Introduction to the lambda calculus

L18: Reduction Strategies
L18: Reduction Strategies

Introduction to the Lambda Calculus
Alonzo Church
1932, “A set of postulates for the foundation of logic”, a formal system
with the aim of providing a foundation for logic which would be more
natural than Russell’s type theory or Zermelo’s set theory, and would
not contain free variables
1936 Church isolated and published just the portion relevant to
computation, what is now called the untyped lambda calculus
1940, he also introduced a computationally weaker, but logically
consistent system, known as the simply typed lambda calculus
Re-discovered as a versatile tool in Computer Science by people like
McCarthy, et al. in the 1960s

Operational Semantics — Syntax
t ::=
x /* variable */
λx.t /* abstraction */
t t /* application */
v ::=
λx.t /* abstraction value */

true = λt. λf. t
false = λt. λf. f
not = λa. a false true

Applying not
(λa. a false true) true
(λa. a false true) (λt. λf. t)
(λt. λf. t) false true
false
(λx.t) v [x ↦ v]t
(from operational semantic rules)

β-reduction
applying functions to their arguments
Normal order reduction – most well-known languages work
from the leftmost, outermost, redex and work in, halting when
the outermost argument can no longer be applied
Redex – reducible expression
Full β-reduction – reduce available redexes in any desired order

Numbers
c0 = λs. λz. z
c1 = λs. λz. s z
c2 = λs. λz. s (s z)
c3 = λs. λz. s (s (s z))

Just like Peano arithmetic

Operations on numbers
iszro = λm. m (λx. fls) tru
plus = λm. λn. λs. λz. m s (n s z)
mult = λm. λn. m (plus n) c0
exp = λm. λn. n (mult m) c1
scc = λn. λs. λz. s (n s z) /* number successor */
/* number predecessor */
prd = λm. fst (m (λp. pair (snd p) (plus c1 (snd p))) (pair c0 c0))
eql = λm. λn. and (iszro (m prd n)) (iszro (n prd m))

Automating assertions
/* it is true that … */
xnor tru (eql c1 (exp c1 c1))
xnor tru (eql c6 (plus c2 c4))
xnor tru (eql c1 (fst (pair c1 tru)))

Recursion
fix (a.k.a the Y-combinator)
= λf. (λx. f (λy. x x y)) (λx. f (λy. x x y))
takes recursive abstraction
λf. λx. __f__

Recursion
(λf. (λx. f (λy. x x y)) (λx. f (λy. x x y))) (λf. λx. __) (bar)
(λx. (λf. λx’. __) (λy. x x y)) (λx. (λf. λx’. __) (λy. x x y)) (bar)
(foo)
(λf. λx’. __) (λy. foo foo y) (bar)
λx’. __ (bar)

Recursion
fact = λf. λx. cond (iszro x) c1 (mult x (f (prd x)))
(λf. (λx. f (λy. x x y)) (λx. f (λy. x x y))) fact c3
/* successive interations */
(λy. (λx. f (λy’.x x y’)) (λx. f (λy’.x x y’)) y) (/* prev recurs*/)

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