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:
* [[!wikipedia Lambda calculus]]
* [[!wikipedia Church encoding]]
* Oleg's [Basic Lambda Calculus Terms](http://okmij.org/ftp/Computation/lambda-calc.html#basic)
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
; pairs
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
; triples
let make_triple = \x y z f. f x y z in
; Church numerals: basic operations
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
; version 3 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) junk 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 reverse builds a left-fold instead
; (make_left_list a (make_left_list b (make_left_list c empty)) ~~> \f z. f c (f b (f a z))
let reverse = (\make_left_list lst. lst make_left_list empty) (\h t f z. t f (f h z)) 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
; 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
; more math with Church numerals
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
; instead:
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
; instead:
; 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.
(\dhead. m (mtail dhead) (\sel. dhead (sel 0 0)))
(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
; instead:
; 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
; Rosenbloom's fixed point combinator
let Y = \f. (\h. f (h h)) (\h. f (h h)) in
; Turing's fixed point combinator
let Theta = (\u f. f (u u f)) (\u f. f (u u f)) in
; length for version 1 lists
let length = Y (\self lst. isempty lst 0 (succ (self (tail lst)))) in
true