In a type theory with function types, given a type$X$, a functional of base type $X$ is a term of type $X^{X^X}$ aka $(X \to X) \to X$. This should be distinguished from (on the one hand) an operator, which is a term of type $(X^X)^{X^X}$ aka $(X \to X) \to (X \to X)$, and (on the other hand) a function, which (in this context) is a term of type $X^X$ aka $X \to X$.

More generally, any term whose type has the form $A^{B^C}$ aka $(C \to B) \to A$ may be called a functional, although usually not if any of these types is very trivial (since any type has this form, up to equivalence, if $B,C \coloneqq 1$).

Although one typically interprets type theory within set theory so that operations between types become functions, one may also use partial functions, which is necessary for many of the examples below.

If we interpret $X$ as the real line, then $X^X$ consists of real-valued maps of a real variable, which form a vector space. The linear maps from $X^X$ to $X$ are the original linear functionals. In functional analysis, we now replace $X^X$ with an arbitrary topological vector space$V$ (originally but no longer necessarily taken to be a subspace? of $X^X$) and consider linear maps from $V$ to $X$ instead; so these linear functionals are actually unstructured operations in a type-theoretic sense.

Last revised on January 10, 2020 at 10:57:06.
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