We are currently working on new rules for what content should and shouldn't be allowed on this website, and are looking for feedback! See Esolang:2026 topicality proposal to view and give feedback on the current draft.
Functional calculus
In the context of computer science, a functional calculus is an algebraic system (a calculus) where functions are considered values.
In a functional calculus there are functions, which may be definable, or provided as primitives. Functions may be passed to other functions, returned from functions, and so on.
Being algebraic systems, functional calculi are expressible as rewriting systems, generally tree rewriting since algebraic expressions are often structured as trees.
The theory of functional calculus serves as the basis for functional programming.