1. Forum
  2. Usenet
  3. COMP.LANG.ADA

  • Discriminant problem

    From Simon Wright@21:1/5 to All on Sun Aug 29 19:51:57 2021
    I have

    Location_Step.Node_Test :=
    (Node_Test => Get_Node_Type_Test (T.Node_Type_Part.all),
    Name => Null_Unbounded_String);

    where

    type Location_Steps is record
    ...
    Node_Test : Node_Test_Specification
    := (Node_Test => No_Node_Test,
    Name => Null_Unbounded_String);
    ...
    end record;

    and

    type Node_Test_Specification
    (Node_Test : Node_Tests := No_Node_Test) is
    record
    Name : Unbounded_String;
    case Node_Test is
    ...
    end case;
    end record;

    Because of that function call in

    Node_Test => Get_Node_Type_Test (T.Node_Type_Part.all)

    the compiler says

    value for discriminant "Node_Test" must be static
    non-static function call (RM 4.9(6,18))

    OK, I get that (tiresome though it is, and I'm amazed I've never come
    across it before), but how to approach it? I seem to be OK with

    case Get_Node_Type_Test (T.Node_Type_Part.all) is
    when Text_Node_Test =>
    Location_Step.Node_Test :=
    (Node_Test => Text_Node_Test,
    Name => Null_Unbounded_String);
    when ...
    end case;

    but this seems very ugly.

    I've only just noticed this: I'd been using -gnatX (so that I could use
    'Image on records), which meant that the original code was OK (in the
    sense that it didn't raise any problems), but of course it's not
    portable even within the GNAT family. I think -gnat2020 might solve the
    issue too, but there's a bit of a skew between CE 2021 and GCC 11.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Simon Wright@21:1/5 to Simon Wright on Mon Aug 30 09:13:31 2021
    Simon Wright <simon@pushface.org> writes:

    I've only just noticed this: I'd been using -gnatX (so that I could use 'Image on records), which meant that the original code was OK (in the
    sense that it didn't raise any problems), but of course it's not
    portable even within the GNAT family. I think -gnat2020 might solve the
    issue too, but there's a bit of a skew between CE 2021 and GCC 11.

    The skew might well be about the semantics of -gnatX - not 100% sure.

    The only way I can see that -gnat2020 would help would be if
    Get_Node_Type_Test was a static expression function[1], but it's not
    even an expression function! It could be, though since it involves tag
    tests and can 'return' an exception this seems unlikely.

    [1] http://www.ada-auth.org/standards/2xaarm/html/AA-4-9.html#p21.1

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jeffrey R. Carter@21:1/5 to Simon Wright on Tue Aug 31 00:03:16 2021
    On 8/29/21 8:51 PM, Simon Wright wrote:

    the compiler says

    value for discriminant "Node_Test" must be static
    non-static function call (RM 4.9(6,18))

    This has always been the rule for aggregates.

    OK, I get that (tiresome though it is, and I'm amazed I've never come
    across it before), but how to approach it? I seem to be OK with

    That is surprising.

    case Get_Node_Type_Test (T.Node_Type_Part.all) is
    when Text_Node_Test =>
    Location_Step.Node_Test :=
    (Node_Test => Text_Node_Test,
    Name => Null_Unbounded_String);
    when ...
    end case;

    but this seems very ugly.

    Agreed. What I usually do is

    declare
    Result : Whatever (D => Non_Static_Value);
    -- Discriminant of an object need not be static
    begin
    Result.F1 := ...;
    ...
    Location_Step.Node_Test := Result;
    end;

    --
    Jeff Carter
    "You can never forget too much about C++."
    115

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Randy Brukardt@21:1/5 to Jeffrey R. Carter on Mon Aug 30 20:53:41 2021
    "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message news:sgjkj4$kn4$1@dont-email.me...
    On 8/29/21 8:51 PM, Simon Wright wrote:

    the compiler says

    value for discriminant "Node_Test" must be static
    non-static function call (RM 4.9(6,18))

    This has always been the rule for aggregates.

    But Ada 202x relaxes it slightly. If the expression has a static nominal subtype, and every value in the subtype selects the same variant, then a dynamic discriminant is allowed in a aggregate. I don't know if that is what
    is happening here or not; this relaxation doesn't come up that often.

    Randy.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Simon Wright@21:1/5 to Randy Brukardt on Thu Sep 9 20:51:39 2021
    "Randy Brukardt" <randy@rrsoftware.com> writes:

    "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message news:sgjkj4$kn4$1@dont-email.me...
    On 8/29/21 8:51 PM, Simon Wright wrote:

    the compiler says

    value for discriminant "Node_Test" must be static
    non-static function call (RM 4.9(6,18))

    This has always been the rule for aggregates.

    But Ada 202x relaxes it slightly. If the expression has a static
    nominal subtype, and every value in the subtype selects the same
    variant, then a dynamic discriminant is allowed in a aggregate. I
    don't know if that is what is happening here or not; this relaxation
    doesn't come up that often.

    That's exactly what's happening here.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)