Sounds like you're looking for takeWhile:

val takeWhile : ('T -> bool) -> seq<'T> -> seq<'T><summary>

Return a sequence that, when iterated, yields elements of the underlying sequence while the

given predicate returns 'true', and returns no further elements

For source, you'll want to download the CTP in addition to the Visual Studio 2010 Beta 1.

[link:www.microsoft.com]

There's a source directory in there.
</summary>

You could replace the loop with a recursive function. Or, you could use a computation expresion: [link:tomasp.net]

I'm a little bit confused by the code though. Why have a seq expression for only one element?

I think the below code will have performance issues. But, it might not matter.

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let rec first f s = if (s |> Seq.hd |> f) then Seq.hd s else first f (Seq.skip 1 s)

or:

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let rec first f s = seq{ if (s |> Seq.hd |> f) then yield Seq.hd s else yield! first f (Seq.skip 1 s) }