In computer science, a function type (also arrow type or exponential) is the type of a variable or parameter to which a function has or can be assigned or the result type of a higher-order function returning a function.
A function type depends on the type of the parameters and the result type of the function (it, or more accurately the unapplied type constructor · → ·, is a higher-kinded type). In theoretical settings and languages where functions are defined in curried form, such as the simply typed lambda calculus, a function type depends on exactly two types, the domain A and the range B. Here a function type is often denoted A → B, following mathematical convention, or BA, based on the fact that there exist exactly BA (exponentially many) set-theoretic functions mapping A to B.
Read more about Function Type: Programming Languages, Denotational Semantics
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