JS-Pattern-Matching

A small library intended to provide simple Pattern Matching capabilities for JavaScript.

var match = require('js-pattern-matching');

const sum = (list) =>  match (list) (
  ([x,...xs]) => x + sum(xs),
  ([]) => 0
)

console.log(sum([]));
// prints 0
console.log(sum([1,2,3]));
// prints 6

Installation

npm install --save js-pattern-matching

JS-Pattern-Matching leverages ES2015 syntax to make more readable and easy-to-use code. Therefore, it can only be run in ES2015-supporting environments (Node ^4 or any platform transpiled with Babel)

Babel

Babel is now supported by JS-Pattern-Matching!

To make use of it, you just need to install the Babel Plugin for JS-Pattern-Matching

npm install --save-dev babel-plugin-js-pattern-matching

Finally, you have to tell Babel to use it by adding it to your .babelrc file:

{
  "presets": [
    "es2015"
  ],
  "plugins": [
      "babel-plugin-js-pattern-matching",
  ]
}

Using JS-Pattern-Matching

We import the powerful match function by doing

var match = require('js-pattern-matching');

General Syntax

The syntax is always of the form:

match (valueToMatch)(
 listOfCommaSeparatedCaseClosures
)

Each case-closure has to be a valid ES2015 closure and is itself of the form (pattern) => closureBody

We explore different patterns in the following section

Matching literal values

The pattern for literal values is always (someVariable= matchingValue). By convention, we use the variable v which simply stands for "value", but any other valid JS identifier will do

• We can match literal values of primitive types: number, string, boolean, null, undefined:

const getValue = (value) =>  match (value) (
  (v= 1) => "The number one",
  (v= "hello") => "A greeting",
  (v= undefined) => "An undefined value",
  (v= null) => "A null value",
  (v= true) => "The true boolean",
  (v= NaN) => "Not a number"
)

getValue(1) //returns "The number one"
getValue("hello") //returns "A greeting"
getValue(parseInt("lala")) //returns "Not a number"
getValue(2 == 2)//returns "The true boolean"
...  

• If no value matches, MatchError is thrown:

const getNumberName = (value) =>  match (value) (
  (v= 1) => "one",
  (v= 2) => "two"
)

getNumberName(1) //returns "one"
getNumberName(5) //throws MatchError

• If more than one case-closures match, the first takes precedence:

const getNumberName = (value) =>  match (value) (
  (v= 1) => "the first one",
  (v= 1) => "another one"
)

getNumberName(1) //returns "the first one"

Matching and binding variables

The pattern for a variable is simply (variableName)

• A variable pattern always matches anything and binds the variable to the matching value

const length = (array) =>  match (array) (
  (array) => array.length
)

length([1,2,3]) //returns 3
length("Hello!") //returns 6

• An annonymous variable (_) pattern always matches anything but doesn't bind the variable. It is usually used as a fallback case-closure

const isVowel = (letter) =>  match (letter) (
  (v= 'A') => true,
  (v= 'E') => true,
  (v= 'I') => true,
  (v= 'O') => true,
  (v= 'U') => true,
   (_) => false
)

isVowel('I') //returns true
isVowel('R') //returns false

Matching and Binding Class instances

The pattern for Classes is simply (ClassName) or ( variable = ClassName) to bind the class instance

• We can match values according to their class:

const hasPermission = (user) =>  match (user) (
  (Admin) => true,
  (FreeUser) => false,
  (PremiumUser) => false
)

hasPermission(new FreeUser()) //returns false

• We can match values according to their superclass:

const readError = (user) =>  match (user) (
  (ReferenceError) => "Something was undeclared!",
  (Error) => "Other Error"
)

readError(new ReferenceError()) //returns "Something was undeclared!"
readError(new SyntaxError()) //returns "Other Error"

• We can bind to variables values that match to a class o superclass:

try {
  x + 1
} catch(error){
  match (error) (
  (e = ReferenceError) => "Reference error:" + e.message,
  (Error) => "Other Error"
)

//As x hasn´t been declared, it returns "Reference Error: x is not defined"

• We can even bing to variables by ES2015 destructuring:

try {
  x + 1
} catch(error){
  match (error) (
  ({ message } = ReferenceError) => "Reference error:" + message,
  (Error) => "Other Error"
)

//As x hasn´t been declared, it returns "Reference Error: x is not defined"

Matching and Binding Array instances:

To simplify array handling, we provide specific Array matchers, based on ES2015 Array destructuring:

• We can match an empty or nonempty array:

const sum = (array) =>  match (array) (
  ([x,...xs]) => x + sum(xs),
  ([]) => 0
)

sum([1,2,3]) // returns 6