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As I understand it, monads help solve the problem of sequential computation in Haskell but the concept is not limited to that. For example, how would you consid

• solomonb 4y ago
  The `Monad` instance for `Maybe` and `Either` is precisely for doing sequential computation! Consider the following: myBigSubroutine :: Maybe Int -> Maybe Int -> Maybe Bool myBigSubroutine ma mb = do a <- ma b <- mb return (a > b) Here we are sequencing the "effect" of optionality. `ma` must be evaluated before `mb` and if it returns a `Nothing` then we short circuit and do not evaluate `mb`.
  • truculent 4y ago
    > `ma` must be evaluated before `mb` No - either one can be evaluated first, with the other being short-circuited. If you swap the order of those lines, the function is exactly the same (in terms of inputs and outputs, at least).
    • solomonb 4y ago
      This `do` syntax desugars to binds like: ma >>= \a -> mb >>= \b -> return (a > b) We can then inline the definition of `>>=` and `return` to get: case ma of Nothing -> Nothing Just a -> case mb of Nothing -> Nothing Just b -> Just (a > b) Imagine that `mb` is actually a really expensive computation that we don't want to perform unless `ma` returns a value. Sequencing our case statements in this way allows us to do that. `mb` will remain an unevaluated thunk until `ma` evaluates to a `Just a` value.
  • gemstones 4y ago
    I think the commenter who mentioned syntax is on to something. If I write this fn my_big_subroutine(ma: Option<isize>, mb: Option<isize>) -> Option<bool> { match (ma, mb) { (Some(a), Some(b)) => Some(a > b), (_, _) => None, } } it’s clearer to me what the intent is. I’m not sure why the other syntax is so hard for me but it feels hard to understand for some reason.
    • solomonb 4y ago
      The `do` block I showed is more like this: fn my_big_subroutine(ma: Option<isize>, mb: Option<isize>) -> Option<bool> { match (ma) { (Some(a)) => match (mb) { (Some(b)) => Some(a > b), _ => None } (_) => None, } } Which in this case is equivalent in this example, however I'm trying to stress the sequencing. Imagine `mb` had some very expensive computation in it, then it will remain an unevaluated thunk if we shortcircuit on `ma`. > it’s clearer to me what the intent is. I’m not sure why the other syntax is so hard for me but it feels hard to understand for some reason. We can write `myBigSubroutine` with case matching: case ma of Nothing -> Nothing Just a -> case mb of Nothing -> Nothing Just b -> Just (a > b) In fact, the `do` notation version desugars to something equivalent to this snippet. The motivation for using the `Monad` instance (and thus `do` notation), is that it allows us to be polymorphic over the effect described by `>>=` (and thus `do`). This lets us have a customized version of sequencing computations specialized to whatever "effect" we need, not just casing on optional values.
• CyberDildonics 4y ago
  monads help solve the problem of sequential computation in Haskell It probably confuses people because this is a problem haskell created for itself.