make a program [in F#]

 fileReplace : string -> needle : string -> replace: string -> unit

which replaces all occurrences of the needle argument with the replace argument in the file with name filename. The solution must as a minimum use the functions System.IO.File.OpenText, ReadLine and WriteLine to access the files. A short test must be included and a short description of the solution with arguments for larger design choices taken to reach the given solution