lambda_calculus_demo.js

function array(lt) {
    let unit = _ => _;
    let no_use = unit;

    let no = _ => f => f(no_use); 
    let when = unit;

    let left = p => p(l => _ => l);
    let right = p => p(_ => r => r);

    let head = left;
    let tail = right;

    let is_nil = l => l(_ => _ => no);
    let isEmpty = is_nil;
        
    function natural(n) {
        return n(i => i + 1)(0);
    }

    function arr(acc, l) {
        return when(isEmpty(l))
                   (() => acc)
                   (() => arr(acc.concat([natural(head(l))]), tail(l)));
    }
    return arr([], lt);
}

let lt =  (f => (x => f(n => x(x)(n)))(x => f(n => x(x)(n))))(m => l => f => (_ => _)((l => l(_ => _ => (_ => f => f(_ => _))))(l))(_ => (_ => (f => _ => f(_ => _))))(_ => (h => t => (l => r => f => f(l)(r))(h)(t))(f((p => p(l => _ => l))(l)))(m((p => p(_ => r => r))(l))(f))))((e => (l => (f => (x => f(n => x(x)(n)))(x => f(n => x(x)(n))))(v => r => l => (_ => _)((l => l(_ => _ => (_ => f => f(_ => _))))(l))(_ => r)(_ => v((h => t => (l => r => f => f(l)(r))(h)(t))((p => p(l => _ => l))(l))(r))((p => p(_ => r => r))(l))))(_ => (f => _ => f(_ => _)))(l))(e(_ => _ => (f => _ => f(_ => _)))))((f => (x => f(n => x(x)(n)))(x => f(n => x(x)(n))))(r => t => h => (_ => _)((n => n(_ => (_ => f => f(_ => _)))(_ => f => f(_ => _)))(h))(_ => t)(_ => r((h => t => (l => r => f => f(l)(r))(h)(t))(h)(t))))(_ => (f => _ => f(_ => _)))(f => x => f(x))(f => x => f(f(x)))(f => x => f(f(f(x))))))(e => (m => n => n(n => (p => p(l => _ => l))(n(p => (l => r => f => f(l)(r))((p => p(_ => r => r))(p))((n => f => x => f(n(f)(x)))((p => p(_ => r => r))(p))))((l => r => f => f(l)(r))(_ => _)(_ => x => x))))(m))(e)(f => x => f(x)));

console.log(array(lt)); // [0, 1, 2]