matches.js

Matches.js brings the power and expressiveness of pattern matching to Javascript.

Install

npm install matches

var pattern = require("matches").pattern;
var mymap = pattern({
  '_, []' : function () { 
    return [];
  },
  'f, [x, xs...]' : function (f, x, xs) {
    return [f(x)].concat(mymap(f, xs));
  }
});

// [2, 4, 6, 8]
mymap(function (x) { return x * 2; }, [1, 2, 3, 4]);

Patterns

Literals

Check for specific values using number or string literals, null, or undefined.

var myfn = pattern({
  // Null
  'null' : function () {...},

  // Undefined
  'undefined' : function () {...},

  // Numbers
  '42'    : function () { ... },
  '12.6'  : function () { ... },
  '1e+42' : function () { ... },

  // Strings
  '"foo"' : function () { ... },

  // Escape sequences must be double escaped.
  '"This string \\n matches \\n newlines."' : function () { ... }
});

Wildcards

Underscores will match successfully on any value but ignore it. A single underscore works well as a catch all pattern.

var myfn = pattern({
  // Match if second argument is 12, ignoring the first
  '_, 12' : function () { ... },

  // Match on anything
  '_' : function () { ... }
});

Identifiers

Patterns that start with lowercase letters will pass the value to the function. Values are passed to the function in the same right to left order they are declared in the pattern.

var myfn = pattern({
  // Pass on the second argument if the first is 12
  '12, x': function (x) { ... },

  // Pass on the first and third argument, ignoring the second
  'x, _, y': function (x, y) { ... }
});

Arrays

Match on the entire array, or only a few values.

var myfn = pattern({
  // Empty array
  '[]' : function () { ... },

  // Strict comparison on contents
  '[1, 2, 3]': function () { ... },

  // Grab the first value, ignoring the rest
  '[x, ...]': function (x) { ... },

  // Split it into a head and tail
  '[head, tail...]': function (head, tail) { ... },

  // Grab the first and last items, ignoring the middle
  '[x, ..., y]': function (x, y) { ... },

  // Grab the last item
  '[..., last]': function (last) { ... },

  // Make a shallow clone
  '[clone...]': function (clone) { ... },

  // Grab the first item, but also pass on the whole array
  'arr@[first, ...]': function (arr, first) { ... }
});

Objects

Like with an array, you can match on an entire object, or just a few keys.

var myfn = pattern({
  // Empty object
  '{}': function () { ... },

  // Check that an object has only two keys 'x' and 'y', and pass to the function
  '{x, y}': function (x, y) { ... },

  // Check that an object has a key 'children' that contains an array
  '{children: [a, b], ...}': function (a, b) { ... },

  // Match on two keys, 'x' and 'y' and copy the rest to another object
  '{x: a@Number, y: b@Number, c...}': function (a, b, c) { ... },

  // Make a shallow clone of an object
  '{clone...}': function (clone) { ... }
})

Core Javascript Types

Typecheck arguments using Number, String, Date, RegExp, Function, Array, or Object.

var myfn = pattern({
  // Takes a function and an array
  'fn@Function, arr@Array': function (fn, arr) { ... },

  // Takes a function and an object
  'fn@Function, obj@Object': function (fn, obj) { ... }
});

Adt.js Types

Matches.js has builtin support for adt.js types. Adt.js is a library for building algebraic data types in Javascript.

// Create a new adt.js type
var Tree = adt.data({
  Empty : adt.single(),
  Node  : adt.record("val", "left", "right")
});

var mytree = Tree.Node(12, Tree.Empty(), Tree.Node(42, Tree.Empty(), Tree.Empty()));

var myfn = pattern({
  // Match on an Empty tree node
  'Empty': function () { ... },

  // Match on a Node with a value of 12
  'Node(12, _, _)': function () { ... },

  // Match on a Node that has non-Empty children
  'Node(_, Node, Node)': function () { ... }

  // Match on a Node that has a left child Node of 42 and an Empty right node
  'Node(val, Node(42, _, _), Empty)': function (val) { ... }
});

Find out more about adt.js: https://github.com/natefaubion/adt.js

Usage

Matches.js exports two functions, pattern and caseOf.

var matches = require("matches");
var pattern = matches.pattern;
var caseOf  = matches.caseOf;

pattern(patternObj)

The simplest way is to give it an object with the keys being patterns, and the values being functions. Each pattern will be tried in order until a match is found.

var arrayElems = pattern({
  '[]': function () { 
    return "This array is empty."; 
  },
  '[x]': function (x) { 
    return "This array has one element: " + x; 
  },
  '[x, y]': function (x, y) { 
    return "This array has two elements: " + x + " and " + y;
  },
  '[x, y, ...]': function (x, y) {
    return "This array is long. The first two elements are: " + x + " and " + y;
  }
});

arrayElems([1, 2, 3]);

pattern(patternStr, successFn)

You can create individual pattern and function pairs.

var emptyArray = pattern('[]', function () { return "Empty array" });

// "Empty array"
emptyArray([]);

// TypeError: "All patterns exhausted"
emptyArray(12);

pattern(patternFn, successFn)

You can also create your own custom pattern functions. The patternFn takes an array of arguments, and should return false for no match, or a new array of arguments to forward on to the successFn.

var greater42 = function (args) {
  if (args[0] >= 42) return [args[0]];
  return false;
};

var customPattern = pattern(greater42, function (x) {
  console.log(x);
});

// Logs 54
customPattern(54);

// TypeError: "All patterns exhausted"
customPattern(12);

caseOf(args..., patternObj)

You can use caseOf to do ad-hoc pattern matching on objects. It's the same as immediately invoking a pattern function, but lets you put the arguments first.

var result = caseOf(42, {
  'x@Number' : function (x) { return x * 2; },
  '_'        : function () { return null; }
});

// Is the same as...
var result = pattern({
  // ...
})(42);

Combinators

You can combine any of these methods to create unique match chains using the alt combinator.

var wildcard = pattern('_', function () { return "No matches."; });
var mychain = pattern('1', function () { return "One"; })
  .alt({
    '2': function () { return "Two"; },
    '3': function () { return "Three"; }
  })
  .alt(wildcard);

// 'One'
mychain(1);

// 'Two'
mychain(2);

// 'No matches.'
mychain(5);

Multiple Arguments

Separate matches for multiple arguments with a comma. Since you can pass any number of arguments to functions in Javascript, Matches.js is not strict and will happily combine patterns for varying numbers of arguments.

var myfn = pattern({
  // Matches on the first three arguments. If more are passed, they are ignored.
  '1, "foo", [a, ...]': function (a) { return a; },

  // Matches on the first two arguments, ignoring the rest
  'a, fn@Function': function (a, fn) { return fn(a); },

  // Matches anything
  '_': function () { return null; }
});

// 12
myfn(6, function (x) { return x * 2; }, "foo", "bar");

// null
myfn(1, 2, 3, 4);

Performance

Pattern strings are compiled to pure Javascript functions and then cached, so in general, they are quite fast.

On a 2GHz core, average compilation time is around .12ms for a pattern comprised of 5-6 sub-patterns. Pattern matched functions are around 3-4x slower to dispatch than an equivalent hand-optimized function that does similar type-checking. Keep in mind, that time is measured in the single microseconds (1µs vs 3µs) to dispatch 5 calls to the same function.

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