kirbyfan64sos kirbyfan64sos - 4 months ago 25
Pascal Question

ISO Pascal record variants without a field name

I'm slightly confused trying to figure a section of ISO Pascal.

The grammar allows you to do this:

type RPoint = Record
Case Boolean of
False : (X,Y,Z : Real);
True : (R,theta,phi : Real);
end;


To construct it, you do:

var p: RPoint;
begin
p.x := 1;
end.


There's one part I don't understand: what's the purpose of the
Case Boolean
part? I understand that you can do
case MyVal: Boolean
; then
MyVal
becomes the field selector. However, what is the purpose when there is no field selector, just a type?

In addition, the standard says:


With each variant-part shall be associated a type designated the selector-type possessed by the
variant-part . If the variant-selector of the variant-part contains a tag-field, or if the case-constant-
list of each variant of the variant-part contains only one case-constant, then the selector-type shall
be denoted by the tag-type, and each variant of the variant-part shall be associated with those
values specified by the selector-type denoted by the case-constants of the case-constant-list of the
variant . Otherwise, the selector-type possessed by the variant-part shall be a new ordinal-type that
is constructed to possess exactly one value for each variant of the variant-part, and no others, and
each such variant shall be associated with a distinct value of that type.


I don't quite understand what the
selector-type
is and why it would be a new
ordinal-type
. Wouldn't the
selector-type
just be the type like in
case Boolean of
? And what does each
case-constant-list
having only one
case-constant
have to do with it?

Answer

Here your variant record has two possible 'personalities'. Boolean is a type with two possible values. So, it seemed like a logical choice. But, it doesn't have to be Boolean.

You could have used some other ordinal type such as Integer or Byte to get the same effect. For example:

type RPoint = Record
  Case Byte of
    0: (X,Y,Z : Real);
    1: (R,theta,phi : Real);
end;
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