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digitalmars.D.bugs - [Issue 11124] New: support "is (T : struct)" syntax

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=11124

           Summary: support "is (T : struct)" syntax
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: monarchdodra gmail.com



As well as:
is (T : class)
is (T : interface)
is (T : union)

This could be useful in two scenarios:

The case where T represents a enum instance of said type. For example:
struct T {}
enum E : T {a = T.init}

In this case, E being an enum, it isn't a struct, however, it would be useful
to know if it can "decay" to a struct type. This syntax would make proper enum
handling correct "by default" rather than explicitly, EG:
is (T : struct)
vs
is (OriginalType!T == struct)

This (I think), is actually quite important, as it increases the chances of
code being correct with no special extra content, as well as lower the overall
complexity.



Another case where this could be useful, is if T defines an alias this that
resolves to another type:

class C
{
    T t;
    alias t this;
}

is (T == struct); //false
is (T : struct); //true

Related conversation:
http://forum.dlang.org/thread/vmfiodpnsuzmxrogdxjg forum.dlang.org

Also, as a general rule, it's more consistent. If:
is (T : U);

is legal, then so should:
is (T : struct);

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next sibling parent d-bugmail puremagic.com writes:
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Also, use case that we might want to support (but I'm not 100% sure on this):

is (typeof(f) : delegate)

idea being that a function is also a delegate (?) Not entirely sure this is
actually true though. Still, a struct might be implicitly castable via alias
this to a delegate, so ": delegate" and ": function" would be useful.

See also:
http://d.puremagic.com/issues/show_bug.cgi?id=11123

This might have been avoided with useage of ": delegate" as opposed to "==
delegate"

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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 Another case where this could be useful, is if T defines an alias this that
 resolves to another type:
 
 class C
 {
     T t;
     alias t this;
 }
 
 is (T == struct); //false
 is (T : struct); //true


Mistake of:

is (C == struct); //false
is (C : struct); //true

?

And, how this will work when multiple alias this is properly implemented?

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Andrej Mitrovic <andrej.mitrovich gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrej.mitrovich gmail.com



09:24:35 PDT ---


 And, how this will work when multiple alias this is properly implemented?


I think we should think about adding a new 'is' expression for alias this
checks, because I've already ran into an issue with them interfering with the
is(T : X) syntax. I'm thinking we could introduce something like the following:

if (is(T : alias X)) => T has a subtype of type X

I can't find my other bug report right now where I mention this, I'll look into
it later.

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 And, how this will work when multiple alias this is properly implemented?


I don't understand the problem? Isn't it the same as, say: "is (C : int)"?

I don't see what is so special about struct:

class C
{
    T t;
    int i;
    alias t this;
    alias i this;
}
is (C == struct); //false
is (C == int); //false
is (C : struct); //true
is (C : int); //true

Or am I missing something obvious? Apologies in advance.


 I've already ran into an issue with them interfering with the


is(T : X) syntax

Isn't the point that they *do* interfere with such syntax?

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 Another case where this could be useful, is if T defines an alias this that
 resolves to another type:
 
 class C
 {
     T t;
     alias t this;
 }
 
 is (T == struct); //false
 is (T : struct); //true


 
 Mistake of:
 
 is (C == struct); //false
 is (C : struct); //true
 
 ?


Yes. Sorry. Your code is correct.

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 And, how this will work when multiple alias this is properly implemented?


 
 I don't understand the problem? Isn't it the same as, say: "is (C : int)"?


Today,
1. is(T U == super) captures the super class list of T in U.

  class C : Object {}
  static if (is(C U == super)) { static assert(is(U[0] == Object)); }

2. If T is implicitly convertible to U, is(T X : U) captures T in X.

Then, if multiple alias this is supported:

  struct S1 {}
  struct S2 {}
  struct S {
    S1 s1;  alias s1 this;
    S2 s2;  alias s2 this;
  }
  static if (is(S U : struct)) { /* What's captured in U? */ }




Which is chosen? Or other choices?

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 And, how this will work when multiple alias this is properly implemented?


 
 I don't understand the problem? Isn't it the same as, say: "is (C : int)"?


 
 Today,
 1. is(T U == super) captures the super class list of T in U.
 
   class C : Object {}
   static if (is(C U == super)) { static assert(is(U[0] == Object)); }
 
 2. If T is implicitly convertible to U, is(T X : U) captures T in X.
 
 Then, if multiple alias this is supported:
 
   struct S1 {}
   struct S2 {}
   struct S {
     S1 s1;  alias s1 this;
     S2 s2;  alias s2 this;
   }
   static if (is(S U : struct)) { /* What's captured in U? */ }
 


 
 Which is chosen? Or other choices?


I'm no expert with "is", but isn't "is(C U == super)" a special case of "is"?
(which would look much better as a trait too?)

For example, when I do this;

//----
struct S
{
    short i;
    alias i this;
}

class C
{
    S s;
    alias s this;
}

void main()
{
    static if (is(C U : int))
        static assert(is(U == C)); //Base type C is captured
}
//----

If C was to actually to be:

class C
{
    S s1;
    S s2;
    alias s1 this;
    alias s2 this;
}

I don't see how it would affect my above static if.

Since I don't really see ": int" as any different from ": struct", then to your


But I'm no "is" expert.

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