Arrows

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 (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.