Pure enums allow using the same name but allow nonqualification with quirky behaviour

[metagn]

type
  X {.pure.} = enum
    red

  Y {.pure.} = enum
    red

echo red # red, with unused warning for Y
let newRed = red # this works too and can be operated on

when false: # undeclared identifier: 'red' for both,
  echo red.ord
  from typetraits import `$`
  echo red.type

[apahl]

I am not sure if this is about Language Design. To me this looks like a bug or a regression, since code from the manual does not work as described anymore:

type
  MyEnum {.pure.} = enum
    valueA, valueB, valueC, valueD

echo valueA

The manual states that this code should give an error because of the pure pragma, but it compiles and runs fine.

KR Axel

[haltcase]

I agree this should be considered a bug, it defeats the whole purpose of using pure on an enum.

[andreaferretti]

There was a discussion not so long ago where @Araq argued that there is no issue in allowing non-qualified access to pure enums, as long as no ambiguity arises.

Hence the condition on pure enums was relaxed a bit. The problem here is that there is an ambiguity.

In my opinion, allowing only qualified access to pure enums is a simple and clear rule to explain and to follow, and I do not see much value in allowing non qualified access.

[Araq]

Yes, you don't see much value. I've actually tried to use the old pure enums though and they are terrible. {Foo.valA, Foo.valB} violates DRY and so you need even more complex rules like "In a set construction, the prefix can be avoided. Oh and in case statements too. Oh and in array constructions..."

[Araq]

And then these rules fail to be applied in DSLs...

[andreaferretti]

@Araq I actually don't have any problems in relaxing the conditions for pure enums, the current issue is just a bug that needs to be ironed out. I was just trying to reconstruct why things changed.

Even if I don't see much value in this particular circumstance, I am mostly agnostic about whatver choice gest implemented for pure enums

[haltcase]

I'd also be alright with it after hearing the rationale, provided this bug gets fixed and the compiler gives useful error messages similar to other ambiguities.

[mratsim]

Relevant discussion in 2017 from @dom96, @PMunch, @Araq and me https://irclogs.nim-lang.org/22-10-2017.html#13:54:23

Time	Person	Text
13:54:23	dom96	I don't get it. You don't have to prefix enum names even when {.pure.} is specified?
14:03:38	Araq	dom96, pure enum field names get their own scope that is queried if the name was not found in any other scope
14:04:09	Araq	so as long as nothing else clashes with it you can use these names without the Enum. prefix
14:04:35	dom96	So now the only difference is that I cannot use a prefix for non-pure enums
14:04:56	Araq	er, sure you can
14:04:56	dom96	This behaviour should be implemented for non-pure enums and pure enums should have been left alone
14:05:19	dom96	so what's the difference between pure/non-pure enums now?
14:05:21	Araq	well the point is to remove the whole distinction between the two enum types
14:05:57	dom96	er, so you've basically removed {.pure.}?
14:07:09	FromGitter	<mratsim> So this should be updated: https://nim-by-example.github.io/types/enums/
14:07:27	PMunch	Pure was nice though..
14:07:44	dom96	It is nice.
14:07:48	Araq	I unified the two concepts, not sure why you don't understand it
14:07:57	dom96	Which is why I'm trying to understand what the purpose of Araq's changes is
14:08:09	FromGitter	<mratsim> It should be named "qualified" instead of pure though.
14:08:09	Araq	"pure was nice though", omg you can still prefix them all you want
14:08:23	PMunch	But you can't force people to..
14:08:27	dom96	indeed
14:08:32	PMunch	I thought that was the entire point of pure
14:08:39	dom96	precisely
14:08:43	Araq	the point was to prevent clashes
14:08:50	PMunch	To say: this symbol doesn't make any sense without the enum name
14:09:10	dom96	But you've just said that you can prefix non-pure enums too
14:09:21	dom96	so if there are clashes you can prevent them
14:09:31	PMunch	And what happens if you have "OneEnum.test" and "AnotherEnum.test", both marked as pure. What will test refer to?
14:09:38	FromGitter	<mratsim> you will still get "ambiguous call" though? I guess itÔøΩs the same discussion as {.this:this.} pragma
14:09:56	Araq	it never was about enforcing rules, it was about preventing clashes
14:10:14	Araq	nobody wants these rules anyway:
14:10:17	Araq	case e
14:10:24	Araq	of SomeEnum.valueA:
14:10:28	Araq	of SomeEnum.valueB:
14:10:38	dom96	I do
14:10:44	Araq	of SomeEnum.valueC: echo "just kill me already"
14:10:57	dom96	Sometimes it makes sense to have a short enum value name
14:11:04	dom96	and then a qualifying prefix that is long
14:11:19	dom96	In that case I want to force the user of my library to prefix the enum
14:11:46	Araq	yeah, that is exactly what Nim is NOT about
14:11:55	Araq	your users are adults.
14:13:22	Araq	if foo in {SomeEnum.short, shortToo, shorter}
14:13:38	Araq	# ^ context is everything
14:16:27	PMunch	Hmm, I still feel this should be handled somehow else..
14:16:51	dom96	okay, then we should get the ability to enforce this when importing enums
14:17:36	dom96	btw my book mentions the pure pragma
14:18:19	PMunch	TBH, that shouldn't be a factor in the language design
14:18:30	PMunch	Of course it's not good, but such is life..
14:19:22	dom96	True.
14:20:01	Araq	people argued that every enum should be pure and then we should have special rules in 'case' and set literals to be able to avoid the prefix
14:21:06	PMunch	That makes more sense..
14:21:13	Araq	but this doesn't work well, what about [value: "a", valueB: "b"] array constructions
14:22:51	PMunch	"SomeEnum.[value: val1, valueB: val2]" for "type SomeEnum = enum val1, val2"
14:23:05	PMunch	Same for set
14:23:49	PMunch	Case is the only special thing
14:24:37	Araq	that's not true at all
14:25:41	Araq	in Karax for example the problem comes up all the time too, we want a CSS "enum" without the prefixes that yet doesn't clash
14:26:11	Araq	registerEvent(onclick, proc ...)
14:26:28	Araq	# ok, onclick is nothing unexpected here
14:26:55	Araq	registerEvent(Event.onclick, proc ...) # some other onclick clashed so I need to write it out
14:27:23	dom96	It's hard to tell that onclick is an enum type
14:27:33	dom96	it could be a procedure
14:27:41	Araq	it's superior to your "I want to enforce it everywhere until I figured out that's a horrible idea"
14:27:43	dom96	forcing the prefix makes it clearer
14:27:49	dom96	It makes the code far more readable
14:27:54	PMunch	Agreed
14:28:03	Araq	no, I disagree and Nim doesn't work this way
14:28:20	miran	prefix sometimes makes things clearer
14:28:33	Araq	everything else in the language doesn't work this way either anyway
14:28:41	dom96	But sadly this indeed isn't how Nim was designed
14:29:37	Araq	Event.onclick # you don't know anything about enums here, could also be ModuleName.onclick
14:29:38	PMunch	Fair enough

I still think if the pragma is here to stay, {.pure.} should be renamed {.qualified.}, and {.qualified.} should always require the enum prefix. {.pure.} is not really descriptive of what the pragma does. (And maybe we should have import qualified Foo instead of from Foo import nil)

But, I think this is unnecessary policing by the compiler, assuming this is an internal enum, you're free to use your preferred style (always qualifying, or qualifying when necessary), if it's a public module, don't force your style on other projects.

All in all I think this should be handled in a Nim linter, with options chosen by the library authors, not in the compiler.

[cooldome]

Just an opinion,
I like that you don't to prefix everything with MyEnum. it saves a lot of strokes. I think just error message needs to be improved for example above. Nim should say that red is ambiguous.

[metagn]

I think the focus was mishandled in this issue, it's also a matter of not having to deal with hungarian notation or the full name of the enum in the enum member like SDL_SCANCODE_UNKNOWN instead of Scancode.Unknown. To better express my point:

type StreetLight {.pure.} = enum
  red, yellow, green

let foo = StreetLight.red

instead of

type StreetLight = enum
  stlRed, stlYellow, stlGreen

let foo = stlRed