Church Encoded List in JS

Charch encodingで書いたリストの動くものがほしかったので書き下した。isnilを実現するためにコンスをisnil、car、cdrからなるペアとして表現した。

// cons list in Church encoding

const tr = t => f => t;
const fl = t => f => f;
const ite = c => t => f => c(t)(f);

const pair = f => s => pi => pi(f)(s);
const fst = p => p(f=>s=>f);
const snd = p => p(f=>s=>s);

const nil = pair(tr)(0);
const cons = x => xs => pair(fl)(pair(x)(xs));
const car = l => fst(snd(l));
const cdr = l => snd(snd(l));
const isnil = l => fst(l);

const fix = f => (x => f(y => x(x)(y)))((x => f(y => x(x)(y))));

const foldr = fix(folD => f => init => l => (ite (isnil(l)) (() => init) (() => f(car(l))(folD(f)(b)(cdr(l)))) ) ());

const len = l => foldr(elm => b => 1+b)(0)(l);
const len2 = fix(leN => l => (ite(isnil(l))( () => 0)( () => 1 + leN(cdr(l)) ))() );

const lst = cons(1)(cons(2)(nil));
len(lst); // -> 2