number_in_month exercise (SML error unbound variable)

问题

I am trying to learn SML, and I am trying to implement two functions. The first function works fine, but when I added the second function it gives me an run-time error:

stdIn:1.2-1.17 Error: unbound variable or constructor: number_in_month

This happens while calling the function number_in_month. My code is:

 fun is_older(d1 :int*int*int,d2 :int*int*int) =
  (#1 d1) < (#1 d2) andalso (#2 d1) < (#2 d2) andalso (#3 d1) < (#3 d2)


fun number_in_month(da :(int * int * int) list ,mo : int) =
    if da = []
    then 0
    else if (#2(hd da)) = mo
     then 1 + number_in_month((tl da),mo)
    else 0 + number_in_month((tl da),mo)

回答1:

Your question sounds surprisingly much like these other questions: SML list iteration (8 months ago), recursion in SML (9 months ago), and Count elements in a list (8 months ago), so you certainly don't get points for asking a creative question.

Some of the questions above have been answered extensively. Look at them.

Here is your code rewritten in a better style:

(* Find better variable names than x, y and z. *)
fun is_older ((x1,y1,z1), (x2,y2,z2)) =
    x1 < x2 andalso y1 < y2 andalso z1 < z2

fun number_in_month ([], mo) = 0
  | number_in_month ((x,y,z)::rest, mo) =
    if y = mo then 1 + number_in_month(rest, mo)
              else 0 + number_in_month(rest, mo)

来源：https://stackoverflow.com/questions/19398097/number-in-month-exercise-sml-error-unbound-variable