Is every method returning `this` a monad?_问答_开发者

Is every method on a class which returns this a mon开发者_JS百科ad?

I'm going to say a very cautious "possibly". A lot of this is contingent on your definitions.

It's worth noting that I'm taking the definition of monad from the category theory construct, not the functional programming construct.

If you think of a method A of class C that maps a C instance to another C instance (i.e. it returns this), then this would appear that C.A() is a functor from the category consisting of C instantiations to itself. Therefore it's an endofunctor, at least. It would appear that this construction obeys the basic identity and associativity properties that we expect, but further inspection would be required to say for sure.

Anyway, I wouldn't stake my life on it, and I'm not certain this is a very helpful way about thinking of such constructions, but it does seem a reasonable assumption on first inspection, at least.

I have limited understanding of monads. I can't tell if that meets the formal definition of a monad (I don't think so, but I don't know for sure), but return this; alone doesn't allow any of the cool things monads allow (fluid interfaces are nice, but not monads imho and nowhere as useful as even simple monads like the option type monad).

This snippet from wikipedia seems to say "no":

Formally, a monad is constructed by defining two operations (bind and return) and a type constructor M [... further restrictions we don't need here]

Edit: Moreover, a monad is a type and not an operation (e.g. method) - the question should rather read "Is a class a monad if all of its methods return this?"</nitpick >

Probably not, at least not in any of the usual ways.

Monads in programming are typically defined over a category of types with functions as arrows. In that case, a method returning this is an arrow from the class to itself--this is an endomorphism with the usual monoid of function composition, but is not a functor.

Note that functors involving function types are certainly possible, but a functor F(A) => (A -> A) doesn't really work because the type appears in both covariant and contravariant position, that is, given a function A -> B you can send A -> A to A -> B, or you can send B -> B to A -> B, but you can't get a B -> B from A -> A or vice versa.

However, there is one way to view instances as having monadic structure. Consider that instance methods effectively have this as an implicit argument. So for some class C, its methods are functions from C to whatever other type. This corresponds roughly to the covariant function functor above. Note that I'm not describing any particular class here, but the entire concept of classes and instances! So, for this mapping from C to instance methods of C:

If we have an instance method returning some type A and a function with type A -> B, we can trivially define a method returning something of type B: that's the rest of the functor definition, a.k.a. 'fmap` in Haskell.
If we have some value of type A, we can add a trivial instance method that just returns that value: that's the monad's "unit" operation, a.k.a. return in Haskell.
If we have an instance method returning a value of type A, and another instance method taking an argument of type A and returning a value of type B, we can define a method that simply returns a value of type B by combining them. That's the monadic bind, a.k.a. (>>=) in Haskell.

Haskell calls the monad of "functions that all take a first argument of some fixed type" the Reader Monad, and the do notation for it lets you write code where that first argument is implicitly available--rather like the way that this is implicitly available inside instance methods.

The difference here is that with class instances, the monadic structure is... sort of at the level of the syntax, not something you can use directly in a program, at least not in most languages.

In my opinion, No.

There are at least two issues I see with it.

A monad is often a glue between two functions. In this case methodA returns a type on which the next methodB is invoked, (and of course methodA and methodB both belonging to the same type).
A monad is supposed to allow type transformations. So if functionA returns TypeX and functionB expects TypeY, the monad needs to provide a bind operation which can convert a Monad(TypeX) into a Monad(TypeY). The monad then goes on to take the return value of the first function, wrap it as a Monad(TypeX), transform it to Monad(TypeY) from which TypeY would get extracted and fed into functionB.

A method which returns this is actually an implementation of Fluent Interface. And while many have argued it to be a monadic as well, I would only say that while it helps resolve problems similar to what monads could otherwise solve, and while the solution would seem similar to how a monadic solution might work (instead of the "." operator, the bind method of the monad has to be invoked without any explicit do block), it is not a monad. In other words it may walk like a monad and talk like a monad, but it is not a monad.

Slight Correction to point 2: The monad needs to provide mechanisms to a) convert TypeX into Monad(TypeX), transform from Monad(TypeX) to Monad(TypeY) and a coercion from Monad(TypeY) to TypeY