$ cat input.txt
abcd%

$ cat main.morganey
succ := \n.f.x.f (n f x)
pair := \x.y.z. z x y
first := \n.n (\a.b.a)
second := \n.n (\a.b.b)
isnil := \l.l (\h.t.d. (\a.b.b)) (\a.b.a)
Y := \f . (\x. f (x x)) (\x . f (x x))

main := Y (\map.xs. (isnil xs) 0 (pair (succ (first xs)) (map (second xs))))

$ sbt "run exe main.morganey" < input.txt
string: "bcde"

$