> Thanks for that, I can see these things would be useful - but are they a 'killer app'? Like it seems a lot of work to figure out the abstraction to build the

> Thanks for that, I can see these things would be useful - but are they a 'killer app'? Like it seems a lot of work to figure out the abstraction to build the monad, when I don't really need to do this, cause I only have to do this one thing. Most of the core monads are already done. You make new ones to define new compositions of new ones and bring in new features. I've only ever needed to make a genuinely novel monad once. You often don't even make custom monads, you make custom typeclasses that explain how to do your work within a monad, and then pick the ones you want. > and I can say when I strike this problem I can use this monad write a few lines of code and go home? Maybe thats the goal? It is. But as I said, a lot of time what you're actually dealing with is a custom transformer stack with either Identity or IO at its root. So you just write generically and specify what you want. In practice, it's not very difficult. > Like Swift can define types as optionals, which sort of looks like the Maybe in haskell (to me any way). They are. Good example. > Monads I just can't see why they're good - you've given a couple of examples and they seem useful, though they're not things I can't do with other language constructs, perhaps not as elegantly. What's incredibly hard to do is write code that's generic in this axis. There are other examples exercising these principles and pretty amazing things can fall out of that. For example, lenses use a weaker abstraction (functors)and they can make a function that depending on how its' called, either gets a value inside a struct or rebuilds the struct with a new value. Pretty amazing! > I'm wondering if I'm missing something huge? :-) Maybe. But as many people have noted, Haskell is a comittee language that is surprisingly good as an accident of its features all happening to stack together in a somewhat unplanned way. So there is more to the story than this, but this IS a pretty unheard-of story.