Normalization property (abstract rewriting)

In mathematical logic and theoretical computer science, a rewrite system has the strong normalization property (in short: the normalization property) if every term is strongly normalizing; that is, if every sequence of rewrites eventually terminates to a term in normal form. A rewrite system may also have the weak normalization property, meaning that for every term, there exists at least one particular sequence of rewrites that eventually yields a normal form.

Lambda calculus

Untyped lambda calculus

The pure untyped lambda calculus does not satisfy the strong normalization property, and not even the weak normalization property. Consider the term λx.xxx. It has the following rewrite rule: For any term t,

$(\mathbf{\lambda} x . x x x) t \rightarrow t t t$

But consider what happens when we apply λx.xxx to itself:

$(\mathbf{\lambda} x . x x x) (\lambda x . x x x)$	$\rightarrow (\mathbf{\lambda} x . x x x) (\lambda x . x x x) (\lambda x . x x x)$
	$\rightarrow (\mathbf{\lambda} x . x x x) (\lambda x . x x x) (\lambda x . x x x) (\lambda x . x x x)$
	$\rightarrow (\mathbf{\lambda} x . x x x) (\lambda x . x x x) (\lambda x . x x x) (\lambda x . x x x) (\lambda x . x x x)$
	$\ldots\,$

Therefore the term (λx.xxx)(λx.xxx) is not strongly normalizing.

Typed lambda calculus

Various systems of typed lambda calculus including the simply typed lambda calculus, Jean-Yves Girard's System F, and Thierry Coquand's calculus of constructions are strongly normalizing.

A lambda calculus system with the normalization property can be viewed as a programming language with the property that every program terminates. Although this is a very useful property, it has a drawback: a programming language with the normalization property cannot be Turing complete. That means that there are computable functions that cannot be defined in the simply typed lambda calculus (and similarly there are computable functions that cannot be computed in the calculus of constructions or System F). As an example, it is impossible to define a self-interpreter in any of the calculi cited above.^[1]

See also

• Typed lambda calculus
• Rewriting
• Total functional programming
• Barendregt–Geuvers–Klop conjecture

References

1. ^ Conor McBride (May 2003), "on termination" (posted to the Haskell-Cafe mailing list).