underline_f v0.0.11

_F

Functional chaining in js.

Usage

1. bower install Hypercubed/_F
2. Include the _F.js script into your app. By default should be at bower_components/_F/_F.js

Testing

Install npm and bower dependencies:

npm install
bower install
npm test

Summary of API

Hypercubed/_F is simply global shortcut for composable "d3 style" data accessors functions. For example:

Accessors

Example

var data = [ { firstname: 'John', lastname: 'Smith', age: 51 }, /* ... */ ];
var _firstname = _F('firstname');

data.map(_firstname);  // Returns a list of first names

Operators

Example

var _johns = _firstname.eq('John');

data.filter(_johns);  // returns a list of John's

Chaining

Example

var _age = _F('age');
var _twenties = _age.gte(20).and().lt(30);

data.filter(_johns.and(_twenties));  // returns a list of John's in their twenties

Sorting

Example

data.filter(_johns.and(_twenties)).sort(_age.order().asc);  // returns a list of John's in their twenties sorted by age in ascending order

Why?

In JavaScript, especially when using d3, we often write accessor functions like this:

function(d) { return d.value; }

This simple function returns the value of the value key when an object is pass to it. For example in the map function:

values = data.map(function(d) { return d.value; });

This is lightweight, simple, and readable. There is nothing wrong with it. Sometimes, however, in order to avoid repeating ourselves so we crete a reusable accessor function like this:

var _value = function(d) { return d.value; };
values = data.map(_value);

Now imagine the object also has a year key whose values are date objects. We may want to filter like this:

var _value = function(d) { return d.value; };
var _year_filter = function(d) { return d.year >= new Date('1980 Jan 1'); };
values = data.filter(_year_filter).map(_value);

However, this has a couple of slight drawbacks. First of all you will need to create a new filter every time the date changes; also the Date constructor is called for every element in the data array. A better approach is an accessor factory:

var _year_filter = function(date) {
  return function(d) { return d.year >= date; };
}

var _filter = _year_filter(new Date('1990 Jan 1'));
values = data.filter(_filter).map(_value);

It's a little ugly but here the Date constructor is only called once and the _year_filter function returns the accessor. An new accessor can be created any time by calling _year_filter

Now what if we want to filter between two dates. We can do modify our accessor factory:

var _year_filter = function(dateA, dateB) {
  return function(d) { return d.year >= new Date(dateA) && d.year < new Date(dateB); };
}

but let's say that you have multidimensional data where dateA and dataB change independently. You might be tempted to do something like this:

var _year_gte = function(dateA) {
  return function(d) { return d.year >= dateA; };
}

var _year_lt = function(dateB) {
  return function(d) { return d.year < dateB; };
}

_year_filter1 = _year_gte(new Date('1980 Jan 1'));
_year_filter2 = _year_lt(new Date('1990 Jan 1'));

values = data
  .filter(_year_filter1)
  .filter(_year_filter2)
  .map(_value);

Ok, no we are getting ridiculous. The date constructor is not that expensive. But you can imagine a situation where the values for filters could be very expensive. For example based on aggregated statistics or reading from the DOM.

Ok, at this point let me introduce _F. _F is simply a shortcut for all this. For example:

var _value = _F('value');
values = data.map(_value);

The value returned from _F() in this case is simply the accessor function function(d) { return d.value; }.

Interesting. How about this:

var _value = _F('value');
var _year_filter = _F('year').gte(new Date('1980 Jan 1'));
values = data.filter(_year_filter).map(_value);

_F('year').gte(somevalue) is essentially a shortcut for function(d) { return d.year >= somevalue; }.

It gets better:

var _value = _F('value');

var _year_filter =
  _F('year')
    .gte(new Date('1980 Jan 1'))
    .and().lt(new Date('1990 Jan 1'));

values = data.filter(_year_filter).map(_value);

or how about this:

var _value = _F('value');
var _value_filter = _value.gt(10);

var _year = _F('year');
var _year_filter =
  _year
    .gte(new Date('1980 Jan 1'))
    .and().lt(new Date('1990 Jan 1'));

var _filter = _value_filter.and(_year_filter);

values = data.filter(_filter).map(_value);

Pretty neat?

Acknowledgments

License

Copyright (c) 2014+ Jayson Harshbarger MIT