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That's one of the things monads do, but they can do more. A monad is defined by an underlying type (roughly, some kind of "container"), and two functions (putti

That's one of the things monads do, but they can do more. A monad is defined by an underlying type (roughly, some kind of "container"), and two functions (putting an object in the container, and creating a new container by applying a function to the contained object). That's all a monad actually is. It simply turns out that given those relatively simply building blocks, you can do all sorts of neato things with them, like encapsulating state, error handling, chaining actions together, etc. An example of a monad which has really nothing necessarily to do with mutable state are Promises in JavaScript. The container, of course, is a Promise, and the two functions I described are `resolve` and `then`.

sago 11y ago
Right, and some of that makes sense to me. It makes sense in some cases (Maybe was the example I gave, a Promised value another) to tie such things directly to a single typed value. But what I was querying was, is this implicitness a good move in general. In particular the issue I was addressing was the encapsulation of an arbitrary state with a single typed value. It's been remarked elsewhere that 'mixing' monads is a problem: I guess that's always been the thing I've felt haven't made them work replicating as a coding pattern. I tend to prefer explicit over implicit state.
- thinkpad20 11y ago
  > I tend to prefer explicit over implicit state. I guess it depends on your definitions, but monadic state is just as explicit as any other. myMonadicFunction :: String -> State Int String myMonadicFunction string = do currentVal <- get put (currentVal + length string) return (reverse string) It's just as clear as what you'd have in an imperative programming language. I'm convinced the main reason monads aren't common in other languages is that most languages just don't have strong enough type systems to support them explicitly. Implicitly they can be found here or there, such as with promises, or just the fact that the semantics of many imperative languages can be viewed as a monad. But of course, probably the biggest reason is that people who are used to mutable state don't see the need. It's one of those things whose value becomes apparent with use.
  - sago 11y ago
    Thanks, I really appreciate your responses. What's to stop you passing a struct or object with the two monadic functions attached, or ignore the function requirement (for the case of state) and pass the struct with the state and result in it? The benefit being that you can package up more than just one value in that (immutable) structure along with the 'specialness'. I don't see why that is less typesafe.
    - thinkpad20 11y ago
      I'm having a tough time trying to understand what you're proposing. First of all, in Haskell there's no real concept of having functions "attached" to objects (in the OO sense). The closest analogue is a type class, which relates a type to a set of functions. For monad, that looks like class Monad m where return :: a -> m a (>>=) :: m a -> (a -> m b) -> m b And it's up to the particular instance to determine how those functions are implemented. For example, instance Monad Maybe where return :: a -> Maybe a return x = Just x (>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b maybe >>= f = case maybe of Nothing -> Nothing Just x -> f x The definition of the State type, and its corresponding Monad instance, is a little more involved (I suggest this blog post for an explanation [1]): newtype State s a = State { runState :: s -> (a, s) } instance Monad (State a) where return x = State $ \s -> (x, s) m >>= k = State $ \s -> let (a, s') = runState m s in runState (k a) s' I'm honestly not quite sure what you mean by the rest of it (for example, "ignoring the function requirement", "specialness" etc). But the bottom line is that you can do any typesafe thing you want to, within the rules of how the language works. [1]: http://brandon.si/code/the-state-monad-a-tutorial-for-the-confused/ http://brandon.si/code/the-state-monad-a-tutorial-for-the-co...
      - sago 11y ago
        Thanks again. Sorry, I wasn't talking about Haskell. Following the OP I was talking about the merits of using these concepts or structures in other languages (C++, as per the OP, in particular). Of course, in a language with strong support and syntax for a particular pattern, you should rely on that pattern. But beyond that, it can be useful to use a pattern in other languages. Carmack is talking about writing C++ in a more functional style. I've written OO plain C, for example, when that pattern has made sense, and much of my C++ is heavily influenced by my experience with Scheme. My point was that I don't get the emphasis on Monads, as a concept, for the range of things it gets emphasised for. In a language which has core syntactic and semantic support, and there are few alternatives, then of course it has to be used widely. But that's a different argument to saying it is an intrinsically good solution for those cases. I might be missing something, but as an architectural pattern in general it doesn't seem appealing. Not enough to reimplement in another language. That's the context to my comment on typesafety. You said that other languages aren't typesafe enough. I was suggesting that, implementing a monad as a structure or an object would be typesafe, so I couldn't see what was insufficient.

sago 11y ago

Right, and some of that makes sense to me. It makes sense in some cases (Maybe was the example I gave, a Promised value another) to tie such things directly to a single typed value. But what I was querying was, is this implicitness a good move in general. In particular the issue I was addressing was the encapsulation of an arbitrary state with a single typed value. It's been remarked elsewhere that 'mixing' monads is a problem: I guess that's always been the thing I've felt haven't made them work replicating as a coding pattern. I tend to prefer explicit over implicit state.

thinkpad20 11y ago
> I tend to prefer explicit over implicit state. I guess it depends on your definitions, but monadic state is just as explicit as any other. myMonadicFunction :: String -> State Int String myMonadicFunction string = do currentVal <- get put (currentVal + length string) return (reverse string) It's just as clear as what you'd have in an imperative programming language. I'm convinced the main reason monads aren't common in other languages is that most languages just don't have strong enough type systems to support them explicitly. Implicitly they can be found here or there, such as with promises, or just the fact that the semantics of many imperative languages can be viewed as a monad. But of course, probably the biggest reason is that people who are used to mutable state don't see the need. It's one of those things whose value becomes apparent with use.
- sago 11y ago
  Thanks, I really appreciate your responses. What's to stop you passing a struct or object with the two monadic functions attached, or ignore the function requirement (for the case of state) and pass the struct with the state and result in it? The benefit being that you can package up more than just one value in that (immutable) structure along with the 'specialness'. I don't see why that is less typesafe.
  - thinkpad20 11y ago
    I'm having a tough time trying to understand what you're proposing. First of all, in Haskell there's no real concept of having functions "attached" to objects (in the OO sense). The closest analogue is a type class, which relates a type to a set of functions. For monad, that looks like class Monad m where return :: a -> m a (>>=) :: m a -> (a -> m b) -> m b And it's up to the particular instance to determine how those functions are implemented. For example, instance Monad Maybe where return :: a -> Maybe a return x = Just x (>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b maybe >>= f = case maybe of Nothing -> Nothing Just x -> f x The definition of the State type, and its corresponding Monad instance, is a little more involved (I suggest this blog post for an explanation [1]): newtype State s a = State { runState :: s -> (a, s) } instance Monad (State a) where return x = State $ \s -> (x, s) m >>= k = State $ \s -> let (a, s') = runState m s in runState (k a) s' I'm honestly not quite sure what you mean by the rest of it (for example, "ignoring the function requirement", "specialness" etc). But the bottom line is that you can do any typesafe thing you want to, within the rules of how the language works. [1]: http://brandon.si/code/the-state-monad-a-tutorial-for-the-confused/ http://brandon.si/code/the-state-monad-a-tutorial-for-the-co...
    - sago 11y ago
      Thanks again. Sorry, I wasn't talking about Haskell. Following the OP I was talking about the merits of using these concepts or structures in other languages (C++, as per the OP, in particular). Of course, in a language with strong support and syntax for a particular pattern, you should rely on that pattern. But beyond that, it can be useful to use a pattern in other languages. Carmack is talking about writing C++ in a more functional style. I've written OO plain C, for example, when that pattern has made sense, and much of my C++ is heavily influenced by my experience with Scheme. My point was that I don't get the emphasis on Monads, as a concept, for the range of things it gets emphasised for. In a language which has core syntactic and semantic support, and there are few alternatives, then of course it has to be used widely. But that's a different argument to saying it is an intrinsically good solution for those cases. I might be missing something, but as an architectural pattern in general it doesn't seem appealing. Not enough to reimplement in another language. That's the context to my comment on typesafety. You said that other languages aren't typesafe enough. I was suggesting that, implementing a monad as a structure or an object would be typesafe, so I couldn't see what was insufficient.