yield and yield! are both keywords used in sequence expressions. To compute a sequence of type seq<'T>, you can use a sequence expression, in which the yield keyword takes a value of type 'T, and adds that value to the computed sequence, while yield! takes a value of type seq<'T> and appends that value to the computed sequence.

You also have keywords "return" and "return!" for F#'s version of monads. In Haskell, "return" is just a function, while "return!" is not needed (though I wish it was present in Scala).