Lambda Calculus

One of the simplest programming languages is lambda calculus.  All components are atoms, lambdas or lists.  Lists can contain other lists, and, can only be definitions or invocations.  Definitions are lists that define one parameter functions.  They contain a lambda, an atom and another element.  Invocations are lists that invoke one parameter functions.  They contain two elements.

Evaluating atoms returns them.  Evaluating lists involves evaluating elements.  Evaluating invocations may also involve various substitutions.