wojons/pun

A small library to enable pattern matching in javascript and coffeescript

Pun

A tiny functional programming library for Javascript/Coffeescript.

Currying

pun.curry allows you to partially apply some values to a function:

f = (a,b,c,d) ->
	[a,b,c,d].join(', ')

fab = pun.curry f, 1, 2

fab(3,4)      # 1, 2, 3, 4
fab("c", "d") # 1, 2, c, d

pun.autocurry allows you to create functions which automatically curry when you apply arguments to them:

# Using f from previous example
autof = pun.autocurry f

autofab = autof(1, 2)

autofab(3,4)      # 1, 2, 3, 4
autofab("c", "d") # 1, 2, c, d

Abstract Data Types

You can use ADTs like so:

List = pun.ADT
	Cons: ['value', 'next']
	Nil:  []

l = List.Cons(1, List.Cons(2, List.Nil()))  # Linked list for [1,2]
l.value                                     # 1
l.next.value                                # 2

And can apply pattern matching!

Pattern Matching

Pun allows for complex pattern matching in javascript/coffeescript. Let's dive in with an example:

Hello, factorial

Coffeescript

$ = pun.$

fac = pun.match(
    0, -> 1
    $, (n) -> n * fac (n-1)
)

Javascript

var $ = pun.$

var fac = pun.match(
    0, function() { return 1 },            // Note required comma at end of line
    $, function(n){ return n * fac(n-1) }
)

This is equivalent to the Haskell:

fac 0 = 1
fac n = n * fac (n-1)

pun.match takes pairs of arguments, the first being the pattern the second being the function applied if the pattern is matched. It will return undefined if there is no match.

Basic Matching

Numbers, strings, bools and undefined/nulls are all matched simply by equality:

f = pun.match(
	110,      -> "one"
	"foobar", -> "two"
	true,     -> "three"
)

f(110)      # "one"
f("foobar") # "two"
f(true)     # "three"

Wildcard

The wild character pun._ can be used to match any value:

_ = pun._

f = pun.match(
    0, -> 0
    1, -> 1
    _, -> "Other"
)

f(0)           # 0
f(1)           # 1
f("foobarbaz") # "Other"

Type Matching

You can pass functions which will be interpreted as type constructors - this allows you to match your own "classes" or the builtin ones:

class Cat

f = pun.match(
	Number,  -> "Number"
	String,  -> "String"
	Boolean, -> "Boolean"
	Cat,      -> "Cat"
)

f(1024)       # "Number"
f("foobar")   # "String"
f(true)       # "Boolean"
cf(new Cat()) # "Cat"

Binding

The $ symbol can be used to 'bind' values so that you can use them in the matching function. You can use it without an argument and the bound value will be passed to match function as an argument, or with an argument of a pattern to match:

$ = pun.$    # alias $ so we can use it more easily

f = pun.match(
    $(Number), (n) -> "Num: #{n}"
    $,         (a) -> "Got: #{a}"
)

f(0)    # "Num: 0"
f(true) # "Got: true"
f({})   # "Got: [object Object]"

Or you can supply it with a string s and it will be avaliable in this.s/@s of the match function:

f = pun.match(
    $('a'), -> "Got: #{@a}"
)

Finally, you can bind to patterns:

f = pun.match(
	$('a',Number), -> "Got: #{@a}"
)

f(1)     # "Got 1"
f(false) # undefined

Arrays

Arrays will be matched item by item and each element of the array is a pattern. The pattern array length and the value array length must be the same.

$ = pun.$

f = pun.match(
	[1,2,3],                             -> "onetwothree"
	[$, $('a'), $('b', Number)], (first) -> "#{first}, #{@a}, #{@b}" 
)

f([0,"lol",2])    # "0, lol, 2"
f([0,"lol",true]) # undefined
f([1,2,3])        # "onetwothree"
f([1,2,3,4])      # undefined

Objects

Objects as a pattern will match each key and value. The value of each key/value pair is a pattern:

$ = pun.$

f = pun.match(
	{a: 0, b: "lol"},          -> "One"
	{a: 0, b: $('n',Number)},  -> "Two #{@n}"
	{a: 0},                    -> "Three"
)

f({a:0})               # "Three"
f({a:0, b:"lol", c:1}) # "One"
f({a:0, b: 4})         # "Two 4"

Abstract Data Types

ADTs can also be matched:

$ = pun.$

from = pun.match(
	List.Nil(),               -> []
	List.Cons($, $),  (x, xs) -> [x].concat from(xs)
)

from(List.Cons 1, List.Cons 2, List.Cons 3, List.Nil())  # [1,2,3]