Variant records (EP)

[Waldek Hebisch]

Frank Heckenbach  wrote:
> A question to the EP experts? Is the following program valid EP, and
> if so, what should its output be?
> 
> program Foo (Output);
> 
> type
>   i23 = 2 .. 3 value 2;
>   a = record
>         b: Integer value 1;
>       case c: i23 of
>         2: (d: Integer value 4);
>         3: (e: Integer value 5)
>       end value [b: 6; case c: 3 of [e: 7]];
> 
> var
>   v: ^a;
> 
> begin
>   New (v, 2);
>   WriteLn (v^.b, ' ', v^.c, ' ', v^.d)
> end.
> 
...
> : 6.7.5.3 Dynamic allocation procedures
> :
> : It shall be an error if a variant of a variant­part within the new variable
> : is active and a different variant of the variant­part is one of the
> : specified variants.
> :

IMHO the above means that the program is incorrect. Since the type
specifies the initial state, variant corresponding to 3 is 
active nad you are not allowed to specify different variant to `new'.

> Or is this `New' statement invalid? If anything, paragraph 3 might
> be a reason, but AFAICS it's meant to forbid invalid combinations of
> selectors in nested variant records, and doesn't apply here.

From 6.4.3.4

: The value of the selector of the variant type shall cause the associated
: variant of the variant-part to be designated active.

So once we apply the inital state corresponding variant is designated
active and by the paragraph 3 if we specify different variant, we have
an error.

> 
> The more general question is whether the initial states of the
> components of a structured type can have any influence when the
> structured type itself has a `value' specification. (Since I don't
> see another way how they could.)
> 

AFAICS no -- it seem that `value' specification is required to be
complete.

-- 
                              Waldek Hebisch
hebisch at math.uni.wroc.pl