Flags.Applicative

Simple flags parsing module, inspired by optparse-applicative.

Sample usage (note the default log level and optional context):

module Main where

import Control.Applicative ((<|>), optional)
import Data.Text (Text)
import Flags.Applicative

data Options = Options
  { rootPath :: Text
  , logLevel :: Int
  , context :: Maybe Text
  } deriving Show

optionsParser :: FlagParser Options
optionsParser = Options <$> textFlag "root" "path to the root"
                        <*> (flag "log_level" "" <|> pure 0)
                        <*> (optional $ textFlag "context" "")

main :: IO ()
main = do
  (opts, args) <- parseSystemFlagsOrDie optionsParser
  print opts

Documentation

type Name = Text

The name of a flag, can use all valid utf-8 characters but = (the value delimiter). In general, it's good practice for flag names to be lowercase ASCII with underscores.

The following names are reserved and attempting to define a flag with the same name will cause an error:

• help, used to display usage when set.

type Description = Text

An human-readable explanation of what the flag does.

data FlagParser a

Flags parser.

There are two types of flags:

• Nullary flags created with 'switch which are True when set and False otherwise. For example --version or --enable_foo.
• Unary flags created with unaryFlag and its convenience variants (e.g. textFlag, flag, repeatedFlag). These expect a value to be passed in either after an equal sign (--foo=value) or as the following input value (--foo value). If the value starts with --, only the first form is accepted.

You can run a parser using parseFlags.

Instances

Functor FlagParser

Defined in Flags.Applicative

Methods

fmap :: (a -> b) -> FlagParser a -> FlagParser b

(<$) :: a -> FlagParser b -> FlagParser a

Applicative FlagParser

Defined in Flags.Applicative

Methods

pure :: a -> FlagParser a

(<*>) :: FlagParser (a -> b) -> FlagParser a -> FlagParser b

liftA2 :: (a -> b -> c) -> FlagParser a -> FlagParser b -> FlagParser c

(*>) :: FlagParser a -> FlagParser b -> FlagParser b

(<*) :: FlagParser a -> FlagParser b -> FlagParser a

Alternative FlagParser

Defined in Flags.Applicative

Methods

empty :: FlagParser a

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

some :: FlagParser a -> FlagParser [a]

many :: FlagParser a -> FlagParser [a]

data FlagError

The possible parsing errors.

Constructors

DuplicateFlag Name

A flag was declared multiple times.

Help Text

The input included the --help flag.

InconsistentFlagValues Name

At least one unary flag was specified multiple times with different values.

InvalidFlagValue Name Text String

A unary flag's value failed to parse.

MissingFlag Name

A required flag was missing.

MissingFlagValue Name

A unary flag was missing a value. This can happen either if a value-less unary flag was the last token or was followed by a value which is also a flag name (in which case you should use the single-token form: --flag=--value).

UnexpectedFlags (NonEmpty Name)

At least one flag was set but unused. This can happen when optional flags are set but their branch is not selected.

UnknownFlag Name

An unknown flag was set.

Instances

Eq FlagError

Defined in Flags.Applicative

Methods

(==) :: FlagError -> FlagError -> Bool

(/=) :: FlagError -> FlagError -> Bool

Show FlagError

Defined in Flags.Applicative

Methods

showsPrec :: Int -> FlagError -> ShowS

show :: FlagError -> String

showList :: [FlagError] -> ShowS

parseFlags :: FlagParser a -> [String] -> Either FlagError (a, [String])

Runs a parser on a list of tokens, returning the parsed flags alongside other non-flag arguments (i.e. which don't start with --). If the special -- token is found, all following tokens will be considered arguments (even if they look like flags).

parseSystemFlagsOrDie :: FlagParser a -> IO (a, [String])

Runs a parser on the system's arguments, or exits with code 1 and prints the relevant error message in case of failure.

Defining flags

switch :: Name -> Description -> FlagParser Bool

Returns a nullary parser with the given name and description.

unaryFlag :: (Text -> Either String a) -> Name -> Description -> FlagParser a

Returns a unary parser using the given parsing function, name, and description.

textFlag :: Name -> Description -> FlagParser Text

Returns a parser for a single text value.

flag :: Read a => Name -> Description -> FlagParser a

Returns a parser for any value with a Read instance. Prefer textFlag for textual values since flag will expect its values to be double-quoted and might not work as expected.

repeatedTextFlag :: Text -> Name -> Description -> FlagParser [Text]

Returns a parser for a multiple text value.

repeatedFlag :: Read a => Text -> Name -> Description -> FlagParser [a]

Returns a parser for multiple values with a Read instance, with a configurable separator.