Chainable Currying

curry-chain is a library for creating literary, chai-style curried functions.

Usage

curry-chain allows to define curried functions: functions with settable arguments and chainable setters. Curried functions are wrapped around ordinary functions, allowing to set their arguments one by one, set individual properties for object arguments, and much more.

In order to create a curried function, call curry on an existing function:

const curry = require('curry-chain');

function greet (who, how) {
  return how + ', ' + who + '!';
}

var curryGreet = curry.fn(greet).where
  .arg(0).has.setter('who').and
  .arg(1).has.setter('how')
  .done();

This creates a chained function, the 0-th argument of which can be set with a who setter, and the 1-st with a how setter:

var greeting = curryGreet.who('universe').how('Hello').done();
// greeting === 'Hello, universe!'

More details of how the above code works:

• fn sets the function for currying
• arg selects the context - the argument index, which further setter and other instructions will describe
• setter creates a chainable setter with the specified name. The setter will set the current argument. By default, a setter replaces the argument, but other behaviors are also available
• done signals that the function is complete
• where, that and and are language chains, they are here just to improve readablility, just like in chai

The created curried function will have the specified chainable setters, and the done function, which will call the original function specified by fn, with the values of arguments set.

Notice that curry looks like a curried function: it is chainable and has the done terminator. This is not a coincidence; curry actually is a curried wrapper around a low-level curry creator function (see the source code for more details).

Argument assignment

Several setters

You can specify several setters for the same argument:

var curryGreet = curry.fn(greet).where
  .arg(0).has.setter('who')
  .arg(1).has.setter('how').and.setter('with')
  .done();
  
curryGreet.with('G\'day').who('world').done();
curryGreet.how('Hello').who('world').done();

Language

You can specify language chains to improve readablility by using the language chain:

var curryAdd = curry.fn((x, y) => x + y).where
  .arg(0).has.setter('x').and
  .arg(1).has.setter('y').and
  .language('with', 'and')
  .done();
  
curryAdd.with.x(10).and.y(5);

The names of language chains may coincide with names of setters.

Setters for properties

It is possible to specify setters for separate properties of an argument using the option and options chains:

function drawFigure(shape, pos, options) {
  // uses pos.x, pos.y, options.color and options.size
}

var curriedDraw = curry.fn(drawFigure).where
  .arg(0).has.setter('shape')
  .arg(1).has.setter('pos').and.option('x').and.option('y')
  .arg(2).has.options('color', 'size')
  .language('with', 'and')
  .done();
  
curriedDraw.shape('circle')
  .with.x(10).and.y(25)
  .color('blue').and.size(10).done();

Option sinks

The sink chain specifies a setter that can assign several properties of an argument using Object.assign:

var curriedDraw = // ...
  .arg(2).has.sink('with').done();

// 'with' may work simultaneously as a language chain and a sink
var shape = curriedDraw.shape('circle').with.x(10).and.color('blue');
// Leaves { color: 'blue' } option intact
shape.with({ size: 10 }).done();

Defaults

You can specify default argument values by using the defaultsTo chain:

var curriedDraw = // ...
  .arg(1).defaultsTo({ x: 0, y: 0 })
  .done();

Partial assignment

As it can be expected from currying, curried functions may have only some arguments assigned.

var curryAdd = // ...
[1, 2, 3].map(curryAdd.with.x(10).done); // returns [11, 12, 13]

Immutability

All generated curry chains are immutable, so you can safely pass them around.

var curriedDraw = // ...
var circle = curriedDraw.type('circle');
var setSize = (shape, sz) => shape.size(sz);

// will draw red circle with size 10
var sizedCircle = setSize(circle.color('red'), 10);
// draws a blue circle with size 25
setSize(circle.color('blue'), 25).done();
// still draws a red circle with size 10
sizedCircle.x(5).y(10).done();

Functional interface

curry is implemented using objects. There is also a purely functional implementation, available via require('curry-chain/fn'). Unlike the default implementation, the functional one does not require the done terminator (although it still works):

var curry = require('curry-chain/fn');

var curryAdd = curry.fn((x, y) => x + y).where
  .arg(0).has.setter('x').and
  .arg(1).has.setter('y').and
  .language('with', 'and')();
  
[1, 2, 3].map(curryAdd.with.x(10)); // returns [11, 12, 13]

Unfortunately, the functional implementation is ~2 times slower than the object one. This is because the object implementation can use prototypes in order to reuse setters during chaining; the functional one needs to create new functions for setters during each chained call.

License

curry-chain is available under the terms of the Apache 2.0 license.