F# stream a text file line by line lazily

open System.IO

let readLines (path: string) : seq<string> =
    seq {
        use reader = new StreamReader(path)
        while not reader.EndOfStream do
            yield reader.ReadLine()
    }

// lazy: nothing is read until the sequence is enumerated
let countNonBlank path =
    readLines path
    |> Seq.filter (fun line -> not (System.String.IsNullOrWhiteSpace line))
    |> Seq.length