q'

         <> help "Whether to be quiet" )

      <*> option auto

          ( long "enthusiasm"

         <> help "How enthusiastically to greet"

         <> showDefault

         <> value 1

         <> metavar "INT" )

```

The parser is built using an [applicative] style starting from a

set of basic combinators. In this example, hello is defined as an

option with a `String` argument, while quiet is a boolean flag

(called a switch) and enthusiasm gets parsed as an `Int` with help

of the `Read` type class.

The parser can be used like this:

```haskell

main :: IO ()

main = greet =<< execParser opts

  where

    opts = info (sample <**> helper)

      ( fullDesc

     <> progDesc "Print a greeting for TARGET"

     <> header "hello - a test for optparse-applicative" )

greet :: Sample -> IO ()

greet (Sample h False n) = putStrLn $ "Hello, " ++ h ++ replicate n '!'

greet _ = return ()

```

The `greet` function is the entry point of the program, while `opts`

is a complete description of the program, used when generating a

help text. The `helper` combinator takes any parser, and adds a

`help` option to it.

The `hello` option in this example is mandatory since it doesn't

have a default value, so running the program without any argument

will display an appropriate error message and a short option summary:

    Missing: --hello TARGET

    Usage: hello --hello TARGET [-q|--quiet] [--enthusiasm INT]

      Print a greeting for TARGET

Running the program with the `--help` option will display the full help text

containing a detailed list of options with descriptions

```

    hello - a test for optparse-applicative

    Usage: hello --hello TARGET [-q|--quiet] [--enthusiasm INT]

      Print a greeting for TARGET

    Available options:

      --hello TARGET           Target for the greeting

      -q,--quiet               Whether to be quiet

      --enthusiasm INT         How enthusiastically to greet (default: 1)

      -h,--help                Show this help text

```

## Basics

### Parsers

optparse-applicative provides a number of primitive parsers,

corresponding to different posix style options, through its *Builder*

interface. These are detailed in their [own section](#builders)

below, for now, here's a look at a few more examples to get a feel

for how parsers can be defined.

Here is a parser for a mandatory option with an argument:

```haskell

target :: Parser String

target = strOption

  (  long "hello"

  <> metavar "TARGET"

  <> help "Target for the greeting" )

```

One can see that we are defining an option parser for a `String`

argument, with *long* option name "hello", *metavariable* "TARGET",

and the given help text. This means that the `target` parser defined

above will require an option like

    --hello world

on the command line. The metavariable and the help text will appear

in the generated help text, but don't otherwise affect the behaviour

of the parser.

The attributes passed to the option are called *modifiers*, and are

composed using the [semigroup] operation `(<>)`.

Options with an argument such as `target` are referred to as *regular

options*, and are very common.  Another type of option is a *flag*,

the simplest of which is a boolean *switch*, for example:

```haskell

quiet :: Parser Bool

quiet = switch ( long "quiet" <> short 'q' <> help "Whether to be quiet" )

```

Here we used a `short` modifier to specify a one-letter name for

the option.  This means that this switch can be set either with

`--quiet` or `-q`.

Flags, unlike regular options, have no arguments. They simply return

a predetermined value. For the simple switch above, this is `True`

if the user types the flag, and `False` otherwise.

There are other kinds of basic parsers, and several ways to configure

them.  These are covered in the [Builders](#builders) section.

### Applicative

Now we may combine the `target` and `quiet` into a single parser that

accepts both options and returns a combined value. Given a type

```haskell

data Options = Options

  { optTarget :: String

  , optQuiet :: Bool }

```

and now it's just a matter of using `Applicative`'s apply operator `(<*>)`

to combine the two previously defined parsers

```haskell

opts :: Parser Options

opts = Options <$> target <*> quiet

```

No matter which parsers appear first in the sequence, options will

still be parsed in whatever order they appear in the command line.

A parser with such a property is sometimes called a *permutation

parser*.

In our example, a command line like:

    --target world -q

will give the same result as

    -q --target world

It is this property which leads us to an Applicative interface

instead of a Monadic one, as all option must be considered in

parallel, and can not depend on the output of other options.

Note, however, that the order of sequencing is still somewhat

significant, in that it affects the generated help text. Customisation

can be achieved easily through a lambda abstraction, with [Arrow

notation](#arrow-interface), or by taking advantage of GHC 8's

[ApplicativeDo](#applicative-do) extension.

### Alternative

It is also common to find programs that can be configured in different

ways through the command line.  A typical example is a program that

can be given a text file as input, or alternatively read it directly

from the standard input.

We can model this easily and effectively in Haskell using *sum types*:

```haskell

data Input

  = FileInput FilePath

  | StdInput

run :: Input -> IO ()

run = ...

```

We can now define two basic parsers for the components of the sum type:

```haskell

fileInput :: Parser Input

fileInput = FileInput <$> strOption

  (  long "file"

  <> short 'f'

  <> metavar "FILENAME"

  <> help "Input file" )

stdInput :: Parser Input

stdInput = flag' StdInput

  (  long "stdin"

  <> help "Read from stdin" )

```

As the `Parser` type constructor is an instance of `Alternative`, we can

compose these parsers with a choice operator `(<|>)`

```haskell

input :: Parser Input

input = fileInput <|> stdInput

```

Now `--file "foo.txt"` will be parsed as `FileInput "foo.txt"`, `--stdin`

will be parsed as `StdInput`, but a command line containing both options,

like

    --file "foo.txt" --stdin

will be rejected.

Having `Applicative` and `Alternative` instances, optparse-applicative

parsers are also able to be composed with standard combinators. For

example: `optional :: Alternative f => f a -> f (Maybe a)` will

mean the user is not required to provide input for the affected

`Parser`.

### Running parsers

Before we can run a `Parser`, we need to wrap it into a `ParserInfo`

structure, that specifies a number of properties that only apply

to top level parsers, such as a header describing what the program

does, to be displayed in the help screen.

The function `info` will help with this step.  In the [Quick Start](#quick-start)

we saw

```haskell

opts :: ParserInfo Sample

opts = info (sample <**> helper)

  ( fullDesc

  <> progDesc "Print a greeting for TARGET"

  <> header "hello - a test for optparse-applicative" )

```

The `helper` parser that we added after `opts` just creates a dummy

`--help` option that displays the help text.  Besides that, we just

set some of the fields of the `ParserInfo` structure with meaningful

values.  Now that we have a `ParserInfo`, we can finally run the

parser.  The simplest way to do so is to simply call the `execParser`

function in your `main`:

```haskell

main :: IO ()

main = do

  options <- execParser opts

  ...

```

The `execParser` function takes care of everything, including getting

the arguments from the command line, displaying errors and help

screens to the user, and exiting with an appropriate exit code.

There are other ways to run a `ParserInfo`, in situations where you

need finer control over the behaviour of your parser, or if you

want to use it in pure code. They will be covered in [Custom parsing

and error handling](#custom-parsing-and-error-handling).

## Builders

Builders allow you to define parsers using a convenient combinator-based

syntax. We have already seen examples of builders in action, like

`strOption` and `switch`, which we used to define the `opts` parser

for our "hello" example.

Builders always take a [modifier](#modifiers) argument, which is

essentially a composition of functions acting on the option, setting

values for properties or adding features.

Builders work by building the option from scratch, and eventually

lifting it to a single-option parser, ready to be combined with

other parsers using normal `Applicative` and `Alternative` combinators.

See the [haddock documentation][hackage] for `Options.Applicative.Builder`

for a full list of builders and modifiers.

There are four different kinds of options in `optparse-applicative`:

regular options, flags, arguments, and commands. In the following,

we will go over each one of these and describe the builders that

can be used to create them.

### Regular options

A *regular option* is an option which takes a single argument,

parses it, and returns a value.

A regular option can have a default value, which is used as the

result if the option is not found in the command line. An option

without a default value is considered mandatory, and produces an

error when not found.

Regular options can have *long* names, or *short* (one-character)

names, which determine when the option matches and how the argument

is extracted.

An option with a long name (say "output") is specified on the command

line as

    --output filename.txt

or

    --output=filename.txt

while a short name option (say "o") can be specified with

    -o filename.txt

or

    -ofilename.txt

Options can have more than one name, usually one long and one short,

although you are free to create options with an arbitrary combination

of long and short names.

Regular options returning strings are the most common, and they can

be created using the `strOption` builder. For example,

```haskell

strOption

   ( long "output"

  <> short 'o'

  <> metavar "FILE"

  <> value "out.txt"

  <> help "Write output to FILE" )

```

creates a regular option with a string argument (which can be

referred to as `FILE` in the help text and documentation), default

value "out.txt", a long name "output" and a short name "o".

A regular `option` can return an object of any type, and takes a

*reader* parameter which specifies how the argument should be parsed.

A common reader is `auto`, which requires a `Read` instance for the

return type and uses it to parse its argument. For example:

```haskell

lineCount :: Parser Int

lineCount = option auto

            ( long "lines"

           <> short 'n'

           <> metavar "K"

           <> help "Output the last K lines" )

```

specifies a regular option with an `Int` argument. We added an

explicit type annotation here, since without it the parser would

have been polymorphic in the output type. There's usually no need

to add type annotations, however, because the type will be normally

inferred from the context in which the parser is used.

Further information on *readers* is available [below](#option-readers).

### Flags

A *flag* is just like a regular option, but it doesn't take any

arguments, it is either present in the command line or not.

A flag has a default value and an *active value*. If the flag is

found on the command line, the active value is returned, otherwise

the default value is used. For example:

```haskell

data Verbosity = Normal | Verbose

flag Normal Verbose

  ( long "verbose"

 <> short 'v'

 <> help "Enable verbose mode" )

```

is a flag parser returning a `Verbosity` value.

Simple boolean flags can be specified using the `switch` builder, like so:

```haskell

switch

  ( long "keep-tmp-files"

 <> help "Retain all intermediate temporary files" )

```

There is also a `flag'` builder, which has no default value. This

was demonstrated earlier for our `--stdin` flag example, and is

usually used as one side of an alternative.

Another interesting use for the `flag'` builder is to count the

number of instances on the command line, for example, verbosity

settings could be specified on a scale; the following parser with

count the number of of instances of `-v` on the command line.

```haskell

length <$> many (flag' () (short 'v'))

```

Flags can be used together after a single hyphen, so  `-vvv` and

`-v -v -v` will both yield 3 for the above parser.

### Arguments

An *argument* parser specifies a positional command line argument.

The `argument` builder takes a reader parameter, and creates a

parser which will return the parsed value every time it is passed

a command line argument for which the reader succeeds. For example

```haskell

argument str (metavar "FILE")

```

creates an argument accepting any string.  To accept an arbitrary

number of arguments, combine the `argument` builder with either the

`many` or `some` combinator:

```haskell

some (argument str (metavar "FILES..."))

```

Note that arguments starting with `-` are considered options by

default, and will not be considered by an `argument` parser.

However, parsers always accept a special argument: `--`. When a

`--` is found on the command line, all the following words are

considered by `argument` parsers, regardless of whether they start

with `-` or not.

Arguments use the same *readers* as regular options.

### Commands

A *command* can be used to specify a sub-parser to be used when a

certain string is encountered in the command line.

Commands are useful to implement command line programs with multiple

functions, each with its own set of options, and possibly some

global options that apply to all of them. Typical examples are

version control systems like `git`, or build tools like `cabal`.

A command can be created using the `subparser` builder (or `hsubparser`,

which is identical but for an additional `--help` option on each

command), and commands can be added with the `command` modifier.

For example

```haskell

subparser

  ( command "add" (info addOptions ( progDesc "Add a file to the repository" ))

 <> command "commit" (info commitOptions ( progDesc "Record changes to the repository" ))

  )

```

Each command takes a full `ParserInfo` structure, which will be

used to extract a description for this command when generating a

help text.

Note that all the parsers appearing in a command need to have the

same type.  For this reason, it is often best to use a sum type

which has the same structure as the command itself. For example,

for the parser above, you would define a type like:

```haskell

data Options = Options

  { optCommand :: Command

  , ... }

data Command

  = Add AddOptions

  | Commit CommitOptions

  ...

```

Alternatively, you can directly return an `IO` action from a parser,

and execute it using `join` from `Control.Monad`.

```haskell

start :: String -> IO ()

stop :: IO ()

opts :: Parser (IO ())

opts = subparser

  ( command "start" (info (start <$> argument str idm) idm)

 <> command "stop"  (info (pure stop) idm) )

main :: IO ()

main = join $ execParser (info opts idm)

```

### Modifiers

*Modifiers* are instances of the `Semigroup` and `Monoid` typeclasses,

so they can be combined using the composition function `mappend`

(or simply `(<>)`).  Since different builders accept different sets

of modifiers, modifiers have a type parameter that specifies which

builders support it.

For example,

```haskell

command :: String -> ParserInfo a -> Mod CommandFields a

```

can only be used with [commands](#commands), as the `CommandFields`

type argument of `Mod` will prevent it from being passed to builders

for other types of options.

Many modifiers are polymorphic in this type argument, which means

that they can be used with any builder.

## Custom parsing and error handling

### Parser runners

Parsers are run with the `execParser` family of functions — from

easiest to use to most flexible these are:

```haskell

execParser       :: ParserInfo a -> IO a

customExecParser :: ParserPrefs -> ParserInfo a -> IO a

execParserPure   :: ParserPrefs -> ParserInfo a -> [String] -> ParserResult a

```

When using the `IO` functions, retrieving command line arguments

and handling exit codes and failure will be done automatically.

When using `execParserPure`, the functions

```haskell

handleParseResult :: ParserResult a -> IO a

overFailure :: (ParserHelp -> ParserHelp) -> ParserResult a -> ParserResult a

```

can be used to correctly set exit codes and display the help message;

and modify the help message in the event of a failure (adding

additional information for example).

### Option readers

Options and Arguments require a way to interpret the string passed

on the command line to the type desired. The `str` and `auto`

*readers* are the most common way, but one can also create a custom

reader that doesn't use the `Read` type class or return a `String`,

and use it to parse the option. A custom reader is a value in the

`ReadM` monad.

We provide the `eitherReader :: (String -> Either String a) -> ReadM a`

convenience function to help create these values, where a `Left` will

hold the error message for a parse failure.

```haskell

data FluxCapacitor = ...

parseFluxCapacitor :: ReadM FluxCapacitor

parseFluxCapacitor = eitherReader $ \s -> ...

option parseFluxCapacitor ( long "flux-capacitor" )

```

One can also use `ReadM` directly, using `readerAsk` to obtain the

command line string, and `readerAbort` or `readerError` within the

`ReadM` monad to exit with an error message.

One nice property of `eitherReader` is how well it composes with

[attoparsec] parsers with

```haskell

import qualified Data.Attoparsec.Text as A

attoReadM :: A.Parser a -> ReadM a

attoReadM p = eitherReader (A.parseOnly p . T.pack)

```

### Preferences

`PrefsMod`s can be used to customise the look of the usage text and

control when it is displayed; turn off backtracking of subparsers;

and turn on [disambiguation](#disambiguation).

To use these modifications, provide them to the `prefs` builder,

and pass the resulting preferences to one of the parser runners

that take an `ParserPrefs` parameter, like `customExecParser`.

### Disambiguation

It is possible to configure optparse-applicative to perform automatic

disambiguation of prefixes of long options. For example, given a

program `foo` with options `--filename` and `--filler`, typing

    $ foo --fil test.txt

fails, whereas typing

    $ foo --file test.txt

succeeds, and correctly identifies `"file"` as an unambiguous prefix

of the `filename` option.

Option disambiguation is *off* by default. To enable it, use the

`disambiguate` `PrefsMod` modifier as described above.

Here is a minimal example:

```haskell

import Options.Applicative

sample :: Parser ()

sample = () <$

  switch (long "filename") <*

  switch (long "filler")

main :: IO ()

main = customExecParser p opts

  where

    opts = info (helper <*> sample) idm

    p = prefs disambiguate

```

### Customising the help screen

optparse-applicative has a number of combinators to help customise

the usage text, and determine when it should be displayed.

The `progDesc`, `header`, and `footer` functions can be used to

specify a brief description or tagline for the program, and detailed

information surrounding the generated option and command descriptions.

Internally we actually use the [ansi-wl-pprint][ansi-wl-pprint]

library, and one can use the `headerDoc` combinator and friends if

additional customisation is required.

To display the usage text, the user may type `--help` if the `helper`

combinator has been applied to the `Parser`.

Authors can also use the preferences `showHelpOnError` or

`showHelpOnEmpty` to show the help text on any parser failure or

when a command is not complete and at the beginning of the parse

of the main program or one of its subcommands respectively.

Even if the help text is not shown for an error, a specific error

message will be, indicating what's missing, or what was unable to

be parsed.

```haskell

myParser :: Parser ()

myParser = ...

main :: IO ()

main = customExecParser p opts

  where

    opts = info (myParser <**> helper) idm

    p = prefs showHelpOnEmpty

```

### Command groups

One experimental feature which may be useful for programs with many

subcommands is command group separation.

```haskell

data Sample

  = Hello [String]

  | Goodbye

  deriving (Eq, Show)

hello :: Parser Sample

hello = Hello <$> many (argument str (metavar "TARGET..."))

sample :: Parser Sample

sample = subparser

       ( command "hello" (info hello (progDesc "Print greeting"))

      <> command "goodbye" (info (pure Goodbye) (progDesc "Say goodbye"))

       )

      <|> subparser

       ( command "bonjour" (info hello (progDesc "Print greeting"))

      <> command "au-revoir" (info (pure Goodbye) (progDesc "Say goodbye"))

      <> commandGroup "French commands:"

      <> hidden

       )

```

This will logically separate the usage text for the two subparsers

(these would normally appear together if the `commandGroup` modifier

was not used). The `hidden` modifier suppresses the metavariable

for the second subparser being show in the brief usage line, which

is desirable in some cases.

In this example we have essentially created synonyms for our parser,

but one could use this to separate common commands from rare ones,

or safe from dangerous.

The usage text for the preceding example is:

```

Usage: commands COMMAND

Available options:

  -h,--help                Show this help text

Available commands:

  hello                    Print greeting

  goodbye                  Say goodbye

French commands:

  bonjour                  Print greeting

  au-revoir                Say goodbye

```

## Bash completion

`optparse-applicative` has built-in support for bash completion of

command line options and arguments. Any parser, when run using

`execParser` (and similar functions), is automatically extended

with a few (hidden) options for bash completion:

 - `--bash-completion-script`: this takes the full path of the program as

   argument, and prints a bash script, which, when sourced into a bash session,

   will install the necessary machinery to make bash completion work. For a

   quick test, you can run something like (for a program called `foo` on the

   `PATH`):

   ```console

   $ source <(foo --bash-completion-script `which foo`)

   ```

   Normally, the output of `--bash-completion-script` should be shipped with

   the program and copied to the appropriate directory (usually

   `/etc/bash_completion.d/`) during installation.

 - `--bash-completion-index`, `--bash-completion-word`: internal options used

   by the completion script to obtain a list of possible completions for a

   given command line.

### Actions and completers

By default, options and commands are always completed. So, for example, if the

program `foo` has an option with a long name `output`, typing

```console

$ foo --ou<TAB>

```

will complete `--output` automatically.

Arguments (either of regular options, or top-level) are not completed by

default. To enable completion for arguments, use one of the following modifiers

on a regular option or argument:

 - `completeWith`: specifies a list of possible completions to choose from;

 - `action`: specifies a completion "action". An action dynamically determines

   a list of possible completions. A full list of actions can be found in the

   [bash documentation];

 - `completer`: a completer is a function `String -> IO [String]`, returning

   all possible completions for a given string. You can use this modifier to

   specify a custom completion for an argument.

Completion modifiers can be used multiple times: the resulting completions will

call all of them and join the results.

### Internals

When running a parser with `execParser`, the parser is extended with

`bashCompletionParser`, which defines the above options.

When completion is triggered, the completion script calls the executable with

the special `--bash-completion-index` and `--bash-completion-word` options.

The original parser is therefore run in *completion mode*, i.e. `runParser` is

called on a different monad, which keeps track of the current state of the

parser, and exits when all arguments have been processed.

The completion monad returns, on failure, either the last state of the parser

(if no option could be matched), or the completer associated to an option (if

it failed while fetching the argument for that option).

From that we generate a list of possible completions, and print them to

standard output. They are then read by the completion script and put into the

`COMPREPLY` variable.

## Arrow interface

It is also possible to use the [Arrow syntax][arrows] to combine basic parsers.

This can be particularly useful when the structure holding parse results is

deeply nested, or when the order of fields differs from the order in which the

parsers should be applied.

Using functions from the `Options.Applicative.Arrows` module, one can write,

for example:

```haskell

data Options = Options

  { optArgs :: [String]

  , optVerbose :: Bool }

opts :: Parser Options

opts = runA $ proc () -> do