[<JavaScript>]
let indent (right: bool) (cm: CodeMirror) =
    let startCursor = cm.GetCursor(true)
    let endCursor = cm.GetCursor(false)
    let startLine = cm.GetCursor(true).Line
    let endLine = cm.GetCursor(false).Line
    if startLine = endLine then
        let ch = startCursor.Ch
        if right then
            cm.ReplaceSelection(String.replicate (4 - ch % 4) " ")
        else
            let line = cm.GetLine(startLine)
            let notSpace i = i < 0 || line.[i] <> ' '
            let removeCh =
                if notSpace (ch - 1) then 0
                elif notSpace (ch - 2) then 1
                elif notSpace (ch - 3) then 2
                elif notSpace (ch - 4) then 3
                else 4
            if removeCh > 0 then
                cm.ReplaceRange("", CharCoords(startLine, ch - removeCh), startCursor)
    else
        for i = startLine to endLine do
            cm.IndentLine(i, if right then 4 else -4)

[<JavaScript>]
let CodeMirrorOptions () =
    CodeMirror.Options(
        IndentUnit = 4,
        TabSize = 4,
        IndentWithTabs = false,
        ExtraKeys = New [
            "Tab" => indent true
            "Shift-Tab" => indent false
        ])