## lodash/fp

The `lodash/fp` module promotes a more
[functional programming](https://en.wikipedia.org/wiki/Functional_programming)
(FP) friendly style by exporting an instance of `lodash` with its methods wrapped
to produce immutable auto-curried iteratee-first data-last methods.

## Installation

In a browser:
```html
<script src='path/to/lodash.js'></script>
<script src='path/to/lodash.fp.js'></script>
<script>
// Loading `lodash.fp.js` converts `_` to its fp variant.
_.defaults({ 'a': 2, 'b': 2 })({ 'a': 1 });
// ➜ { 'a': 1, 'b': 2 }

// Use `noConflict` to restore the pre-fp variant.
var fp = _.noConflict();

_.defaults({ 'a': 1 }, { 'a': 2, 'b': 2 });
// ➜ { 'a': 1, 'b': 2 }
fp.defaults({ 'a': 2, 'b': 2 })({ 'a': 1 });
// ➜ { 'a': 1, 'b': 2 }
</script>
```

In Node.js:
```js
// Load the fp build.
var fp = require('lodash/fp');

// Load a method category.
var object = require('lodash/fp/object');

// Load a single method for smaller builds with browserify/rollup/webpack.
var extend = require('lodash/fp/extend');
```

## Mapping

Immutable auto-curried iteratee-first data-last methods sound great, but what
does that really mean for each method? Below is a breakdown of the mapping used
to convert each method.

#### Capped Iteratee Arguments

Iteratee arguments are capped to avoid gotchas with variadic iteratees.
```js
// The `lodash/map` iteratee receives three arguments:
// (value, index|key, collection)
_.map(['6', '8', '10'], parseInt);
// ➜ [6, NaN, 2]

// The `lodash/fp/map` iteratee is capped at one argument:
// (value)
fp.map(parseInt)(['6', '8', '10']);
// ➜ [6, 8, 10]
```

Methods that cap iteratees to one argument:<br>
<%= toFuncList(_.keys(_.pickBy(mapping.iterateeAry, _.partial(_.eq, _, 1)))) %>

Methods that cap iteratees to two arguments:<br>
<%= toFuncList(_.keys(_.pickBy(mapping.iterateeAry, _.partial(_.eq, _, 2)))) %>

The iteratee of `mapKeys` is invoked with one argument: (key)

#### Fixed Arity

Methods have fixed arities to support auto-currying.
```js
// `lodash/padStart` accepts an optional `chars` param.
_.padStart('a', 3, '-')
// ➜ '--a'

// `lodash/fp/padStart` does not.
fp.padStart(3)('a');
// ➜ '  a'
fp.padCharsStart('-')(3)('a');
// ➜ '--a'
```

Methods with a fixed arity of one:<br>
<%= toFuncList(_.difference(mapping.aryMethod[1], _.keys(mapping.skipFixed))) %>

Methods with a fixed arity of two:<br>
<%= toFuncList(_.difference(mapping.aryMethod[2], _.keys(mapping.skipFixed))) %>

Methods with a fixed arity of three:<br>
<%= toFuncList(_.difference(mapping.aryMethod[3], _.keys(mapping.skipFixed))) %>

Methods with a fixed arity of four:<br>
<%= toFuncList(_.difference(mapping.aryMethod[4], _.keys(mapping.skipFixed))) %>

#### Rearranged Arguments

Method arguments are rearranged to make composition easier.
```js
// `lodash/filter` is data-first iteratee-last:
// (collection, iteratee)
var compact = _.partial(_.filter, _, Boolean);
compact(['a', null, 'c']);
// ➜ ['a', 'c']

// `lodash/fp/filter` is iteratee-first data-last:
// (iteratee, collection)
var compact = fp.filter(Boolean);
compact(['a', null, 'c']);
// ➜ ['a', 'c']
```

##### Most methods follow these rules

A fixed arity of two has an argument order of:<br>
<%= toArgOrder(mapping.aryRearg[2]) %>

A fixed arity of three has an argument order of:<br>
<%= toArgOrder(mapping.aryRearg[3]) %>

A fixed arity of four has an argument order of:<br>
<%= toArgOrder(mapping.aryRearg[4]) %>

##### Exceptions to the rules

Methods that accept an array of arguments:<br>
<%= toFuncList(_.keys(mapping.methodSpread)) %>

Methods with unchanged argument orders:<br>
<%= toFuncList(_.keys(mapping.skipRearg)) %>

Methods with custom argument orders:<br>
<%= _.map(_.keys(mapping.methodRearg), function(methodName) {
  var orders = mapping.methodRearg[methodName];
  return ' * `_.' + methodName + '` has an order of ' + toArgOrder(orders);
}).join('\n') %>

#### New Methods

Not all variadic methods have corresponding new method variants. Feel free to
[request](https://github.com/lodash/lodash/blob/master/.github/CONTRIBUTING.md#feature-requests)
any additions.

Methods created to accommodate Lodash’s variadic methods:<br>
<%= toFuncList(_.keys(mapping.remap)) %>

#### Aliases

There are <%= _.size(mapping.aliasToReal) %> method aliases:<br>
<%= _.map(_.keys(mapping.aliasToReal).sort(), function(alias) {
  var realName = mapping.aliasToReal[alias];
  return ' * `_.' + alias + '` is an alias of `_.' + realName + '`';
}).join('\n') %>

## Placeholders

The placeholder argument, which defaults to `_`, may be used to fill in method
arguments in a different order. Placeholders are filled by the first available
arguments of the curried returned function.
```js
// The equivalent of `2 > 5`.
_.gt(2)(5);
// ➜ false

// The equivalent of `_.gt(5, 2)` or `5 > 2`.
_.gt(_, 2)(5);
// ➜ true
```

## Chaining

The `lodash/fp` module **does not** convert chain sequence methods. See
[Izaak Schroeder’s article](https://medium.com/making-internets/why-using-chain-is-a-mistake-9bc1f80d51ba)
on using functional composition as an alternative to method chaining.

## Convert

Although `lodash/fp` & its method modules come pre-converted, there are times
when you may want to customize the conversion. That’s when the `convert` method
comes in handy.
```js
// Every option is `true` by default.
var _fp = fp.convert({
  // Specify capping iteratee arguments.
  'cap': true,
  // Specify currying.
  'curry': true,
  // Specify fixed arity.
  'fixed': true,
  // Specify immutable operations.
  'immutable': true,
  // Specify rearranging arguments.
  'rearg': true
});

// The `convert` method is available on each method too.
var mapValuesWithKey = fp.mapValues.convert({ 'cap': false });

// Here’s an example of disabling iteratee argument caps to access the `key` param.
mapValuesWithKey(function(value, key) {
  return key == 'a' ? -1 : value;
})({ 'a': 1, 'b': 1 });
// => { 'a': -1, 'b': 1 }
```

Manual conversions are also possible with the `convert` module.
```js
var convert = require('lodash/fp/convert');

// Convert by name.
var assign = convert('assign', require('lodash.assign'));

// Convert by object.
var fp = convert({
  'assign': require('lodash.assign'),
  'chunk': require('lodash.chunk')
});

// Convert by `lodash` instance.
var fp = convert(lodash.runInContext());
```

## Tooling

Use [eslint-plugin-lodash-fp](https://www.npmjs.com/package/eslint-plugin-lodash-fp)
to help use `lodash/fp` more efficiently.
