Elixir's Enum.reduce_while in TypeScript

How to use

Install via yarn add locutus and import: import { reduce_while } from 'locutus/elixir/Enum/reduce_while'.

Or with CommonJS: const { reduce_while } = require('locutus/elixir/Enum/reduce_while')

Use a bundler that supports tree-shaking so you only ship the functions you actually use. Vite, webpack, Rollup, and Parcel all handle this. For server-side use this is less of a concern.

Examples

These examples are extracted from test cases that automatically verify our functions against their native counterparts.

#	code	expected result
1	reduce_while([1, 2, 3, 4], 0, (acc, value) => (value < 4 ? ['cont', acc + value] : ['halt', acc]))	6
2	reduce_while([1, 3, 4, 6], null, (acc, value) => (value % 2 === 0 ? ['halt', value] : ['cont', acc]))	4
3	reduce_while([], 'done', (acc, _value) => ['cont', acc])	'done'

Notes

• Mirrors Elixir Enum.reduce_while/3 using [‘cont’, acc] and [‘halt’, acc] control tuples.

Here's what our current TypeScript equivalent to Elixir's Enum.reduce_while looks like.

TSTypeScript JSJavaScript TSStandalone TS JSStandalone JS

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type ReduceWhileTag = 'cont' | 'halt'
type ReduceWhileTuple<TAccumulator> = [ReduceWhileTag, TAccumulator]
type ReduceWhileObject<TAccumulator> = { tag: ReduceWhileTag; value: TAccumulator }
type ReduceWhileStep<TAccumulator> = ReduceWhileTuple<TAccumulator> | ReduceWhileObject<TAccumulator>
type ReduceWhileReducer<TAccumulator, TValue> = (
  accumulator: TAccumulator,
  value: TValue,
  index: number,
  array: TValue[],
) => ReduceWhileStep<TAccumulator>

function normalizeReduceWhileStep<TAccumulator>(value: ReduceWhileStep<TAccumulator>): ReduceWhileTuple<TAccumulator> {
  if (Array.isArray(value)) {
    const tag = value[0]
    if ((tag === 'cont' || tag === 'halt') && value.length >= 2) {
      return [tag, value[1] as TAccumulator]
    }
  } else if (value && typeof value === 'object') {
    const tag = value.tag
    if (tag === 'cont' || tag === 'halt') {
      return [tag, value.value]
    }
  }

  throw new TypeError("reduce_while(): reducer must return ['cont', value], ['halt', value], or { tag, value }")
}

export function reduce_while<TAccumulator, TValue>(
  values: TValue[] | unknown,
  initial: TAccumulator,
  reducer: ReduceWhileReducer<TAccumulator, TValue>,
): TAccumulator {
  //      discuss at: https://locutus.io/elixir/reduce_while/
  // parity verified: Elixir 1.18
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Mirrors Elixir Enum.reduce_while/3 using ['cont', acc] and ['halt', acc] control tuples.
  //       example 1: reduce_while([1, 2, 3, 4], 0, (acc, value) => (value < 4 ? ['cont', acc + value] : ['halt', acc]))
  //       returns 1: 6
  //       example 2: reduce_while([1, 3, 4, 6], null, (acc, value) => (value % 2 === 0 ? ['halt', value] : ['cont', acc]))
  //       returns 2: 4
  //       example 3: reduce_while([], 'done', (acc, _value) => ['cont', acc])
  //       returns 3: 'done'

  if (typeof reducer !== 'function') {
    throw new TypeError('reduce_while(): reducer must be a function')
  }

  if (!Array.isArray(values) || values.length === 0) {
    return initial
  }

  let accumulator = initial
  for (let i = 0; i < values.length; i++) {
    const [tag, nextAccumulator] = normalizeReduceWhileStep(reducer(accumulator, values[i] as TValue, i, values))
    accumulator = nextAccumulator
    if (tag === 'halt') {
      break
    }
  }

  return accumulator
}

function normalizeReduceWhileStep(value) {
  if (Array.isArray(value)) {
    const tag = value[0]
    if ((tag === 'cont' || tag === 'halt') && value.length >= 2) {
      return [tag, value[1]]
    }
  } else if (value && typeof value === 'object') {
    const tag = value.tag
    if (tag === 'cont' || tag === 'halt') {
      return [tag, value.value]
    }
  }

  throw new TypeError("reduce_while(): reducer must return ['cont', value], ['halt', value], or { tag, value }")
}

export function reduce_while(values, initial, reducer) {
  //      discuss at: https://locutus.io/elixir/reduce_while/
  // parity verified: Elixir 1.18
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Mirrors Elixir Enum.reduce_while/3 using ['cont', acc] and ['halt', acc] control tuples.
  //       example 1: reduce_while([1, 2, 3, 4], 0, (acc, value) => (value < 4 ? ['cont', acc + value] : ['halt', acc]))
  //       returns 1: 6
  //       example 2: reduce_while([1, 3, 4, 6], null, (acc, value) => (value % 2 === 0 ? ['halt', value] : ['cont', acc]))
  //       returns 2: 4
  //       example 3: reduce_while([], 'done', (acc, _value) => ['cont', acc])
  //       returns 3: 'done'

  if (typeof reducer !== 'function') {
    throw new TypeError('reduce_while(): reducer must be a function')
  }

  if (!Array.isArray(values) || values.length === 0) {
    return initial
  }

  let accumulator = initial
  for (let i = 0; i < values.length; i++) {
    const [tag, nextAccumulator] = normalizeReduceWhileStep(reducer(accumulator, values[i], i, values))
    accumulator = nextAccumulator
    if (tag === 'halt') {
      break
    }
  }

  return accumulator
}

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