http://d.puremagic.com/issues/show_bug.cgi?id=10816

           Summary: Support for is(T1 != T2) syntax
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2013-08-13 07:03:37 PDT ---
Created an attachment (id=1242)
Hundreds of usage cases found in dmd/Phobos

In Phobos I have counted about 240 cases (even if some of them are possibly false positives, see the attach) of usages of the pattern:

!is(T1 == T2)

So I suggest to add to D a handy syntax like this (that in past sometimes I even have used by mistake):

is(T1 != T2)

It's similar to the handy "!is" and "!in" syntaxes.

- - - - - - - - - - - - - - - - - -

Extra: in some cases you can even think about a comparison, but this is left to other enhancement requests:

class Klass1 {}
class Klass2 : Klass1 {}
enum bool b1 = is(Klass1 < Klass2);

alias T1 = Tuple!(int, "x");
alias T2 = Tuple!(int);
enum bool b2 = is(T1 < T2);

--------------------------

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timon.gehr@gmx.ch changed:

           What    |Removed                     |Added
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                 CC|                            |timon.gehr@gmx.ch


--- Comment #1 from timon.gehr@gmx.ch 2013-08-13 07:29:42 PDT ---
What is the meaning of is(T1!=T2) ?

Is it

!is(T1==T2)

or

is(T1) && !is(T1==T2)

IMO the way to go is to use T1==T2 and T1!=T2. (In this case, both T1 and T2
should be valid types.)

---

Extra: What would be the difference to is(T1:T2) ? I think this would again make more sense without 'is': T1<T2.

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Dicebot <public@dicebot.lv> changed:

           What    |Removed                     |Added
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                 CC|                            |public@dicebot.lv


--- Comment #2 from Dicebot <public@dicebot.lv> 2013-08-13 08:49:44 PDT ---
I like the idea. Both because of consistency and because that '!' near to 'i' is so hard to notice.

@Timon

I don't think "is(T1) && !is(T1==T2)" is any useful. It will make "is(T1 != T2)
return false for invalid T1 which is rather surprising. I can't imagine the use
case for it.

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--- Comment #3 from timon.gehr@gmx.ch 2013-08-13 09:01:05 PDT ---
(In reply to comment #2)
> I like the idea. Both because of consistency and because that '!' near to 'i' is so hard to notice.
> 
> @Timon
> 
> I don't think "is(T1) && !is(T1==T2)" is any useful. It will make "is(T1 != T2)
> return false for invalid T1 which is rather surprising. I can't imagine the use
> case for it.

Are you aware that is(T1==T2) gives false for invalid T1 and error for invalid
T2?

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--- Comment #4 from Dicebot <public@dicebot.lv> 2013-08-13 09:07:06 PDT ---
(In reply to comment #3)
> Are you aware that is(T1==T2) gives false for invalid T1 and error for invalid
> T2?

Was aware about the former, not about the latter.

But I don't see how it is relevant. When i compare types via is(T1 == T2), information that matters is "are those two the same?". It does not matter why are they not the same. Same goes for is(T1 != T2), but other way around - "are those types not the same"?

Making "!is(T1 != T2) != is(T1 == T2)" breaks common sense quite a lot in my
opinion.

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--- Comment #5 from bearophile_hugs@eml.cc 2013-08-13 09:36:29 PDT ---
(In reply to comment #1)

> What is the meaning of is(T1!=T2) ?
> 
> Is it
> 
> !is(T1==T2)
> 
> or
> 
> is(T1) && !is(T1==T2)

The most important use case is to test if two already existing types differ.


> IMO the way to go is to use T1==T2 and T1!=T2. (In this case, both T1 and T2
> should be valid types.)

I agree. But I think this is for another enhancement request. Do you want to open it?

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