F#: The type 'MatchCollection' is not compatible with the type 'seq<'a>'.

I was trying to use regular expressions with sequences, and ran across this problem.

(new Regex ".*").Matches("foo") |> Seq.iter (fun s -> printfn "s is %A" s);;

-----------------------------------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

stdin(4,36): error FS0001: The type 'MatchCollection' is not compatible with the type 'seq<'a>'.

> (new Regex "f.*").Matches("foo") |> Seq.cast |> Seq.iter (fun s -> printfn "s is %A" s);;

s is foo

val it : unit = ()

>

Turns out MatchCollection implements System.Collections.IEnumerable, not System.Collections.Generic.IEnumerable<T>. You need to cast it with Seq.cast to use it:

(new Regex "f.*").Matches("foo") |> Seq.cast |> Seq.iter (fun s -> printfn "s is %A" s);;

I got the solution from hubfs - it's a good place for F# information.