Here are a bunch of pre-tested operations for the untyped lambda calculus. In some cases multiple versions are offered.

Some of these are drawn from:

and all sorts of other places. Others of them are our own handiwork.

Spoilers! Below you'll find implementations of map and filter for v3 lists, and several implementations of leq for Church numerals. Those were all requested in Assignment 2; so if you haven't done that yet, you should try to figure them out on your own. (You can find implementations of these all over the internet, if you look for them, so these are no great secret. In fact, we'll be delighted if you're interested enough in the problem to try to think through alternative implementations.)

;; booleans
let true = \y n. y  in ; aka K
let false = \y n. n  in ; aka K I
let and = \p q. p q false  in ; or
let and = \p q. p q p  in ; aka S C I
let or = \p q. p true q  in ; or
let or = \p q. p p q  in ; aka M
let not = \p. p false true  in ; or
let not = \p y n. p n y  in ; aka C
let xor = \p q. p (not q) q  in
let iff = \p q. not (xor p q)  in ; or
let iff = \p q. p q (not q)  in

;; tuples
let make_pair = \x y f. f x y  in
let get_fst = \x y. x  in ; aka true
let get_snd = \x y. y  in ; aka false
let make_triple = \x y z f. f x y z  in

;; Church numerals
let zero = \s z. z  in ; aka false
let one = \s z. s z  in ; aka I
let succ = \n s z. s (n s z)  in
; for any Church numeral n > zero : n (K y) z ~~> y
let iszero = \n. n (\x. false) true  in

let add = \m n. m succ n  in ; or
let add = \m n s z. m s (n s z)  in
let mul = \m n. m (\z. add n z) zero  in ; or
let mul = \m n s. m (n s)  in
let pow = \b exp. exp (mul b) one  in ; or
; b succ : adds b
; b (b succ) ; adds b b times, ie adds b^2
; b (b (b succ)) ; adds b^2 b times, ie adds b^3
; exp b succ ; adds b^exp
let pow = \b exp s z. exp b s z  in

; three strategies for predecessor
let pred_zero = zero  in
let pred = (\shift n. n shift (make_pair zero pred_zero) get_snd)
; where shift is
(\p. p (\x y. make_pair (succ x) x))  in ; or
; from Oleg; observe that for any Church numeral n: n I ~~> I
let pred = \n. iszero n zero
; else
(n (\x. x I ; when x is the base term, this will be K zero
; when x is a Church numeral, it will be I
(succ x))
; base term
(K (K zero))
)  in
; from Bunder/Urbanek
let pred = \n s z. n (\u v. v (u s)) (K z) I  in ; or

; inefficient but simple comparisons
let leq = \m n. iszero (n pred m)  in
let lt = \m n. not (leq n m)  in
let eq = \m n. and (leq m n) (leq n m)  in ; or

; more efficient comparisons, Oleg's gt provided some simplifications
let leq = (\base build consume. \m n. n consume (m build base) get_fst)
; where base is
(make_pair true junk)
; and build is
(\p. make_pair false p)
; and consume is
(\p. p get_fst p (p get_snd))  in
let lt = \m n. not (leq n m)  in
let eq = (\base build consume. \m n. n consume (m build base) get_fst)
; 2nd element of a pair will now be of the form (K sthg) or I
; we supply the pair being consumed itself as an argument
; getting back either sthg or the pair we just consumed
; base is
(make_pair true (K (make_pair false I)))
; and build is
(\p. make_pair false (K p))
; and consume is
(\p. p get_snd p)  in

; -n is a fixedpoint of \x. add (add n x) x
; but unfortunately Y that_function doesn't normalize
let sub = \m n. n pred m  in ; or
; how many times we can succ n until m <= result
let sub = \m n. (\base build. m build base (\cur fin sofar. sofar))
; where base is
(make_triple n false zero)
; and build is
(\t. t (\cur fin sofar. or fin (leq m cur)
(make_triple cur true sofar) ; enough
(make_triple (succ cur) false (succ sofar)) ; continue
))  in
; or
let sub = (\base build consume. \m n. n consume (m build base) get_fst)
; where base is
(make_pair zero I) ; see second defn of eq for explanation of 2nd element
; and build is
(\p. p (\x y. make_pair (succ x) (K p)))
; and consume is
(\p. p get_snd p)  in

let min = \m n. sub m (sub m n)  in
let max = \m n. add n (sub m n)  in

; (m/n) is a fixedpoint of \x. add (sub (mul n x) m) x
; but unfortunately Y that_function doesn't normalize
; how many times we can sub n from m while n <= result
let div = \m n. (\base build. m build base (\cur go sofar. sofar))
; where base is
(make_triple m true zero)
; and build is
(\t. t (\cur go sofar. and go (leq n cur)
(make_triple (sub cur n) true (succ sofar)) ; continue
(make_triple cur false sofar) ; enough
))  in
; what's left after sub n from m while n <= result
let mod = \m n. (\base build. m build base (\cur go. cur))
; where base is
(make_pair m true)
; and build is
(\p. p (\cur go. and go (leq n cur)
(make_pair (sub cur n) true) ; continue
(make_pair cur false) ; enough
))  in

; or
let divmod = (\base build mtail. \m n.
(n build base (\x y z. z junk))
(\t u x y z. make_pair t u) )
; where base is
(make_triple succ (K 0) I) ; see second defn of eq for explanation of 3rd element
; and build is
(\t. make_triple I succ (K t))
; and mtail is
(\dhead d. d (\dz mz df mf drest sel. drest dhead (sel (df dz) (mf mz))))  in
let div = \n d. divmod n d get_fst  in
let mod = \n d. divmod n d get_snd  in

; sqrt n is a fixedpoint of \x. div (div (add n (mul x x)) 2) x
; but unfortunately Y that_function doesn't normalize

; (log base b of m) is a fixedpoint of \x. add (sub (pow b x) m) x
; but unfortunately Y that_function doesn't normalize
; how many times we can mul b by b while result <= m
let log = \m b. (\base build. m build base (\cur go sofar. sofar))
; where base is
(make_triple b true 0)
; and build is
(\t. t (\cur go sofar. and go (leq cur m)
(make_triple (mul cur b) true (succ sofar)) ; continue
(make_triple cur false sofar) ; enough
))  in

;; fixed point combinators
; Curry/Rosenbloom's
let Y = \f. (\h. f (h h)) (\h. f (h h))  in
; Turing's
let Theta = (\u f. f (u u f)) (\u f. f (u u f))  in

; now you can search for primes, do encryption :-)
let gcd = Y (\gcd m n. iszero n m (gcd n (mod m n)))  in ; or
let gcd = \m n. iszero m n (Y (\gcd m n. iszero n m (lt n m (gcd (sub m n) n) (gcd m (sub n m)))) m n)  in
let lcm = \m n. or (iszero m) (iszero n) 0 (mul (div m (gcd m n)) n)  in

;; version 1 lists
let empty = make_pair true junk  in
let make_list = \h t. make_pair false (make_pair h t)  in
let isempty = \lst. lst get_fst  in
let head = \lst. isempty lst err (lst get_snd get_fst)  in
let tail_empty = empty  in
let tail = \lst. isempty lst tail_empty (lst get_snd get_snd)  in

let length = Y (\length lst. isempty lst 0 (succ (length (tail lst))))  in
let fold = Y (\fold lst f z. isempty lst z (f (head lst) (fold (tail lst) f z)))  in
let map = \f. Y (\map lst. isempty lst empty (make_list (f (head lst)) (map (tail lst))))  in
let filter = \f. Y (\filter lst. isempty lst empty (f (head lst) (make_list (head lst)) I (filter (tail lst))))  in

;; version 3 (right-fold) lists
let empty = \f z. z  in
let make_list = \h t f z. f h (t f z)  in
let isempty = \lst. lst (\h sofar. false) true  in
let head = \lst. lst (\h sofar. h) err  in
let tail_empty = empty  in
let tail = \lst. (\shift. lst shift (make_pair empty tail_empty) get_snd)
; where shift is
(\h p. p (\t y. make_pair (make_list h t) t))  in
let length = \lst. lst (\h sofar. succ sofar) 0  in
let map = \f lst. lst (\h sofar. make_list (f h) sofar) empty  in
let filter = \f lst. lst (\h sofar. f h (make_list h sofar) sofar) empty  in ; or
let filter = \f lst. lst (\h. f h (make_list h) I) empty  in
let singleton = \x f z. f x z  in
; append [a;b;c] [x;y;z] ~~> [a;b;c;x;y;z]
let append = \left right. left make_list right  in
; very inefficient but correct reverse
let reverse = \lst. lst (\h sofar. append sofar (singleton h)) empty  in ; or
; more efficient revappend, reverse
; revappend [a;b;c] [x;y] ~~> [c;b;a;x;y]
; make_left_list a (make_left_list b (make_left_list c empty)) ~~> \f z. f c (f b (f a z))
let revappend = (\make_left_lst left right. left make_left_list right) (\h t f z. t f (f h z))  in
; from Oleg, of course it's the most elegant
let revappend = \left. left (\hd sofar. \right. sofar (make_list hd right)) I  in
let rev = \lst. revappend lst empty  in
; zip [a;b;c] [x;y;z] ~~> [(a,x);(b,y);(c,z)]
let zip = \left right. (\base build. reverse left build base (\x y. reverse x))
; where base is
(make_pair empty (map (\h u. u h) right))
; and build is
(\h sofar. sofar (\x y. isempty y
sofar
(make_pair (make_list (\u. head y (u h)) x)  (tail y))
))  in
let all = \f lst. lst (\h sofar. and sofar (f h)) true  in
let any = \f lst. lst (\h sofar. or sofar (f h)) false  in

;; left-fold lists
let make_list = \h t f z. t f (f h z)  in
let head = \lst. lst (\h sofar. (K (sofar (K h))) ) (\k. k err) I  in
let tail = \lst. (\shift. lst shift (\a b. a tail_empty) I I)
(\h p. p (\j a b. b empty) (\t a b. b (\f z. f h (t f z))) )  in

;; version 5 (CPS right-fold) lists
; [] is \f z c a. c z
;  is \f z c a. f 1 z c a
; [1;2] is \f z c a. f 2 z (\z. f 1 z c a) a
; [1;2;3] is \f z c a. f 3 z (\z. f 2 z (\z. f 1 z c a) a) a
let empty = \f2 z continue_handler abort_handler. continue_handler z  in
let isempty = \lst larger_computation. lst
; here's our f2
(\hd sofar continue_handler abort_handler. abort_handler false)
; here's our z
true
; here's the continue_handler for the leftmost application of f2
larger_computation
; here's the abort_handler
larger_computation  in
let make_list = \h t. \f2 z continue_handler abort_handler.
t f2 z (\sofar. f2 h sofar continue_handler abort_handler) abort_handler  in
let head = \lst larger_computation. lst
; here's our f2
(\hd sofar continue_handler abort_handler. continue_handler hd)
; here's our z
err
; here are our continue_handler and abort_handler
larger_computation unused  in
let tail_empty = empty  in
let tail = \lst larger_computation. lst
; here's our f2
(\h sofar continue_handler abort_handler. continue_handler (sofar (\t y. make_pair (make_list h t) t)))
; here's our z
(make_pair empty tail_empty)
; here are our continue_handler and abort_handler
(\sofar. sofar (\x y. larger_computation y)) unused  in

;; CPS left-fold lists
; [] is \f z c a. c z
;  is \f z c a. f 1 z  (\z. c z) a
; [1;2] is \f z c a. f 1 z (\z. f 2 z (\z. c z) a) a
; [1;2;3] is \f z c a. f 1 z (\z. f 2 z (\z. f 3 z (\z. c z) a) a) a
let make_right_list = make_list  in
let make_list = \h t. \f2 z continue_handler abort_handler.
f2 h z (\z. t f2 z continue_handler abort_handler) abort_handler  in
let head = \lst larger_computation. lst
; here's our f2
(\hd sofar continue_handler abort_handler. abort_handler hd)
; here's our z
err
; here are our continue_handler and abort_handler
larger_computation larger_computation  in
let tail = \lst larger_computation. lst
; here's our f2
(\h sofar continue_handler abort_handler. continue_handler (sofar (\j a b. b empty) (\t a b. b (make_right_list h t)) ) )
; here's our z
(\a b. a tail_empty)
; here are our continue_handler and abort_handler
(\sofar. sofar larger_computation larger_computation) unused  in

true