TRANSducers!

transducers.js

// function reducing(acc, next) {
//   return next_acc
// }

// function transducing(reducing) {
//   return more_reducing
// }

// Example reducer (without transducers)
// function incrPrintConcat() {
//   var i = 0
//   return function (acc, val) {
//     console.log(i, val, acc)
//     return (acc||[]).concat(val+1)
//   }
// }
//
// With transducers:
// tcomp(map(function (x) { return x+1 }),
//       forEach(function (acc, val, i) { console.log(i, val, acc) }),
//       concat)

// Perform transduction
Array.prototype.transduce = function (f, init) {
  return this.reduce(f(), init)
}

Object.prototype.transduce = function (f, init) {
  var obj = this
  return Object.keys(obj).transduce(tcomp(map(function (key) { return [key, obj[key]] }), f), init)
}

// Compose transducers
function tcomp(f) {
  var gs = Array.prototype.slice.call(arguments, 1)
  while (gs.length)
    f = (function (ff, g) {
      return function (x) { return ff(g(x)) }
    })(f, gs.shift())

  return f
}

// Modifier transducers

function forEach(f) {
  return function (next) {
    return function (acc, val, i) {
      f(acc, val, i)
      return next(acc, val, i)
    }
  }
}

function map(f) {
  return function (next) {
    return function (acc, val, i) {
      return next(acc, f(val, i), i)
    }
  }
}

function filter(f) {
  return function (next) {
    return function (acc, val, i) {
      if (f(val, i))
        return next(acc, val, i)
      else
        return acc
    }
  }
}

function take(n) {
  return filter(function (val, i) {
    return i < n
  })
}

function takeWhile(f) {
  var taking = true
  return filter(function (val, i) {
    return taking && (taking = f(val, i))
  })
}

function drop(n) {
  return filter(function (val, i) {
    return i >= n
  })
}

function dropWhile(f) {
  var taking = false
  return filter(function (val, i) {
    return taking || (taking = f(val, i))
  })
}

function takeNth(n) {
  return filter(function (val, i) {
    return i === n
  })
}

function attrgetter(name) {
  return map(function (obj) {
    return obj[name]
  })
}

function itemgetter(i) {
  return map(function (arr) {
    return arr[i]
  })
}

// Accumulating transducers

function concat() {
  return function () {
    return function (acc, val) {
      return (acc||[]).concat(val)
    }
  }
}

function objectify(keyf) {
  return function () {
    return function (acc, val, i) {
      acc = acc || {}
      acc[keyf(val, i)] = val
      return acc
    }
  }
}

console.log([1,2,3].transduce(tcomp(map(function(x){return x+1}), concat()), []))
console.log({a:'b',c:'d',e:'f'}.transduce(
  tcomp(itemgetter(1),
        drop(1),
        map(function (letter) { return letter.toUpperCase() }),
        objectify(function(x) {return x.toLowerCase()}))))