Gentoo - Generator tools

Tools for ES6 generators.

• accum
• compose
• dedupe
• filter
• forEach
• lastValue
• map
• nthValue
• partition
• pluck
• skip
• take
• loop
• [everyN] (#everyN)
• [reduce] (#reduce)
• [range] (#range)
• [limit] (#limit)
• [takeWhile] (#takeWhile)
• [chain] (#chain)

accum

accum(gen) -> Generator

Returns a new generator which yields an array accumulating the values from gen. Every time next() is called, the newest value from gen is added to the end of the array the generator yields.

function * genInfinite () {
  let i = 0
  while (true) {
    yield ++i
  }
}

const accumGen = gentoo.accum(genInfinite())

accumGen.next().value
// [1]

accumGen.next().value
// [1, 2]

accumGen.next().value
// [1, 2, 3]

compose

compose(...generators) -> Generator

Compose any number of generators. The returned generator will yield the values of the first generator until it's done, then the values of the second until it's done, etc.

function * gen1 () {
  yield '🍕'
}

function * gen2 () {
  yield '🍤'
}

function * gen3 () {
  yield '🍓'
}

const composed = gentoo.compose(gen1(), gen2(), gen3())

[...composed]

// ['🍕', '🍤', '🍓']

dedupe

dedupe(gen [, eqFn]) -> Generator

Returns a generator which iterates over the values from gen and yields values which are different than the previous value. eqFn may optionally be passed, to evaluate the equality of two values. By default, === is used.

function * dupeGenerator () {
  yield '😎'
  yield '😎'
  yield '😳'
  yield '😳'
  yield '😅'
  yield '😅'
}

const dedupeGen = gentoo.dedupe(dupeGenerator())

[...dedupeGen]
// ['😎', '😳', '😅']

filter

filter(gen, fn [, thisValue]) -> Generator

Returns a generator which will iterate over values from gen until fn returns a truthy result.

Every time next() is called, a value is retrieved fromgen and passed to fn, until the result of calling fn is truthy. At that point, the value from gen will be yielded.

thisValue can optionally be passed in, for the context fn is called in.

function * gen () {
  yield 1 
  yield 2 
  yield 3 
}

function even (n) {
  return n % 2 === 0
}

const filterGen = gentoo.filter(gen(), even)

filterGen.next().value
// 2

forEach

forEach(gen, fn [, thisValue]) -> void

Calls fn for each value of gen.

thisValue can optionally be passed in, for the context fn is called in.

function * gen () {
  yield '🍉'
  yield '🍜'
  yield '🍔'
}

gentoo.forEach(gen(), (n) => console.log(n))

// logs "🍉🍜🍔"

lastValue

lastValue(gen) -> [any]

Returns the last value from gen. NOTE: you should only pass lastValue a generator which will end. If gen has an infinite number of values, lastValue will never finish.

function * gen () {
  yield '🚤'
  yield '🚁'
  yield '👑'
}

gentoo.lastValue(gen())
// '👑'

map

map(gen, fn [, thisValue]) -> Generator

Returns a generator that maps fn over the values of gen. Every time next() is called, the next value of gen will be passed to fn, and the result will be yielded.

thisValue can optionally be passed in, for the context fn is called in.

function * gen () {
  yield '🍪'
  yield '🍩'
  yield '🍟'
}

function repeat (n) {
  return n + n
}

const repeatGen = gentoo.map(gen(), repeat)

[...repeatGen]
// ['🍪🍪', '🍩🍩', '🍟🍟']

nthValue

nthValue(gen, n) -> [any]

Returns the nth value (zero-based) from gen.

function * gen () {
  yield '📀'
  yield '📹'
  yield '🎈'
}

gentoo.nthValue(gen(), 1)
// '📹'

partition

partition(gen, fn) -> Generator

Returns a generator that partitions the values from gen into two arrays: those for which fn returns a truthy value, and those for which fn returns a falsey value.

function * gen () {
  yield 1
  yield 2
  yield 3
}

function even (n) {
  return n % 2 === 0
}

const partitionGen = gentoo.partition(gen(), even)

partitionGen.next().value
// [[], [1]]

partitionGen.next().value
// [[2], [1]]

partitionGen.next().value
// [[2], [1, 3]]

pluck

pluck(gen, name) -> Generator

Returns a generator that plucks the property name from each of gen's values. Every time next() is called, the next value from gen is retrieved, and the name property of that value is yielded.

function * gen () {
  yield {animal: '🐮', flower: '🌷', tree: '🌲'}
  yield {animal: '🐗', flower: '🌹', tree: '🌳'}
  yield {animal: '🐵', flower: '🌺', tree: '🌴'}
}

const pluckGen = gentoo.pluck(gen(), 'flower')

pluckGen.next().value
// '🌷'

pluckGen.next().value
// '🌹'

pluckGen.next().value
// '🌺'

skip

skip(gen, n) -> Generator

Skips the first n values from gen.

function * genInfinite () {
  let i = 0
  while (true) {
    yield ++i
  }
}

const gen = genInfinite()

const skippedGen = gentoo.skip(gen, 2)

skippedGen.next().value
// 3

skippedGen.next().value
// 4

skippedGen.next().value
// 5

take

take(gen, n) -> Array

Takes n values from gen and returns the values in an array.

function * gen () {
  yield '🐍'
  yield '🌀'
  yield '🌊'
}

gentoo.take(gen(), 2)
// ['🐍', '🌀']

loop

loop(gen) -> Generator

Yields the values from gen until gen is done, then yields those values forever in a loop.

function * gen () {
  yield 1
  yield 2
  yield 3
}

const looped = gentoo.loop(gen())

looped.next().value
// 1

looped.next().value
// 2

looped.next().value
// 3

looped.next().value
// 1

looped.next().value
// 2

looped.next().value
// 3

looped.next().value
// 1

everyN

everyN(gen, n, [, takeFirst = true]) -> Generator

Yields every n values from gen. takeFirst determines whether to yield the first value from gen. Default is true.

function * gen () {
  let i = 0

  while (true) {
    yield i++
  }
}

const even = gentoo.everyN(gen(), 2)         // yields 0, 2, 4, 6...
const odd = gentoo.everyN(gen(), 2, false)   // yields 1, 3, 5, 7...

reduce

reduce(gen, fn, initial) -> [any]

Reduces the generator into a single value. The fn is invoked with (memo, value).

function * makeGenerator () {
  yield 1
  yield 2
  yield 3
}

const sum = gentoo.reduce(makeGenerator(), (memo, val) => memo + val, 0) // yields 6

range

range(start, stop [, step]) -> Generator

Creates a generator that yields from start (inclusive) to stop (exclusive) with steps (optional, defaults to 1).

gentoo.range(0, 5) // yields 0, 1, 2, 3, 4
gentoo.range(2, 10. 2) // yields 2, 4, 6, 8

You can easily make an infinite stream by passing Number.POSITIVE_INFINITY to stop.

const infinite = gentoo.range(0, Number.POSITIVE_INFINITY)

limit

limit(gen, n) -> Generator

Limits the underlying generator to at most n values.

function * makeGenerator () {
  yield 1
  yield 2
  yield 3
}

const sum = gentoo.limit(makeGenerator(), 2) // yields 1, 2

takeWhile

takeWhile(gen, fn) -> Generator

Takes the underlying generator to the first time the fn returns false.

const positives = gentoo.range(1, Number.POSITIVE_INFINITY);

const sum = gentoo.takeWhile(positives, (num) => num < 5) // yields 1, 2, 3, 4

chain

chain([val]) -> gentoo

Allows chaining the operations, Underscore.js style. Call value() at the end of the chain to extract the value.

function * makeGenerator () {
  yield 1
  yield 2
  yield 3
}

gentoo.chain(makeGenerator())
  .filter((num) => num < 3)
  .loop()
  .limit(5)
  .reduce((memo, val) => memo + val, 0)
  .value(); // 7