let rec foldStr (s : string) acc fn =

    if s.Length > 0 then foldStr s.[1..] (fn acc s.[0..0]) fn

    else acc

​

let inversion acc s1 = 

    s1 + acc

let count acc s1 =

    if s1 = "l" then acc + 1

    else acc

let removeSpaces acc s1 =

    if s1 <> " " then acc + s1

    else acc

printfn "%s" (foldStr "Hello world!" "" inversion)

printfn "%i" (foldStr "Hello world!" 0 count)

printfn "%s" (foldStr "Hello world!" "" removeSpaces)