That rumour seems like it's false, because not only is "implemented using monads" a rather vague qualification, Linq has currently been implemented by requiring

That rumour seems like it's false, because not only is "implemented using monads" a rather vague qualification, Linq has currently been implemented by requiring objects to implement the SelectMany function, which has signature: MyObject<B> SelectMany(MyObject<A>, Func<A, MyObject<B>>) or perhaps more familiar to the Haskell folks: SelectMany :: M a -> (a -> M b) -> M b i.e. exactly the 'bind' operator for Monads. I think you also need to implement a 'Select :: M a -> M b' operator, rather than a 'return :: a -> M a' operator, which would be closer to a true Monad, but the 'bind' operator is the one that really matters anyway.