Data.Functor.Contravariant.Ticker

This module defines the Ticker cofunctor, useful for 'ticking off' a prefix of the input.

The Ticker type

newtype Ticker x

This is a contra-functor data type for selecting a prefix of an input stream. If the next input item is acceptable, the ticker function returns the ticker for the rest of the stream. If not, it returns Nothing.

Constructors

Ticker (x -> Maybe (Ticker x))

Instances

Contravariant Ticker

Using a Ticker

splitTicked :: Ticker x -> [x] -> (Ticker x, [x], [x])

Extracts a list prefix accepted by the Ticker argument. Returns the modified ticker, the prefix, and the remainder of the list.

class Contravariant f where

Any instance should be subject to the following laws:

 contramap id = id
 contramap f . contramap g = contramap (g . f)

Note, that the second law follows from the free theorem of the type of contramap and the first law, so you need only check that the former condition holds.

Methods

contramap :: (a -> b) -> f b -> f a

Instances

Contravariant Predicate

A Predicate is a Contravariant Functor, because contramap can apply its function argument to the input of the predicate.

Contravariant Comparison

A Comparison is a Contravariant Functor, because contramap can apply its function argument to each input to each input to the comparison function.

Contravariant Equivalence

Equivalence relations are Contravariant, because you can apply the contramapped function to each input to the equivalence relation.

Contravariant Ticker

Contravariant (Const a)

Control.Applicative.Const

Contravariant (Op a)

Contravariant (Constant a)

Data.Functor.Constant

(Contravariant f, Contravariant g) => Contravariant (Product f g)

Data.Functor.Product

Ticker constructors

tickNone :: Ticker x

A ticker that accepts no input.

tickOne :: Ticker x

A ticker that accepts a single input item.

tickCount :: Int -> Ticker x

A ticker that accepts a given number of input items.

tickPrefixOf :: Eq x => [x] -> Ticker x

A ticker that accepts the longest prefix of input that matches a prefix of the argument list.

tickWhilePrefixOf :: [x -> Bool] -> Ticker x

A ticker that accepts a prefix of input as long as each item satisfies the predicate at the same position in the argument list. The length of the predicate list thus determines the maximum number of acepted values.

tickWhile :: (x -> Bool) -> Ticker x

A ticker that accepts all input as long as it matches the given predicate.

tickUntil :: (x -> Bool) -> Ticker x

A ticker that accepts all input items until one matches the given predicate.

tickAll :: Ticker x

A ticker that accepts all input.

Ticker combinators

andThen :: Ticker x -> Ticker x -> Ticker x

Sequential concatenation ticker combinator: when the first argument ticker stops ticking, the second takes over.

and :: Ticker x -> Ticker x -> Ticker x

Parallel conjunction ticker combinator: the result keeps ticking as long as both arguments do.

or :: Ticker x -> Ticker x -> Ticker x

Parallel choice ticker combinator: the result keeps ticking as long as any of the arguments does.