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EQUALOP error message with SML

I am trying to create a simple function that takes two dates of int * int * int format and returns if the first one is older than the second or not.

fun is_older (date1: (int*int*int), date2: (int*int*int)) =
    val in_days1 = (#1 (date1) * 365) + (#2 (date1) * 30) + #3 date1;
    val in_days2 = (#1 (date2) * 365) + (#2 (date2) * 30) + #3 date1;

    if in_days1 < in_days2
    then true
    else false

I am getting this error:

hwk_1.sml: 1.53 Error: syntax error: insert EQUALOP

uncaught exception Compilation [Compilation: syntax error]]

lifted to: ../ compiler / Parse / main / smlfile.sml: 15.24-15.46

../compiler/TopLevel/interact/evalloop.sml: 44.55

../compiler/TopLevel/interact/evalloop.sml: 296.17-296.20

Can anyone please help?

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3 answers


In addition to what has already been mentioned, you must also use pattern matching to decompose this 3-tuple. By doing this, you can also drop the type annotations as it is now clear that it is a 3-tuple (as for the reader, but more importantly also for the type system).

fun is_older ((y1, m1, d1), (y2, m2, d2)) =
    let
      val days1 = y1 * 365 + m1 * 30 + d1
      val days2 = y2 * 365 + m2 * 30 + d2
    in
      days1 < days2
    end


However, you could make it a little smarter. If you have multiple date functions, you can create a small helper function toDays

. In the example below, I just included the function isOlder

, but you can put it on the top level or inside the local

-declaration if you don't hide it

fun isOlder (date1, date2) =
    let
      fun toDays (y, m, d) = y * 365 + m * 30 + d
    in
      toDays date1 < toDays date2
    end
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val in_days1 = (#1 (date1) * 365) + (#2 (date1) * 30) + #3 date1;
val in_days2 = (#1 (date2) * 365) + (#2 (date2) * 30) + #3 date1;


Local definitions val

must be between let

and in

.

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FWIW, I got the same error on one of the other exercises in this same homework:

Error: syntax error: inserting EQUALOP

but in my case it was happening on the first line. This was confusing me because I come from Python where the error usually happens after the error.

Bottom line, and what I wanted to know is this: This error means that it cannot compile the code as written.

PS if you use let

and in

, you also need to use end

. (You don't need to use val

or let

to solve the problem is_older

- there is a way to do it with just installed logic).

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