Declaring a type as a subset of a set

Question

I can easily declare a enumeration and a set.
But sometimes I want to work with only part of the enumeration and I'd like the compiler to check for me if values in the sub-enum and its subset stay within the bounds.

type
  TDay = (mon, tue, wen, thu, fri, sat, sun);
  TWeekday = (mon..fri); //not allowed;

  TDays = set of TDay;
  TWeekdays = set of TDay[mon..fri]; //not allowed

Can I declare TWeekday and TWeekdays as a derivative of TDay, if so, how?

_{Funny enough google does not yield anything (for me) on this issue, just plain old sets.}

Answer 1

You've got the wrong syntax for the subrange. Drop the brackets () and it will work.

type
  TDay = (mon, tue, wen, thu, fri, sat, sun);
  TWeekday = mon..fri; // A subrange of TDay

  TDays = set of TDay;
  TWeekdays = set of TWeekDay; 

More about Subrange Types and Sets.