fix #16815, showing an Enum value should use single-line repr of its type

[JeffBezanson]

No description provided.

[vtjnash]

I wonder if this hints that multiline shouldn't have been sticky on the IO stream. E.g. that there should be a show_multiline function (default is to dispatch to show), and otherwise show assumes it should print a single line representation. Are there any examples where having this property silently propagate gives better output?

[nalimilan]

Also, isn't there a simpler way of representing the name of a type? Why isn't repr or string enough?

[JeffBezanson]

Yes, I think I could have used repr or string, but when printing I prefer to use I/O functions directly instead of converting things to strings first and then printing them.

@vtjnash That's a good point; show functions almost always want to turn multiline off for printing components of an object. But I can't quite stomach adding a function just for this; it seems simpler just to use the mechanism we have.

[nalimilan]

Well, the new version is terribly obscure and complex IMHO. I don't think anybody is going to find out that's the preferred way of printing components of an object.

[JeffBezanson]

OK, I can imagine handling verbosity via separate functions. Arguably we are already doing that with print vs. show. Previously writemime in some cases was used to mean "most verbose, multiline output", but it doesn't make sense to couple that to MIME types. We could add a third basic printing function, but I think a good test is whether we can think of a good name for it. Maybe something like replshow?

I don't think it's fair to dismiss the current approach as obscure and complex. Issues on this were open for months or years. Before, we had a completely inscrutable nest of output functions, and it wasn't even possible to control output in a general way. The fact that some cases could be handled with concise code didn't prevent loud and frequent complaints.

[nalimilan]

The new system is clearly a big improvement, not doubt about that. I was just speaking about the specific case of representing the name of a type, which is very simple, and for which I think we need a simple solution. replshow sounds like it's limited to interactive use, which isn't necessarily the case. Maybe a name similar to showcompact, but referring to multiline? Or maybe showcompact could also set :multiline=>false, since that's the most common need?

[JeffBezanson]

Setting multiline to false in showcompact sounds like a good idea.

[nalimilan]

Found another case of this when printing Nullable, see #16842. I suspect this pattern is quite common when printing custom types.