[fpc-pascal] evaluation of set constant

[Torsten Bonde Christiansen]

On 2010-12-22 14:50, Jonas Maebe wrote:
>
> On 22 Dec 2010, at 14:15, Torsten Bonde Christiansen wrote:
>
>> Is it possible to create a constant of a set that is based on an 
>> evaluation of other constants? (using fpc 2.4.2)
>>
>> A basic case could look like this:
>>
>> type
>>  TMyType = (a, b, c ,d);
>>  TMyTypes = set of TMyTypes;
>>
>> const
>>  SetX:        TMyTypes = (a, b);
>>  SetY:        TMyTypes = (c, d);
>
> These definitions should not compile, a set is defined as [a, b], not 
> (a, b).
Sorry - that was my bad, i just made a typo in the previous email. It's 
written with [] in my code.
>
>>  SetCombined: TMyTypes = SetX + SetY;   // this gives me an "Error: 
>> Illegal expression"
>>
>> I have been reading throught the FPC docs, but it is unclear whether 
>> this is possible at all. The set operators allow this and according 
>> to FPC doc section on constants some expression evaluation is possible.
>
> So-called "typed constants" are actually initialised variables (for 
> historical reasons). The compiler cannot evaluate expressions that 
> involve variables (initialised or not).
>
> Change the constants into symbol constants if you want to use them in 
> constant expressions:
>
> const
>  SetX = [a, b];
>  SetY = [c, d];
>  SetCombined = SetX + SetY;
Ahh... great. I never thought of the difference between typed constants 
and symbol constants. Now it works, Thank!

Kind regards,
Torsten Bonde Christiansen.
EpiData Association.