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The latest version of grab is 0.0.0.8-1.

grab

Version 0.0.0.4 revision 0 uploaded by chris_martin.

Package meta

Synopsis
Applicative non-linear consumption
Description

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])
Author
Chris Martin
Bug reports
https://github.com/typeclasses/grab/issues
Category
Control
Copyright
2019 Typeclass Consulting, LLC
Homepage
https://github.com/typeclasses/grab
Maintainer
Chris Martin, Julie Moronuki
Package URL
n/a
Stability
n/a

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