A concept that exists in functional and semi-functional languages including [Scheme], [Javascript], [Haskell], [ML] ... and is introduced partially to Tcl with [lambdas] for [apply].

The principle is that functions are "first class citizens", like numbers, strings or any other value.  Thus, they can be:

   * stored in variables
   * passed to functions
   * returned from functions

like any other value.  Note that functions are usually [opaque objects], so [introspection] is not available.

First-class functions permit coding styles that are not available in languages without them ([C] is a good example):

   * [higher order functions], like [lmap] and other [fold]s
   * [static variables] become natural
   * [curry] becomes easy when functions are first class
   * [closures] become important with first-class functions

<<categories>>Category Concept | Category Functional Programming