number_in_month exercise (SML error unbound variable)

Question

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)

Are you sure the error you get points to this code? Also, unbound variables are certainly no runtime errors in SML. — Andreas Rossberg, Oct 16 '13 at 10:48
Code works fine. That error means that the function is not defined. Maybe you just forgot to reload your file. — Nick Barnes, Oct 16 '13 at 14:14

score 1 · Answer 1 · edited May 23 '17 at 12:21

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)

edited May 23 '17 at 12:21

Community

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answered Oct 17 '13 at 08:59

sshine

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Or even better: `(if y = mo then 1 else 0) + number_in_month(rest, mo)` :) – Andreas Rossberg Oct 17 '13 at 09:43
Or even better: `fun number_in_month (dates, mo) = foldl (fn ((_,mo',_), n) => (if mo = mo' then 1 else 0) + n) 0 dates` – sshine Oct 17 '13 at 10:17

number_in_month exercise (SML error unbound variable)

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