# grab: Applicative non-linear consumption

### The Grab type

A grab consumes some portion (none, part, or all) of
its input `bag`

, and returns a `residue`

consisting of
the unconsumed input, some monoidal `log`

(e.g. a list
of error messages), and some `desideratum`

(the object
of desire) produced from the consumed input, or
`Nothing`

if the grab failed.

newtype Grab bag residue log desideratum = Grab ( bag -> (residue, log, Maybe desideratum) )

Grabs are useful as parsers for inputs such as JSON objects or lists of form parameters, where the input data is not necessarily given linearly in the same order in which we want to consume it.

### Applicative composition

A `Simple`

grab (where the `bag`

and `residue`

are the
same type) has an `Applicative`

instance.

instance (bag ~ residue, Monoid log) => Applicative (Grab bag residue log)

For example, we can create two simple list grabs, one that grabs multiples of two, and the other that grabs multiples of three:

twos, threes :: Monoid log => Control.Grab.Simple [Integer] log [Integer] twos = partition (Data.List.partition (\x -> mod x 2 == 0)) threes = partition (Data.List.partition (\x -> mod x 3 == 0))

λ> runGrabMaybe ((,) <$> twos @() <*> threes @()) [1..10] Just ([2,4,6,8,10],[3,9])

Notice that the second part of the resulting tuple contains only
the *odd* multiples of three. Because `twos`

runs first, it
consumes `6`

before the `threes`

can get it.

### Pipeline composition

`a / b`

is a pipeline of two grabs, where the desideratum from
`a`

is the `bag`

for `b`

.
.
> (/) :: Semigroup log
> => Grab bag residue log x
> -> Grab x _residue log desideratum
> -> Grab bag residue log desideratum

λ> runGrabMaybe (twos @() / threes @()) [1..10] Just [6]

λ> runGrabMaybe ((,) <$> (twos @() / threes @()) <*> threes @()) [1..10] Just ([6],[3,9])

## Downloads

- grab-0.0.0.8.tar.gz [browse] (Cabal source package)
- Package description (as included in the package)

Versions [RSS] | 0.0.0.1, 0.0.0.4, 0.0.0.6, 0.0.0.7, 0.0.0.8 |
---|---|

Change log | changelog.md |

Dependencies | base (>=4.12 && <4.17) [details] |

License | MIT |

Copyright | 2021 Mission Valley Software LLC |

Author | Chris Martin |

Maintainer | Chris Martin, Julie Moronuki |

Category | Control |

Home page | https://github.com/typeclasses/grab |

Bug tracker | https://github.com/typeclasses/grab/issues |

Uploaded | by chris_martin at 2022-06-20T19:18:02Z |

Distributions | NixOS:0.0.0.8 |

Downloads | 756 total (12 in the last 30 days) |

Rating | 2.0 (votes: 1) [estimated by Bayesian average] |

Your Rating | |

Status | Docs available [build log] Last success reported on 2022-06-20 [all 1 reports] |