Benefits of first-class functions
Before we talk about “first-class functions,” let’s first define what it means for anything to be called “first-class” in programming language design. When we talk about a “first-class citizen,” we mean an entity (object, primitive, or function) for which all the common language operations are available. These are operations such as assignment, passing it to a function, returning from a function, or storing it in another data type such as a map.
Looking at this list, we can see how all of those operations typically apply to the structs that we are defining in our language. Objects and primitives can be passed around between functions. They are often returned as the results of a function and we definitely assign them to variables. When we say that functions are first-class citizens, you can simply think of this as treating functions like objects. Their equivalence will help us create all future constructs...