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λ-Calculus Explained via CoffeeScript

Pull requests please to improve this document.

Basic concepts of the λ-calculus implemented in CoffeeScript. Because of CoffeeScript's elegant, Haskell-like function declrations and its easy transformation to javascript, it is great at demonsrating implementations of λ-calculus.

The lambda abstraction λx.x is equivalent to the mathematical function f(x) = x is equivalent to the CoffeeScript function definition (x) -> x.

The application of (λx.x)(2) is equivalent to the mathematical function f(x) = x where x=2 is equivalent to the CoffeeScript ((x) -> x)(2).

We can use the λ-calculus to encode data, predicate logic, and recursion. For a programmer this is one way to hint at the λ-calculus's ability to compute all that is computable. Alonzo Church's methods of doing this encoding are not immediately intuitive. By giving parallel examples in an easily executable and familiar programming environment, it becomes easier to adopt these concepts.

Predicate Logic

Church booleans are an elegant way of representing "true" and "false".

TRUE  = (x) -> (y) -> x # λx.λy.x
FALSE = (x) -> (y) -> y # λx.λy.y

We implement a function b that converts Church Booleans to literal Javscript booleans.

b = (f) -> f(true)(false)

This can be printed out like so:

console.log b(TRUE)  # --> true
console.log b(FALSE) # --> false

We can then construct logic operators

AND = (p) -> (q) -> p(q)(p)               # λp.λq.p q p
OR  = (p) -> (q) -> p(p)(q)               # λp.λq.p p q

NOT = (p) -> (a) -> (b) -> p(b)(a)        # λp.λa.λb.p b a
IFTHENELSE = (p) -> (a) -> (b) -> p(a)(b) # λp.λa.λb.p a b

Note the symmetry between "AND" <-> "OR" and "NOT" <-> "IFTHENELSE".

We can apply the operators like so

AND(TRUE)(TRUE) # --> TRUE

Note that the return value of AND(TRUE)(TRUE) is a "lambda abstraction" (aka function) whose η-conversion is equivalent to TRUE. To verify this we can use our javascript boolean converter to see the full truth table output.

console.log b AND(TRUE)(TRUE)   # --> true
console.log b AND(TRUE)(FALSE)  # --> false
console.log b AND(FALSE)(TRUE)  # --> false
console.log b AND(FALSE)(FALSE) # --> false

We can also create predicates that return Church Booleans

ISZERO = (n) -> n((x) -> FALSE)(TRUE) # λn.n (λx.FALSE) TRUE

We can use IFTHENELSE to perform logic

IFTHENELSE(TRUE)(THREE)(FOUR)  # --> THREE
IFTHENELSE(FALSE)(THREE)(FOUR) # --> FOUR
IFTHENELSE(ISZERO(ZERO))(THREE)(FOUR) # --> THREE
IFTHENELSE(ISZERO(ONE))(THREE)(FOUR)  # --> FOUR

Church Numerals

We can represent the natural numbers using Church Numerals.

$0 = ZERO  = (f) -> (x) -> x            # λf.λx.x
$1 = ONE   = (f) -> (x) -> f x          # λf.λx.f x
$2 = TWO   = (f) -> (x) -> f f x        # λf.λx.f (f x)
$3 = THREE = (f) -> (x) -> f f f x      # λf.λx.f (f (f x))
$4 = FOUR  = (f) -> (x) -> f f f f x    # λf.λx.f (f (f (f x)))
$5 = FIVE  = (f) -> (x) -> f f f f f x  # λf.λx.f (f (f (f (f x))))

We implement a function int that converts a Church Numeral into a literal javascript integer.

int   = (n) -> n((x) -> ++x)(0)

We can then define operations that can use our Church Numerals to do math.

SUCC = (n) -> (f) -> (x) -> f (n(f)(x)) # λn.λf.λx.f (n f x)

PLUS = (m) -> (n) -> (f) -> (x) -> m(f)(n(f)(x)) # λm.λn.λf.λx.m f (n f x)
PLUS = (m) -> (n) -> m(SUCC)(n) # λm.λn.m SUCC n

MULT = (m) -> (n) -> (f) -> m n f # λm.λn.λf.m (n f)

POW  = (b) -> (e) -> e b  # λb.λe.e b

# λn.λf.λx.n (λg.λh.h (g f)) (λu.x) (λu.u)
PRED = (n) -> (f) -> (x) ->
         n((g) -> (h) -> h g f)((u) -> x)((u) -> u)
SUB = (m) -> (n) -> n(PRED)(m) # λm.λn.n PRED m,

Above we have defined the successor SUCC, addition PLUS, multiplication MULT, power POW, predecessor PRED, and subtraction SUB operators. The additive operators work by repeating an operation m times. The subtraction operators are more complex.

These operators, when given Church Numerals perform as expected

PLUS(ONE)(TWO) # --> THREE

We can use our int operator to convert a Church Numeral into a Javascript integer.

console.log int PLUS(ONE)(TWO)  # --> 3
console.log int SUCC(ONE)       # --> 2
console.log int MULT(TWO)(FIVE) # --> 10
console.log int POW(TWO)(FIVE)  # --> 32
console.log int PRED(TWO)       # --> 1
console.log int SUB(THREE)(TWO) # --> 1
console.log int SUB(TWO)(THREE) # --> 0

Combinators

We can also define combinators, which are lambda expressions with no free variables. The simplest is the Identity combinator

I = (x) -> x # λx.x

A more useful combinator is the Y-Combinator. This allows us to use an anonymous function recursively.

Y = (g) -> ((x) -> g (x x))((x) -> g (x x)) # λg.(λx.g (x x)) (λx.g (x x))

For example, say we had the following FACTORIAL function

# λr.λn.(1, if n = 0; else n × (r (n−1)))
FACTORIAL = (r) -> (n) -> IFTHENELSE(ISZERO n)(ONE)(MULT(n)(r PRED(n)))

Since it is illegal to use our function name in the function body (since λ-calculus abstractions are anonymous), we need to use the Y-Combinator.

Y(FACTORIAL)(FOUR) # --> 24
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