Re: Enum alias members: yay or nay?
On Sunday, September 08, 2013 12:08:07 monarch_dodra wrote:
> On Sunday, 8 September 2013 at 09:43:02 UTC, Kenji Hara wrote:
> > I also agree that the compiler enhancement is overkill.
> >
> > Kenji Hara
>
> Let's not throw this away quite yet: There is *another*
> fundamental difference:
>
> enum S
> {
> a,
> b = a,
> }
>
> This creates an enum with *two* entries.
>
> enum S
> {
> a,
> alias b = a,
> }
>
> This would create an enum with a *single* entry, which can be
> accessed via two different names.
>
> *This*, in itself, I think is a good idea. It helps distinguish
> between "an enum that has multiple entries, some of which have
> the same values" and "an enum whose entry 'a' can also be refered
> to as 'b', fo rconvenience".
>
> For starters, the distinction would be self documenting.
>
> Second, once you involve things like `EnumMembers`, it becomes a
> pretty interesting distinction to make.
But then things get weird, because EnumMembers wouldn't return everything, and
presumably final switch wouldn't have every member. It's not necessarily a bad
idea, but I think that it would have to be thought through very thoroughly,
and ultimately, I'm not sure that it's all that valuable.
The main feature that enums lack that would be nice would be the ability to
deprecate their members (presumably with the intention of replacing them with
new names). Aliases of some kind might be beneficial there, but again, I think
that it all would have to be thought through quite thoroughly. And ultimately,
it might be that it's just better to deprecate the entire enum at once and
come up with a new name, much as that's often not what you want to do, because
you just want to rename some of the values rather than replace the whole
thing.
So, all in all, I think that any changes to enums along these lines really
need to be thought through carefully before we consider doing anything, and
whatever changes we make would have to pull their weight (which Andrej's
suggestion doesn't do - and he seems to now agree with us on that).
- Jonathan M Davis
Re: Enum alias members: yay or nay?
On Sunday, 8 September 2013 at 23:24:32 UTC, Walter Bright wrote: On 9/7/2013 9:45 PM, Daniel Murphy wrote: tl;dr I don't think this justifies a new feature. A lint rule, absolutely. A warning, possibly. But not a new feature. I agree with the reasoning of the others here - not worth it. It seems my last message got lost, but I reached the same conclusion: Anyway after some more thought I think it's overkill, since not resetting the counter could be just as confusing as resetting it.
Re: Enum alias members: yay or nay?
On 9/7/2013 9:45 PM, Daniel Murphy wrote: tl;dr I don't think this justifies a new feature. A lint rule, absolutely. A warning, possibly. But not a new feature. I agree with the reasoning of the others here - not worth it.
Re: Enum alias members: yay or nay?
On 9/8/13, Andrej Mitrovic wrote: > Does the alias member feature pull its weight? Or is it overkill and > we should drop it? Anyway after some more thought I think it's overkill, since not resetting the counter could be just as confusing as resetting it.
Re: Enum alias members: yay or nay?
On 9/8/13, monarch_dodra wrote:
> enum S
> {
> a,
> alias b = a,
> }
>
> This would create an enum with a *single* entry, which can be
> accessed via two different names.
Yeah I've thought about this separately from that enhancement, I think
this feature *alone* would help to avoid issues with duplicate enum
members, e.g. where you're generating a switch case table via
EnumMembers. You can use NoDuplicates to for this, but it still
doesn't change that the following fails:
enum E
{
a,
b = a
}
void main()
{
static assert(is(E.a == E.b)); // fails
}
This may or may not be an issue.. Also, currently we don't have a way
to get the length of enum members except via EnumMembers, but if we
ever implemented "E.length" then maybe the alias behavior could be
useful.
Re: Enum alias members: yay or nay?
On 9/8/13, Daniel Murphy wrote: > Two strategies that will prevent this bug are: > > 1) Put the 'alias' members directly after the member they reference > 2) Put the 'alias' members at the end There is another strategy, which I currently use, is to explicitly initialize all enums. As for #1 or #2, that won't work for me, because I want the alias to be in a location that makes sense in the documentation, this is why "click" is right above "double_click", "triple_click", etc.
Re: Enum alias members: yay or nay?
On Sunday, 8 September 2013 at 09:43:02 UTC, Kenji Hara wrote:
I also agree that the compiler enhancement is overkill.
Kenji Hara
Let's not throw this away quite yet: There is *another*
fundamental difference:
enum S
{
a,
b = a,
}
This creates an enum with *two* entries.
enum S
{
a,
alias b = a,
}
This would create an enum with a *single* entry, which can be
accessed via two different names.
*This*, in itself, I think is a good idea. It helps distinguish
between "an enum that has multiple entries, some of which have
the same values" and "an enum whose entry 'a' can also be refered
to as 'b', fo rconvenience".
For starters, the distinction would be self documenting.
Second, once you involve things like `EnumMembers`, it becomes a
pretty interesting distinction to make.
I for one support this proposal (but not necessarilly for the
"counter" proposal: I think it is "a nice plus", but not an
essential feature).
Re: Enum alias members: yay or nay?
2013/9/8 Jonathan M Davis > On Sunday, September 08, 2013 14:45:21 Daniel Murphy wrote: > > Honestly, it seems like overkill to me. > > > > I can understand it would be an annoying bug to hit, but I doubt it > would be > > that common. > > Agreed. I also agree that the compiler enhancement is overkill. Kenji Hara
Re: Enum alias members: yay or nay?
On 9/8/13 8:08, Andrej Mitrovic wrote:
I've recently ran into a bug that was very hard to track down for me.
I've had a good set of unittests, but I kept getting the wrong results
out of my functions, which was very bizarre.
To boil it down, when you introduce a member in an enum which
initializes itself to another member of that enum, the next member
after it will be initialized to + 1 of the previous member. So:
enum E
{
a,
b = a,
c
}
Here, a equals 0, b equals 0 as well, and c equals b + 1, so it's 1.
There's nothing problematic here.
The real problem appears when you need to make sure each member is
unique, except for any members which are either convenience members or
are deprecated members (there's no "deprecated" member feature
yet[2]).
Here's an example:
enum MouseAction
{
///
press,
///
release,
/** Convenience - equal to $(D press). */
click = press,
///
double_click,
}
Notice how introducing the convenience member has re-set the enum
initializer counter, meaning double_click will be equal to (click +
1), which is really (press + 1), which becomes (1). But (1) is also
the intializer for "release", so by mistake I made "double_click"
equal "release", and hence my bug.
So to work around this, I thought about introducing an alias feature to enums:
enum MouseAction
{
press,
release,
alias click = press, // does not reset the counter!
double_click, // equals release + 1
}
The alias member would not re-set the counter and instead the next
non-alias member would initialize itself to the previous non-alias
member + 1.
This would also lend itself well with the "deprecated" keyword, where
we could add deprecated aliases for old enum members when we want to
rename the members. For example, if you want to rename a Phobos enum
you can currently do :
enum SpanMode
{
shallow,
deep,
depth = deep; /// $(RED Deprecated, please use .deep)
breadth,
}
I won't deny the possibility of human error here, but I don't think it's
worth complicating the language for this case. It's easily fixed by
putting the 'alias' after the 'good' enum value.
The case for 'deprecate' is much stronger, though. It's impossible to
deprecate enum values now.
Re: Enum alias members: yay or nay?
On Sunday, September 08, 2013 14:45:21 Daniel Murphy wrote:
> "Andrej Mitrovic" wrote in message
> news:mailman.980.1378598947.1719.digitalmar...@puremagic.com...
>
> > enum MouseAction
> > {
> >
> >press,
> >
> >release,
> >
> >alias click = press, // does not reset the counter!
> >
> >double_click, // equals release + 1
> >
> > }
> >
> > Does the alias member feature pull its weight? Or is it overkill and
> > we should drop it?
>
> Honestly, it seems like overkill to me.
>
> I can understand it would be an annoying bug to hit, but I doubt it would be
> that common.
>
> Two strategies that will prevent this bug are:
>
> 1) Put the 'alias' members directly after the member they reference
> 2) Put the 'alias' members at the end
Agreed. Both of those easily avoid this problem, and I would have argued that
the examples which ran into this problem were bad practice precisely because
they didn't do either of these two. Personally, I would argue that an enum
that is intended to be have the same value as another should pretty much
always be listed right after the one it shares a value with.
- Jonathan M Davis
Re: Enum alias members: yay or nay?
"Andrej Mitrovic" wrote in message
news:mailman.980.1378598947.1719.digitalmar...@puremagic.com...
> enum MouseAction
> {
>press,
>
>release,
>
>alias click = press, // does not reset the counter!
>
>double_click, // equals release + 1
> }
>
> Does the alias member feature pull its weight? Or is it overkill and
> we should drop it?
>
Honestly, it seems like overkill to me.
I can understand it would be an annoying bug to hit, but I doubt it would be
that common.
Two strategies that will prevent this bug are:
1) Put the 'alias' members directly after the member they reference
2) Put the 'alias' members at the end
A possible alternative would be to warn/error on having two implicitly
initialized enum members that get the same value.
eg
enum E
{
a,
b,
c = a,
d, // Warning: Enum member E.d has been automatically assigned the same
value as E.a
}
tl;dr I don't think this justifies a new feature. A lint rule, absolutely.
A warning, possibly. But not a new feature.
Re: Enum alias members: yay or nay?
Andrej Mitrovic wrote: > Or is it overkill and we should drop it? This source of errors is the exact counterpart of a missing `break' in a `switch'-statement, which may result in an unintended fallthrough, thereby resetting some counter. Therefore syntactical assurance of correctness in `enum's should be accompanied by syntactical assurance in `switch'es. Until then and in those cases, where the implicit counter is indead decremented, an error might be exclaimed, because the subtraction indicates that the coder has lost the conciousness to code a _linear ordered_ enum. -manfred
