I believe that he's referring to the fact that Scala's Set is not a monad, so it "shouldn't" have a .flatMap operation.
Scala has other containers that have a .flatMap operation that are not real monads, such as Future and Try.
That can lead to surprising behavior on for/yeld loops.
Disclaimer: I do work in Scala and enjoy the language.
I mean if you write something like:
(for {
x <- List(1, 1, 1)
y <- Set(x)
z <- List(y, y)
} yield z).length
It's only surprising if you ascribe monadic properties to that syntax versus understanding it as literal syntactic sugar and absolutely nothing else.
I would say: it is `Set` and `List` that are wrong in that example, not `for`. That said, `for` really does have some issues (enter https://github.com/oleg-py/better-monadic-for/blob/master/RE...)
Not sure what you're talking about, but for/yield is syntactic sugar for map/flatMap, just like the "do notation" is in Haskell. Of course it has theoretical grounding, because it wouldn't work without flatMap being the monadic bind.
> mixing lists and sets
Again, not entirely sure what you're talking about, but if true, it's entirely unrelated to for/yield.