// I am going to use a system where a sequence object has two methods:

//

// * next(), which returns a boolean indicating whether there are more

// elements in the sequence, and moves it forward to the next

// element when there are.

//

// * current(), which returns the current element, and should only be

// called after next() has returned true at least once.

function logFive(sequence) {

for (var i = 0; i < 5; i++) {

if (!sequence.next())

break;

console.log(sequence.current());

}

}

function ArraySeq(array) {

this.pos = -1;

this.array = array;

}

ArraySeq.prototype.next = function() {

if (this.pos >= this.array.length - 1)

return false;

this.pos++;

return true;

};

ArraySeq.prototype.current = function() {

return this.array[this.pos];

};

function RangeSeq(from, to) {

this.pos = from - 1;

this.to = to;

}

RangeSeq.prototype.next = function() {

if (this.pos >= this.to)

return false;

this.pos++;

return true;

};

RangeSeq.prototype.current = function() {

return this.pos;

};

logFive(new ArraySeq([1, 2]));

// → 1

// → 2

logFive(new RangeSeq(100, 1000));

// → 100

// → 101

// → 102

// → 103

// → 104

// This alternative approach represents the empty sequence as null,

// and gives non-empty sequences two methods:

//

// * head() returns the element at the start of the sequence.

//

// * rest() returns the rest of the sequence, or null if there are no

// elemements left.

//

// Because a JavaScript constructor can not return null, we add a make

// function to constructors of this type of sequence, which constructs

// a sequence, or returns null if the resulting sequence would be

// empty.

function logFive2(sequence) {

for (var i = 0; i < 5 && sequence != null; i++) {

console.log(sequence.head());

sequence = sequence.rest();

}

}

function ArraySeq2(array, offset) {

this.array = array;

this.offset = offset;

}

ArraySeq2.prototype.rest = function() {

return ArraySeq2.make(this.array, this.offset + 1);

};

ArraySeq2.prototype.head = function() {

return this.array[this.offset];

};

ArraySeq2.make = function(array, offset) {

if (offset == null) offset = 0;

if (offset >= array.length)

return null;

else

return new ArraySeq2(array, offset);

};

function RangeSeq2(from, to) {

this.from = from;

this.to = to;

}

RangeSeq2.prototype.rest = function() {

return RangeSeq2.make(this.from + 1, this.to);

};

RangeSeq2.prototype.head = function() {

return this.from;

};

RangeSeq2.make = function(from, to) {

if (from > to)

return null;

else

return new RangeSeq2(from, to);

};

logFive2(ArraySeq2.make([1, 2]));

// → 1

// → 2

logFive2(RangeSeq2.make(100, 1000));

// → 100

// → 101

// → 102

// → 103

// → 104