Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!oliveb!pyramid!nsc!icldata!altnet!altos86!elxsi!beatnix!corbett
From: corbett@beatnix.UUCP (Bob Corbett)
Newsgroups: comp.lang.pascal
Subject: Re: An ISO Pascal question...
Message-ID: <2560@elxsi.UUCP>
Date: 22 Mar 89 22:42:13 GMT
References: <18654@adm.BRL.MIL> <2824@kalliope.rice.edu> <216@m2xenix.UUCP> <2158@virginia.acc.virginia.edu>
Sender: news@elxsi.UUCP
Reply-To: uunet!elxsi!corbett (Bob Corbett)
Organization: ELXSI, San Jose
Lines: 76
Distribution:

In article <2158@virginia.acc.virginia.edu> scl@virginia.acc.Virginia.EDU
(Steve Losen) writes:

>>mootaz@titan.rice.edu (Elmootazbellah Nabil Elnozahy) asks if the
>>following is legal Pascal:
>>
>>  T = real;
>>  B = Record
>>    T: integer;
>>    C: Record
>>      D: T;
>>      end;
>>    end;
>
>The above is not legal ISO standard Pascal.  While it is legal to
>redefine an identifier used in an enclosing scope, under no
>circumstances is it legal for an identifier to have two "meanings"
>in the same scope.

So far, so good.  But...

>For some real fun, lets change the field orderings around and see what
>happens.
>
>  B = Record
>    C: Record
>      D: T;       { This reference to type T irrevocably keeps this }
>                  { anonymous record within the scope of type T.  Record B }
>                  { (excluding this anonymous record) can still be }
>                  { removed from the scope of type T as we shall see.}
>      end;
>    T: integer;   { This is perfectly legal.  The scope of field T is }
>                  { now record B (excluding the anonymous record, which }
>                  { is within the scope of type T). }
>

This construction is not legal.  The following statements are from ISO 7185:

    6.2.2.5  When an identifier or label has a defining-point for region A
    and another identifier or label having the same spelling has a defining
    point for some region B enclosed by A, then region B and all regions
    enclosed by B shall be excluded from the scope of the defining-point
    for region A.

Note the phrase "and all regions enclosed by B."  In the example above, the
inner record-type is a region enclosed by the region that is the outer
record-type.

    From Section 6.4.3.3:
    The occurrence of an identifier in the identifier-list of a record-section
    of a fixed-part of a field-list shall constitute its defining-point as a
    field-identifier for the region that is the record-type closest-containing
    the field-list, and shall associate the field-identifier with a distinct
    component, which shall be designated a field, of the record-type and of
    the field-list.

The region associated with the defining-point of the field-identifier T is
the outer record-type and all regions enclosed by the outer record-type,
which includes the inner record-type.  Therefore, the inner record-type is
excluded from the scope of the defining-point of the type-identifier T.
Therefore, the use of T as a type-identifier in the inner record-type is
illegal.

>If you don't believe me, did you just finish writing an ISO Standard
>Pascal compiler?

No, I just started.

>-- 
>Steve Losen     scl@virginia.edu
>University of Virginia Academic Computing Center

					Yours truly,
					Bob Corbett
					uunet!elxsi!corbett
					ucbvax!sun!elxsi!corbett