[enhanced enums] - end of the road?

[Jean-Baptiste Giraudeau]

Hi,

enhanced enums are basically generalized algebraic data types limited to
nullary constructors.
Maybe the focus should be shifted to proper support for GADT (in the
context of structural pattern matching)?
That way the intended use-cases could be served without the burden of
being compatible with the Enum machinery.

-
jb

ps: heterogeneous lists could properly encode collections of enhanced
enums, though the types would be something like:
HCons<EnhancedEnum<String>, HCons<EnhancedEnum<Integer>, HNil>>
(but of course not in JDK, here using FunctionalJava)

On 23. 05. 17 19:49, Maurizio Cimadamore wrote:
> Hi,
> over the last few weeks we've been carrying out experiments to test
> the feasibility of the enhanced enums feature. As described in the
> JEP, this feature is particularly powerful as it allows enums
> constants to be carrier of generic type information, which can then be
> fed back to the inference machinery.
>
> One experiment we wanted to make was to see if enhanced enums could
> make javac's own Option [1] enum better. This enum defines a bunch of
> constants, one for each supported javac command line arguments (e.g
> -classpath, -d, etc.). Furthermore, the enum defines method so that
> each constant can be parsed given the javac command line and
> processed, accordingly to some OptionHelper. Most options, along with
> the value of their arguments would simply be stored into a Map<String,
> String>, which is the backbone of the Options class [2].
>
> One problem with storing option values as Strings is that clients need
> to do the parsing. So, if an option has an integer argument, it's up
> to the client to get the value of that option off the map, parse it
> into a number and (maybe) check as to whether the range makes sense.
>
> With enhanced enums it should be possible to do better than this; more
> specifically, if enums supported generics, each option could specify a
> type argument - that is, the type of the argument that javac expects
> for that particular option.
>
> So, an option with a plain String argument would be encoded as
> Option<String>, as follows:
>
> D<String>("-d", ...)
>
> While an option for which multiple choices are available, could be
> encoded using an enum as a type-argument - for instance:
>
> PROC<ProcOption>("-proc", ...)
>
> where ProcOption would be defined as follows:
>
> enum ProcOption {
>    NONE, ONLY;
> }
>
> Finally, for an option whose argument can be a set of values, we would
> use the following encoding:
>
> G_CUSTOM<EnumSet<DebugOption>>("-g:",  ...)
>
> where DebugOption would be defined as follows:
>
> enum DebugOption {
>     LINES, VARS, SOURCE;
> }
>
> So, instead of storing all options into a Map<String, String>, we
> could store them into a Map<Option, Object>. Then, we would turn the
> Options.get method from this:
>
> public String get(String option) { ... }
>
> to something like this:
>
> public Z get(Option<Z> option) { ... }
>
> granted, there will be some unchecked operations carried out by the
> body of this method, but the map should be well-constructed by design,
> so it should be safe. What we get back is that now clients can do
> things like:
>
> boolean g_vars = options.get(Option.G_CUSTOM).contains(DebugOption.VARS);
>
> Note how we raised the expressiveness level of the client, which no
> longer has to do parsing duties (and domain conversions). So, that was
> the experiment we wanted to carry out - ultimately, this is the kind
> of stuff you'd like to be writing with enhanced enums, so this seemed
> like a reasonably comprehensive test for the feature.
>
>
> Unfortunately, the results of the experiment were not as successful as
> we'd hoped. As soon as we turned the Option enum into a generic class
> (by merely adding a type parameter in its declaration), we immediately
> started hitting dozens of compile-time errors. The errors were rather
> cryptic, all pointing to some obscure failure when calling
> EnumSet.noneOf or EnumSet.allOf with the newly generified Option
> class. In other words, code like this:
>
> EnumSet.noneOf(Option.class)
>
> Was now failing. The issue that was underpinning all these failures is
> - in retrospect - rather obvious: the following type:
>
> EnumSet<Option>
>
> is *not* a well-formed type if Option is a generic class. Why? Well,
> EnumSet is declared like this:
>
> class EnumSet<*E extends Enum<E>*> { ... }
>
> which means the type parameter has an f-bound. In concrete terms, we
> have to check that:
>
> Option <: [E:=Option]Enum<E>
>
> That is, the actual type-argument must conform to its declared bound.
> But if we follow that check, we obtain:
>
> Option <: [E:=Option]Enum<E>
> Option <: Enum<Option>
> Enum (*) <: Enum<Option>
> false
>
> (*) note that Option is now a 'raw' type - and a raw type has all
> supertypes erased, as per JLS 4.8.
>
> In other words, there's no way to write down the type of an enum set
> which contains heterogeneous options - the wildcard path doesn't help
> either:
>
> EnumSet<Option<?>>
>
> As, the above check would develop in the following way:
>
> Option<?> <: [E:=Option<?>]Enum<E>
> Option<?> <: Enum<Option<?>>
> Enum<#CAP> (**) <: Enum<Option<?>>
> false
>
> (**) the supertype of a wildcard-parameterized type is obtained by
> first capturing, and then recursing to the supertype, as per JLS 4.10.2
>
>
> In other words, generic enums are not interoperable with common data
> structures such as enum sets (and, more generally, with any f-bounded
> generic data structure).
>
> While we could just deliver the part of JEP 301 regarding sharper
> typing of enum constants and leave generic enums alone, we feel
> there's not much value into pursuing that path alone. After all, the
> benefits of enhanced enums were exactly in combining sharper typing
> with generic type information, so that enum constants could be used as
> type carriers. If generic enums are not viable, then much of the
> usefulness of this JEP is lost.
>
> It is unclear at this point in time if type system improvements (which
> we are pursuing as part of a separate activity [Dan is there a link
> for this??]) would ameliorate the situation.
>
> Until we figure this out, I suggest that we put this JEP on hold for
> the time being
>
> [1] -
> http://hg.openjdk.java.net/jdk10/jdk10/langtools/file/tip/src/jdk.compiler/share/classes/com/sun/tools/javac/main/Option.java
> [2] -
> http://hg.openjdk.java.net/jdk10/jdk10/langtools/file/tip/src/jdk.compiler/share/classes/com/sun/tools/javac/util/Options.java#l48
>
>
>
> Cheers
> Maurizio
>
>