alias:: Transducer: ??? ?? ?? ??
???:: ???: 一種強大的函數組同模式

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?? ??? '??'???. ?? ??? ?? ??? ?? ? ? ??? ????? ?????.

  const list = [1, 2, 3, 4, 5]

  list.map(x => x + 1)
  // [ 2, 3, 4, 5, 6 ]

  function map(f, xs) {
    const ret = []
    for (let i = 0; i < xs.length; i++) {
      ret.push(f(xs[i]))
    }
    return ret
  }

  map(x => x + 1, [1, 2, 3, 4, 5])
  // [ 2, 3, 4, 5, 6 ]

???? ?? ??? ??? ??? ????? ?? ???? ???? ?? ????? for ?? ??????.
??? ??;
???? ?? ???? ??.
??? ???????.

  function filter(f, xs) {
    const ret = []
    for (let i = 0; i < xs.length; i++) {
      if (f(xs[i])) {
        ret.push(xs[i])
      }
    }
    return ret
  }

  var range = n => [...Array(n).keys()]

  filter(x => x % 2 === 1, range(10))
  // [ 1, 3, 5, 7, 9 ]

????? ?? ??? ?? ??? ??? ?? ???? ?? ????? xs? ????? ???.
??? ??? ??? ??? ??? ??? ? ???? ?? ?? ??, ?? ? ?? ??? ??? ????.
???? ??? ????. ???? ?????(????)? ?????.
???? ?? ??? ??? JS? ?? ???? ?? ??? ????? ???? ?? ?????.
Symbol.iterator? ???? ?????.
???? ???? Object#contractor? ?????.
???? ???? ??? ??? ??? ????? ????? ??? ?? ????
ramdajs ?????? ???? ?? @@transducer/step ??? ??? ? ????.

  function map(f, xs) {
    const ret = new xs.constructor()  // 1. construction
    for (const x of xs) { // 2. iteration
      ret['@@transducer/step'](f(x))  // 3. collection
    }
    return ret
  }

  Array.prototype['@@transducer/step'] = Array.prototype.push
  // [Function: push]

  map(x => x + 1, [1, 2, 3, 4, 5])
  // [ 2, 3, 4, 5, 6 ]

  Set.prototype['@@transducer/step'] = Set.prototype.add
  // [Function: add]

  map(x => x + 1, new Set([1, 2, 3, 4, 5]))
  // Set (5) {2, 3, 4, 5, 6}

? ??? ???? ??, ?? ? ?? ??? ??? ??? ? ????.
??? ??, ??, ???? ?? ??? ?? ??? ???? ???? ????. ???? ??? ???? ??? ??? ???? ??? ?? ?? ?????.

  function filter(f, xs) {
    const ret = new xs.constructor()
    for (const x of xs) {
      if (f(x)) {
        ret['@@transducer/step'](x)
      }
    }
    return ret
  }

  filter(x => x % 2 === 1, range(10))
  // [ 1, 3, 5, 7, 9 ]

  filter(x => x > 3, new Set(range(10)))
  // Set (6) {4, 5, 6, 7, 8, 9}

????

?? ?? ??? ?? ???? ? ?? ??? ??? ? ????.

  range(10)
    .map(x => x + 1)
    .filter(x => x % 2 === 1)
    .slice(0, 3)
  // [ 1, 3, 5 ]

5?? ??? ????? ???? ?? ??? ?????.
? ????? ?? ??? ??? ?????.
??? Compose? ???? ? ??? ?? ?????

  function compose(...fns) {
    return fns.reduceRight((acc, fn) => x => fn(acc(x)), x => x)
  }

??? ???? ?? ?, ?? ?? ??? ?? ??? ?????.

  function curry(f) {
    return (...args) => data => f(...args, data)
  }

  var rmap = curry(map)
  var rfilter = curry(filter)

  function take(n, xs) {
    const ret = new xs.constructor()
    for (const x of xs) {
      if (n <= 0) {
        break
      }
      n--
      ret['@@transducer/step'](x)
    }
    return ret
  }
  var rtake = curry(take)

  take(3, range(10))
  // [ 0, 1, 2 ]

  take(4, new Set(range(10)))
  // Set (4) {0, 1, 2, 3}

  const takeFirst3Odd = compose(
    rtake(3),
    rfilter(x => x % 2 === 1),
    rmap(x => x + 1)
  )

  takeFirst3Odd(range(10))
  // [ 1, 3, 5 ]

???? ??? ??? ??? ???? ????? ????? ?????.

??? ??

?? ??

curry ??? ?? ??? ??? ????.

  const map = f => xs => ...

?, map(x => ...)? ?? ???? ??? ?????.

  const list = [1, 2, 3, 4, 5]

  list.map(x => x + 1)
  // [ 2, 3, 4, 5, 6 ]

??? ????? ??? ?? ??? ? ????.
????? ?? ??? ??? '???', ??? ??? ???, ??? ??? ???(Transformer)??.

  function map(f, xs) {
    const ret = []
    for (let i = 0; i < xs.length; i++) {
      ret.push(f(xs[i]))
    }
    return ret
  }

  map(x => x + 1, [1, 2, 3, 4, 5])
  // [ 2, 3, 4, 5, 6 ]

  function filter(f, xs) {
    const ret = []
    for (let i = 0; i < xs.length; i++) {
      if (f(xs[i])) {
        ret.push(xs[i])
      }
    }
    return ret
  }

Transformer? ?? ???? ??? ?? ??? ?????.

  var range = n => [...Array(n).keys()]

???

???? ? ??? ??? ???? ? ??? ? ?? 2?? ???? ?????.

  filter(x => x % 2 === 1, range(10))
  // [ 1, 3, 5, 7, 9 ]

???

  function map(f, xs) {
    const ret = new xs.constructor()  // 1. construction
    for (const x of xs) { // 2. iteration
      ret['@@transducer/step'](f(x))  // 3. collection
    }
    return ret
  }

??

  Array.prototype['@@transducer/step'] = Array.prototype.push
  // [Function: push]

  map(x => x + 1, [1, 2, 3, 4, 5])
  // [ 2, 3, 4, 5, 6 ]

??

  Set.prototype['@@transducer/step'] = Set.prototype.add
  // [Function: add]

????

take? ??? ?????? Break? ??? ??? ???? Reduce? ?????.

  map(x => x + 1, new Set([1, 2, 3, 4, 5]))
  // Set (5) {2, 3, 4, 5, 6}

  function filter(f, xs) {
    const ret = new xs.constructor()
    for (const x of xs) {
      if (f(x)) {
        ret['@@transducer/step'](x)
      }
    }
    return ret
  }

  filter(x => x % 2 === 1, range(10))
  // [ 1, 3, 5, 7, 9 ]

???

??? ??? ???? ???
?? ?? ?? ??? ?? ?????

  filter(x => x > 3, new Set(range(10)))
  // Set (6) {4, 5, 6, 7, 8, 9}

??? ??? ??(Array)? ???? ??? ???? ???? ????? ??? ??? ???.

  range(10)
    .map(x => x + 1)
    .filter(x => x % 2 === 1)
    .slice(0, 3)
  // [ 1, 3, 5 ]

??? ????, ??? ????, ??? ??? ??????.
???? ?? ???? ???? ??? ?????.
??? ?? ?????

  function compose(...fns) {
    return fns.reduceRight((acc, fn) => x => fn(acc(x)), x => x)
  }

?? rfilter? rmap? ?? ??? ?????.

  function curry(f) {
    return (...args) => data => f(...args, data)
  }

???? ????? ?? ?? ?? ???? ??????.
Transformer? ?? ????? Transducer? ?? ?????.

  var rmap = curry(map)
  var rfilter = curry(filter)

  function take(n, xs) {
    const ret = new xs.constructor()
    for (const x of xs) {
      if (n <= 0) {
        break
      }
      n--
      ret['@@transducer/step'](x)
    }
    return ret
  }
  var rtake = curry(take)

?? ? ??

??? ???? ??? ??????

  take(3, range(10))
  // [ 0, 1, 2 ]

  take(4, new Set(range(10)))
  // Set (4) {0, 1, 2, 3}

???? ???? ??? ??? ???? ???.

  const takeFirst3Odd = compose(
    rtake(3),
    rfilter(x => x % 2 === 1),
    rmap(x => x + 1)
  )

  takeFirst3Odd(range(10))
  // [ 1, 3, 5 ]

?? ??? ? ??? ??? '???'??? ?? ??????. ???? 100?? ??? ??? ?? 10?? ??? ???????.
???? ?? ??? ??? ????????.

  const map = f => xs => ...

  type Transformer = (xs: T) => R

??

???? ???.

??? ?? ???? ???? ?? ??? ??? ??? ??? ??? ??? ?????.

  data ->> map(...) ->> filter(...) ->> reduce(...) -> result

  function pipe(...fns) {
    return x => fns.reduce((ac, f) => f(ac), x)
  }

  const reduce = (f, init) => xs => xs.reduce(f, init)

  const f = pipe(
    rmap(x => x + 1),
    rfilter(x => x % 2 === 1),
    rtake(5),
    reduce((a, b) => a + b, 0)
  )

  f(range(100))
  // 25

? ??? ??? ???? into ??? ???? ???.
??? ??? ?? ??? ?? ?????.

  type Transformer = (x: T) => T

?? ??? ??? ????.

  type Reducer = (ac: R, x: T) => R

?? ??? ????? ?? ??? ????.

  // add is an reducer
  const add = (a, b) => a + b
  const sum = xs => xs.reduce(add, 0)

  sum(range(11))
  // 55

?? ??? ???? ??? ??? ?????.

??

?? ?? ??? ?? ??? Compose ??? ??? ?? ??? ???? ??? ???? ?? ?? ? ?? ????.

?? ??

  const list = [1, 2, 3, 4, 5]

  list.map(x => x + 1)
  // [ 2, 3, 4, 5, 6 ]

  function map(f, xs) {
    const ret = []
    for (let i = 0; i < xs.length; i++) {
      ret.push(f(xs[i]))
    }
    return ret
  }

?? ??? ??? ?????.

??? ??

  map(x => x + 1, [1, 2, 3, 4, 5])
  // [ 2, 3, 4, 5, 6 ]

  function filter(f, xs) {
    const ret = []
    for (let i = 0; i < xs.length; i++) {
      if (f(xs[i])) {
        ret.push(xs[i])
      }
    }
    return ret
  }

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