Subtyping issue

[valentin tihomirov]

I would like to restrict the space of characters.:
subtype TMYCHAR is character ('a', '1', '.'); -- not supported

I would like the interpriter to understand that the elements are characters
so that character'pos attribute were applicable to them for ASCII code
retreival. Like
constant C: character := TMYCHAR'val(1);
constant I: intger := character'pos(C); -- must be x61

Unfortunately, VHDL supports only ranges rather than enumerations in the
specification of subtype. How would you perform the convertion:
type TMYCHAR is ('a', '1', '.');
constant C: character := TMYCHAR'val(1);



I fail to write a function TMYCHAR elements with characters

function GET_ASCII(MC: TMYCHAR) return character is
begin
for C in character'left to character'right loop
if C = MC then -- incompareble types
return character'pos(C);
end if;
end loop;
end;



thanks in advance

 

[Jonathan Bromley]

I would like to restrict the space of characters.:
subtype TMYCHAR is character ('a', '1', '.'); -- not supported

I would like the interpriter to understand that the elements are characters
so that character'pos attribute were applicable to them for ASCII code
retreival. Like
constant C: character := TMYCHAR'val(1);
constant I: intger := character'pos(C); -- must be x61

Unfortunately, VHDL supports only ranges rather than enumerations in the
specification of subtype.

You are trying to specify a *set* of character. Modula-2 and Pascal
supported sets. VHDL does not.

How would you perform the convertion:
type TMYCHAR is ('a', '1', '.');
constant C: character := TMYCHAR'val(1);

One possible solution...

function To_Character(C: TMYCHAR) return CHARACTER is
begin
return CHARACTER'VALUE(TMYCHAR'IMAGE(C));
end

Alternatively, try a lookup table:

type Char_Lookup is array (TMYCHAR) of CHARACTER;
constant To_Character: Char_Lookup :=
( 'a' => 'a'
, '1' => '1'
, '.' => '.'
);

In either case (function or lookup table) you can then write...

constant C: CHARACTER := To_Character('1');

HTH
--
Jonathan Bromley, Consultant

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