I'm trying to match an integer expression against character literals, and the compiler complains about type mismatch.

let rec read file includepath =
    let ch = ref 0
    let token = ref 0
    use stream = File.OpenText file

    let readch() =
        ch := stream.Read()
    let lex() =
        match !ch with
        | '!' ->
            readch()
        | _ -> token := !ch

ch has to be an int because that's what stream.Read returns in order to use -1 as end of file marker. If I replace '!' with int '!' it still doesn't work. What's the best way to do this?