Demo for small bug in transducers.js 0.2.3

mapcat_bug.js

'use strict';

var t = require('transducers.js');


// -- some hacks to let us copy and paste most of the original library code

var Reduced = t.Reduced;
var reduce  = t.reduce;
var compose = t.compose;
var map     = t.map;

function isReduced(x) {
  return (x instanceof t.Reduced) || (x && x.__transducers_reduced__);
}

function bound(f, ctx, count) {
  count = count != null ? count : 1;

  if(!ctx) {
    return f;
  }
  else {
    switch(count) {
    case 1:
      return function(x) {
        return f.call(ctx, x);
      }
    case 2:
      return function(x, y) {
        return f.call(ctx, x, y);
      }
    default:
      return f.bind(ctx);
    }
  }
}


// -- modified Cat, cat and mapcat

function Cat(xform) {
  this.xform = xform;
}

Cat.prototype.init = function() {
  return this.xform.init();
};

Cat.prototype.result = function(v) {
  return this.xform.result(v);
};

Cat.prototype.step = function(result, input) {
  var xform = this.xform;
  var newxform = {
    init: function() {
      return xform.init();
    },
    result: function(v) {
      return v;
    },
    step: function(result, input) {
      var val = xform.step(result, input);
      // changed this:
      //   return isReduced(val) ? deref(val) : val;
      // to this:
      return isReduced(val) ? new Reduced(val) : val;
    }
  }

  return reduce(input, newxform, result);
};

function cat(xform) {
  return new Cat(xform);
}

function mapcat(f, ctx) { 
  f = bound(f, ctx);
  return compose(map(f), cat);
}


// -- test with original and modified versions

var xf_original = t.compose(
  t.mapcat(function(x) { return [x, x*x]; }),
  t.map(function(x) { console.log('.'+x+'.'); return x; }),
  t.take(3)
);

console.log('Original:');
console.log(t.seq([1,2,3,4], xf_original));


var xf_modified = t.compose(
  mapcat(function(x) { return [x, x*x]; }),
  t.map(function(x) { console.log('.'+x+'.'); return x; }),
  t.take(3)
);

console.log();
console.log('Modified:');
console.log(t.seq([1,2,3,4], xf_modified));

Output

Original:
.1.
.1.
.2.
.4.
.3.
.9.
.4.
.16.
[ 1, 1, 2 ]

Modified:
.1.
.1.
.2.
[ 1, 1, 2 ]