optparse-applicative
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
The core type in optparse-applicative is a Parser
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.
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:
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
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:
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:
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 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
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
opts :: Parser Options
opts = Options <$> target <*> quiet
No matter which parsers appear first in the sequence, options will
still be parsed in whatever order they appear in the command line.
A parser with such a property is sometimes called a permutation
parser.
In our example, a command line like:
--target world -q
will give the same result as
-q --target world
It is this property which leads us to an Applicative interface
instead of a Monadic one, as all option must be considered in
parallel, and can not depend on the output of other options.
Note, however, that the order of sequencing is still somewhat
significant, in that it affects the generated help text. Customisation
can be achieved easily through a lambda abstraction, with Arrow
notation, or by taking advantage of GHC 8's
ApplicativeDo extension.
Alternative
It is also common to find programs that can be configured in different
ways through the command line. A typical example is a program that
can be given a text file as input, or alternatively read it directly
from the standard input.
We can model this easily and effectively in Haskell using sum types:
data Input
= FileInput FilePath
| StdInput
run :: Input -> IO ()
run = ...
We can now define two basic parsers for the components of the sum type:
fileInput :: Parser Input
fileInput = FileInput <$> 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 (<|>)
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
we saw
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
:
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.
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 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 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,
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:
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.
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:
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:
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.
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
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:
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
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:
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
.
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,
command :: String -> ParserInfo a -> Mod CommandFields a
can only be used with 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:
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
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.
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
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.
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:
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
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.
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.
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
):
$ 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
$ 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 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:
data Options = Options
{ optArgs :: [String]
, optVerbose :: Bool }
opts :: Parser Options
opts = runA $ proc () -> do
verbosity <- asA (option auto (short 'v' <> value 0)) -< ()
let verbose = verbosity > 0
args <- asA (many (argument str idm)) -< ()
returnA -< Options args verbose
where parsers are converted to arrows using asA
, and the resulting
composed arrow is converted back to a Parser
with runA
.
See tests/Examples/Cabal.hs
for a slightly more elaborate example
using the arrow syntax for defining parsers.
Note that the Arrow
interface is provided only for convenience. The
API based on Applicative
is just as expressive, although it might be
cumbersome to use in certain cases.
Applicative do
Some may find using optparse-applicative easier using do notation.
However, as Parser
is not an instance of Monad
, this can only
be done in recent versions of GHC using the ApplicativeDo extension.
For example, a parser specified in this manner might be
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE ApplicativeDo #-}
data Options = Options
{ optArgs :: [String]
, optVerbose :: Bool }
opts :: Parser Options
opts = do
optVerbose <- switch (short 'v')
optArgs <- many (argument str idm)
pure Options {..}
Here we've also used the RecordWildCards extension to make the
parser specification cleaner. Compilation errors referring to Monad
instances not being found are likely because the Parser
specified
can not be implemented entirely with Applicative
(Note however,
there were a few desugaring bugs regarding ApplicativeDo in GHC
8.0.1, function application with ($)
in particular may not work,
and the pure
value should instead be wrapped parenthetically).
FAQ
-
Monadic parsing?
If a Monadic style were to be used, there would be no possible
way to traverse the parser and generate a usage string, or for
us to allow for options to be parsed in any order. Therefore it
is intentionally unsupported to write a Parser
in this manner
with optparse-applicative, and the Parser
type does not have
an instance for Monad
.
-
Overlapping flags and options / options with optional arguments?
This is not supported as it can lead to an ambiguous parse.
For example, if we supported and had an optional value option
"--foo" and a flag "--bar", is "--foo --bar" the option with value
"--bar", or the default value with the flag switched on? What if
instead of a switch we had many positional string arguments, is
the first string the option's value or the first positional?
It is suggested to instead use the Alternative
instance of
Parser
and create a flag', an option, and a pure value for the
default (with different names for the flag and option).
-
Backtracking on ReadM
errors?
Parser structures are predetermined at parse time. This means
that if a ReadM
fails, the whole parse must also fail, we can't
consider any alternatives, as there can be no guarantee that the
remaining structure will fit. One occasionally confusing side
effect of this is that two positional arguments for different
constructors of a sum type can't be composed at the parser level;
rather, this must be done at the ReadM
level. For example:
import Options.Applicative
data S3orFile = S3 BucketKey | File FilsePath
s3Read, fileRead :: ReadM S3orFile
s3Read = S3 <$> ...
fileRead = File <$> ...
correct :: Parser S3orFile
correct = argument (s3Read <|> fileRead) idm
incorrect :: Parser S3orFile
incorrect = argument s3Read idm <|> argument fileRead idm
How it works
An applicative Parser
is essentially a heterogeneous list or tree
of Option
s, implemented with existential types.
All options are therefore known statically (i.e. before parsing,
not necessarily before runtime), and can, for example, be traversed
to generate a help text. Indeed, when displaying the usage text for
a parser, we use an intermediate tree structure.
When we examine the user's input, each argument is examined to
determine if it's an option or flag, or a positional argument. The
parse tree is then searched for a matching term, and if it finds
one, that leaf of the tree is replaced with the value itself. When
all input has been processed, we see if we can generate the complete
value, and if not issue an error.
See this blog post for a more detailed explanation based on a
simplified implementation.