[fpc-pascal]typed constants

[Vinzent Hoefler]

On Thursday 02 January 2003 09:31, Michael.VanCanneyt at Wisa.be wrote:
> On Thu, 2 Jan 2003, Vinzent Hoefler wrote:
> > On Thursday 02 January 2003 08:43, Peter Vreman wrote:
>
> > > Go and tell Borland about it, they use '=' for assigning enums.
> > > FPC already supported that with ':='.
> >
> > *eeek* In Delphi you can write the following?
> >
> > |var
> > |  X : some_enum = enum_value;
> >
> > I thought, this kind of syntax is only allowed for typed
> > "constants"?
>
> No:
>
> Type
>   TEnum = (enOne,entwo,enThree=4);

Oh, this one. To assign specific values to an enum position. Well, this 
can be considered as a constant value, so the "=" would be quite ok for 
me.

> In Free pascal, it was:
>
> Type
>   TEnum = (enOne,entwo,enThree:=4);
>
> Its main purpose is to support translation of C enums

The intention is clear (in C, enums are just named integer constants 
anyway).

[I just wouldn't like to have declaration and implementation at the same 
place. The Ada approach with its representation clauses looks more 
appealing to me.]

> > > > with the "=" operator. Consider this:
> > > > |Var
> > > > |  A : boolean = C = D;
> > > >
> > > > Looks a little bit crazy to me.
> > >
> > > That is also not allowed. You need to specify a constant value.

[I think this one is clarified now, thanks]

> program  tini;
>
> Const
>   A = 1;
>   B = 2;
>
> Procedure ProcA;
>
> Var
>   C : Integer = A+B;
>   D : Boolean = (A=B);

To get back to the point, I understood that this will "probably be 
implemented as initialized variables."
[see <Pine.LNX.4.44.0301021020000.871-100000 at home.wisa.be>]

Rereading that message again seems to reveal to me, that this means 
_both_ syntactic forms will be implemented as initalized variables? 
*dizzled*

I thought, the only difference between local typed constants and 
initialized variables will be that

(a) the "const" declaration implements them as global with just local 
scope and

(b) "var" would initialize it each time the subprogram is called?

And especially in case of (b), why should it not be allowed to use 
variable parameters as initialization value?

Anyway, I would think that an assignment operator ":=" is more 
appropriate for a variable assignment in those cases. It would also 
help to distinguish between both cases then, because people tend to 
write declarations in single const/var blocks, so you cannot tell from 
the first look, if this one is a local typed constant or an initialized 
variable. With the distinction between "=" and ":=" you could do that.

> With correct output :
> home: >ppc386-1.1 tini.pp
> home: >tini
> 3
> FALSE

I did not expect otherwise. :-)


Vinzent.

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