Options.Applicative.Arrows

This module contains an arrow interface for option parsers, which allows to define and combine parsers using the arrow notation and arrow combinators.

The arrow syntax is particularly useful to create parsers of nested structures, or records where the order of fields is different from the order in which the parsers should be applied.

For example, an arguments parser often needs to be applied last, and that makes it inconvenient to use it for a field which is not the last one in a record.

Using the arrow syntax and the functions in this module, one can write, e.g.:

data Options = Options
  { optArgs :: [String]
  , optVerbose :: Bool }

opts :: Parser Options
opts = runA $ proc () -> do
  verbose <- asA (switch (short 'v')) -< ()
  args <- asA (arguments str idm) -< ()
  returnA -< Options args verbose

Parser arrows, created out of regular Parser values using the asA function, are arrows taking () as argument and returning the parsed value.

Documentation

module Control.Arrow

newtype A f a b

For any Applicative functor f, A f is the Arrow instance associated to f.

The A constructor can be used to convert a value of type f (a -> b) into an arrow.

Constructors

A

Fields

unA :: f (a -> b)

Instances

Applicative f => Category * (A f)

Applicative f => Arrow (A f)

asA :: Applicative f => f a -> A f () a

Convert a value of type f a into an arrow taking () as argument.

Applied to a value of type Parser, it turns it into an arrow that can be used inside an arrow command, or passed to arrow combinators.

runA :: Applicative f => A f () a -> f a

Convert an arrow back to an applicative value.

This function can be used to return a result of type Parser from an arrow command.

type ParserA = A Parser

The type of arrows associated to the applicative Parser functor.