F# equivalent of LINQ Single

Question

Ok, so for most LINQ operations there is a F# equivalent. (Generally in the Seq module, since Seq= IEnumerable)

I can\'t find the equiv of IEmumerable.Single

Answer 1

What about

let Single source f =
    let worked = ref false
    let newf = fun a -> 
        match f a with
        |true -> 
            if !worked = true then failwith "not single"
            worked := true
            Some(a)
        |false -> None
    let r = source |> Seq.choose newf
    Seq.nth 0 r 

Very unidiomatic but probably close to optimal

EDIT:

Solution with exactlyOne

let only2 f s= (Seq.filter f s) |> exactlyOne