F#: Seq.forall weirdness?

Question

Given let ra = ResizeArray<int> ():

Seq.forall (fun i ->
                    let q = i % 2
                    if 0 = q then ra.Add i
                    true ) <| seq { 1..10 }

If I do that, ra.Count returns 5.

Seq.forall (fun i ->
                    let q = i % 2
                    if 0 = q then ra.Add i
                    0 = q ) <| seq { 1..10 }

If I do that, ra.Count returns 0.

So, what, unless every iteration of the lambda function evaluates to true, then effectively none of the code in the function is executed, or what??

What's going on, here?

Answer 1

You use a value of "false" to cease the Seq.forall function from processing further elements.

Since 1 % 2 = 0 is false, this stops the evaluation on the first iteration.

Answer 2

The following approach would be more functional:

let (anyOdds, evens) =
    seq {1..10}
    |> Seq.fold (fun (anyOdds, xs) x ->
        if x % 2 = 0 then
            anyOdds, x :: xs
        else true, xs) (false, [])

Feel comfortable to ditch ResizeArray's in favor of F# lists they are generally efficient unless you have specific requirements.