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Thanks! > From the OOP perspective, though, monad is a pattern where you only have one behavior (bind) that you can customize, and you have to do everything through that one behavior. That looks like a straightjacket. Sure, it composes well, but... why limit yourself like that? That's an excellent question. Thinking of monad as a pattern itself, with bind as its API, made me realize that OOP patterns (as a categorical concept) aren't the best analogy for the different behaviors monads encode, but rather monad is just one of the patterns. Makes sense -- there are obviously behaviors not generalized by monads but are in the realm of OOP patterns. For instance, the visitor pattern is covered by functors. (At the time of writing I was thinking about how bind can relate seemingly disparate behaviors: error handling, async, state, etc. What other useful structures seem disparate? OOP patterns! And so I fell into the black pits of "Monads are just like <thing>" myself, where thing = OOP patterns.) As for the question: One could call it a straightjacket but one could also call it "the law". Limiting yourself is exactly how Straightjacketed Sam can ensure correctness properties of the program. Monads force Sam to work within the monad, so they can only write "a depend on b" in a specific and well-defined way. This is analogous to how singleton pattern would force Sam to work with this one instance of an object, or how the factory/smart constructor pattern forces ensures only certain (valid) objects can exist. If any of these restrictions are too limiting, there's no need to use the patterns, but you'll also lose the laws (guarantees)! |
And, as you point out, there are some OO patterns that we can also provide useful guarantees and constraints.