f# - Applying a function returned from a subparser with fparsec -

noob alert!

ok, i'm trying build simple math expression parser in fparsec. right want handle strings "1+2-3*4/5" , return double result of evaluation. no spaces, newlines, or parens, , left right order of operations fine.

here's have far:

let number = many1 digit |>> fun ds -> int <| string.concat(ds)  let op : parser<int -> int -> int, unit> =     charreturn '+' (+) <|>     charreturn '-' (-) <|>     charreturn '*' (*) <|>     charreturn '/' (/)  let expression, expressionimpl = createparserforwardedtoref() expressionimpl :=     choice[         attempt(number .>> op >>. expression);         number]  let test p str =     match run (p .>> eof) str     | success(result, _, _) -> printfn "success: %a" result     | failure(result, _, _) -> printfn "failure: %a" result  [<entrypoint>] let main argv =       test expression "1+1/2*3-4"     console.read() |> ignore      0 

in first choice of expression parser, i'm not sure how apply function returned op parser.

as usual, find answer right after posting question (after 3 hours of searching).

i changed line:

attempt(number .>> op >>. expression); 

to this:

attempt(pipe3 number op expression (fun x y z -> y x z)); 

however, realized expressions parse backwards. drawing board.