Question

If I have an infinite sequence, I cannot use a normal Seq.tryFind.

However, if the sequence is ordered, that means the search could be cancelled when I detect that no other element in the sequence can ever satisfy my condition.

Is there an elegant way to express such a search?

Answer 1

Here is another possible solution. It is quite similar to what @desco posted, but it is using sequence expressions instead of Seq.tryPick and it also does not need nested option types:

let has element input = 
  seq { for v in input do
          if v = element then yield Some v
          if v > element then yield None }
  |> Seq.head

Or even nicer and simpler solution using built-in functions. Just use Seq.takeWhile to take the beginning of the sequence containing only elements that are smaller or equal than the one you’re looking for and then use Seq.tryFind on this part of the sequence:

let has element input = 
  input |> Seq.takeWhile (fun x -> x <= element)
        |> Seq.tryFind (fun x -> x = element)

Or if you fancy the point-free style (which I do not, because it gets harder to read IMHO):

let has element = Seq.takeWhile ((>=) element) >> Seq.tryFind ((=) element)