What is the difference between a Functor and the Command pattern?

Question

I am very familiar with the Command pattern, but I don't yet understand the difference in theory between a Functor and a command. In particular, I am thinking of Java implementations. Both are basically programming "verbs" represented as objects. However, in the case of functors, as I have seen from some examples anonymous inner class implementations seem common. Can anyone out there clear this up for me nicely?

Answer 1

From the description of the Apache Commons Functor page:

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A functor is a function that can be manipulated as an object, or an object representing a single, generic function.

Functors support and encourage a number of powerful programming techniques including:

• programming in a functional style
• higher order functions
• internal iterators
• reuse and specialization through composition rather than inheritance and overloading
• generic "callback" or "extension point" APIs
• generic "filters" or predicate APIs
• many "behavioral" design patterns, such as Visitor, Strategy, Chain of Responsibility, etc.

Answer 2

A functor is an implementation, a way of making an object behave like a function.

The 'Command Pattern' is a design pattern.
The functor is one way to implement the 'Command Pattern'.

Answer 3

I think of a functor as being a component of the command pattern, which also involves other infrastructure such as the invoker and command recipients.

Answer 4

A functor is a 'syntax level' concept - it packages up code in an object that can be treated syntactically like a function pointer - i.e. it can be 'called' by putting parameter list in brackets after it. In C++ you could make a class a functor by overriding operator().

A Command in the command pattern is an object that packages up some runnable functionality, but there's no requirement for it to be a functor. For example, it could be a class that implements an interface ICommand, allowing its command to be run by calling Do().

Answer 5

The Command pattern is used in Java because we don't have closures in Java. Functors is an attempt to implement closures.

A language with closures doesn't need the Command pattern.

Answer 6

Here comes the answer from the GOF:

Coplien describes how to implement functors, objects that are functions, in C++ [Cop92]. He achieves a degree of transparency in their use by overloading the function call operator (operator()). The Command pattern is different; its focus is on maintaining a binding between a receiver and a function (i.e., action), not just maintaining a function.