Transducers CoffeeScript

transducers.coffee

class Reduced

  @isReduced = (obj) -> obj instanceof Reduced

  constructor: (@wraped) ->

  unwrap: -> @wraped


reduce = (collection, fn, seed) ->
  result = seed;

  for item in collection
    result = fn result, item
    if Reduced.isReduced result then return result.unwrap()

  return result


append = (result, item) ->
  return []                   if not result? and not item?
  return result               if result? and not item?
  return result.concat [item] if result? and item?


transduce = (collection, transducer, append) ->
  step = transducer append
  seed = step()
  result = reduce collection, step, seed
  return step result



# constructors
map = (fn) -> (step) -> (result, item) ->
  return step()                 if not result? and not item?
  return step(result)           if result? and not item?
  return step(result, fn(item)) if result? and item?


filter = (predicate) -> (step) -> (result, item) ->
  return (if predicate item then step result, item else result) if result? and item?
  return step(result) if result? and not item?
  return step()       if not result? and not item?


take = (n) -> (step) -> 
  count = 0
  (result, item) ->
    return (if count++ < n then step result, item else new Reduced result) if result? and item?
    return step(result) if result? and not item?
    return step()       if not result? and not item?


flatten = -> (step) -> (result, item) ->
  if result? and item?
    for x in item
      result = step result, x
    return result
  return step(result) if result? and not item?
  return step()       if not result? and not item?


partition = (n) -> 
  if n < 1 then throw new Error "n > 1"
  (step) ->
    current = []
    (result, item) ->
      if result? and item?
        current.push item
        if current.length is n
          result = step result, current
          current = []
        return result
        
      if result? and not item?
        if current.length > 0
          result = step result, current
          return result

      if not result? and not item?
        return step()


# samples

addOneT     = map (x) -> x + 1
mulTwoT     = map (x) -> x * 2
lessTnan4T  = filter (x) -> x < 4
flattenT    = flatten()
first5T     = take 5
by3ItemsT   = partition 3


addOneMulTwoT = (step) -> addOneT mulTwoT(step)


console.log(transduce [1..20], addOneT, append)
console.log(transduce [1..20], mulTwoT, append)
console.log(transduce [1..20], addOneMulTwoT, append)
console.log(transduce [[1, 2], [], [3], [4, 5]], flattenT, append)
console.log(transduce [1..20], lessTnan4T, append)
console.log(transduce [1..20], first5T, append)
console.log(transduce [1..20], by3ItemsT, append)

transducers.js

"use strict";

function Reduced(wrapped) {
  this._wrapped = wrapped;
}

Reduced.isReduced = function(obj) {
  return (obj instanceof Reduced);
};

Reduced.prototype.unwrap = function() {
  return this._wrapped;
};


function curryArgs() {
  return function(first) {
    return function(all) {
      return function(last) {
        return function() {
          var args = Array.prototype.slice.call(arguments, 0);
          if (arguments.length === 0) {
            return first.apply(null, args);
          }
          if (arguments.length === 1) {
            return last.apply(null, args);
          }
          if (arguments.length === 2) {
            return all.apply(null, args);
          }
        };
      };
    };
  };
}


function prepareArgs(first, all, last) {
  return function() {
    var args = Array.prototype.slice.call(arguments, 0);
    if (arguments.length === 0) {
      return first.apply(this, args);
    }
    if (arguments.length === 1) {
      return last.apply(this, args);
    }
    if (arguments.length === 2) {
      return all.apply(this, args);
    }
  };
}


function reduce(collection, fn, seed) {
  var result = seed;
  for (var i = 0; i < collection.length; i++) {
    result = fn(result, collection[i]);
    if (Reduced.isReduced(result)) {
      return result.unwrap();
    }
  }
  return result;
}


function append(result, item) {
  if (arguments.length === 2) {
    return result.concat([item]);
  }
  if (arguments.length === 1) {
    return result;
  }
  if (arguments.length === 0) {
    return [];
  }
}


function transduce(collection, transducer, append) {
  var step   = transducer(append),
      seed   = step(),
      result = reduce(collection, step, seed);
  return step(result);
}


// transducers constructors:
function take(n) {
  return function(step) {
    var count   = 0,
        onFirst = function()       { return step(); },
        onLast  = function(result) { return step(result); },
        onAll   = function(result, item) {
            if (count++ < n) {
              return step(result, item);
            } else {
              return new Reduced(result);
            }          
        };

    return prepareArgs(onFirst, onAll, onLast);
  };
}


function map(fn) {
  return function(step) {
    return function(result, item) {
      if (arguments.length === 2) {
        return step(result, fn(item));
      }
      if (arguments.length === 1) {
        return step(result);
      }
      if (arguments.length === 0) {
        return step();
      }
    };
  };
}


function filter(predicate) {
  return function(step) {
    var onFirst = function()       { return step(); },
        onLast  = function(result) { return step(result); },
        onAll   = function(result, item) {
          if (predicate(item)) {
            return step(result, item);
          } else {
            return result;
          }
        };
    return prepareArgs(onFirst, onAll, onLast);
  };
}


function flatten() {
  return function(step) {
    var onFirst = function()       { return step(); },
        onLast  = function(result) { return step(result); },
        onAll   = function(result, item) {
          for (var i = 0; i < item.length; i++) {
            result = step(result, item[i]);
          }
          return result;
        };
    return prepareArgs(onFirst, onAll, onLast);
  };
}


function partition(n) {
  if (n < 1) {
    throw new Error("n > 1");
  }

  return function(step) {
    var current = [],
        onFirst = function() { return step(); },
        onLast  = function(result) {
          if (current.length > 0) {
            result = step(result, current);
            return step(result);
          }
        },
        onAll   = function(result, item) {
          current.push(item);
          if (current.length === n) {
            result  = step(result, current);
            current = [];
          }
          return result;
        };
    return prepareArgs(onFirst, onAll, onLast);
  };
}

// sample
var initial = [1,  2,  3,  4,  5,  6,  7,  8,  9,  10,
               11, 12, 13, 14, 15, 16, 17, 18, 19, 20];

var addOneT     = map(function(x) { return x + 1; }),
    lessTnan4T  = filter(function(x) { return x < 4; }),
    flattenT    = flatten(),
    first5T     = take(5),
    by3ItemsT   = partition(3);

var addOne_lessTnan4 = function(step) {
  return addOneT(lessTnan4T(step));
};

console.log(transduce(initial, addOneT, append));
console.log(transduce(initial, lessTnan4T, append));
console.log(transduce(initial, addOne_lessTnan4, append));
console.log(transduce([[1, 2], [], [3]], flattenT, append));
console.log(transduce(initial, first5T, append));
console.log(transduce(initial, by3ItemsT, append));