Classfile API proposal to integrate basic print functionality directly to ClassModel and MethodModel

[Michael van Acken]

Am Di., 26. Juli 2022 um 21:43 Uhr schrieb Brian Goetz <
brian.goetz at oracle.com>:

>
> To be more specific about the actual classes:
>
> Mapping accepts only fragments, because it must render as single line and
> for example in XML it renders as single element with attributes.
>
>
> OK, so this unearths a previously unstated requirement: that we be able to
> turn certain maps into attributes, rather than embedded elements, when
> converting to XML.  Does this requirement correspond to anything in other
> formats?  Do we gain anything by dropping this, and formatting always to
> elements, and/or using other heuristics to determine when we can get away
> with this format optimization?
>
> In general, I'd like to unearth more of this sort of requirements.
> Looking at the current hierarchy, I can't help but feel we've not yet "hit
> bottom"; it doesn't feel factored into separate concerns yet.  But I'm
> optimistic we can get there.
>
> Here's a naive decomposition, which looks a lot like the JSON spec if you
> squint:
>
>     Value = SimpleValue(ConstantDesc) | ListValue(List<Value>) |
> MapValue(Map<String, Value>)
>
> Converting to this form is useful separate from formatting -- it provides
> a basis for query/traversal using simple string keys.  But it's not good
> enough for getting good formatting yet, because you need formatting hints
> to determine how to lay out / indent, right?  So let's consider how we
> might add these in as hints, that are more transparent. [...]
>

If it's a question of prettyprinting where indentation and line breaks
are added automatically, then there is the option to layer all
whitespace manipulation on top of the printed data.  I use this to
prettyprint Clojure data structures, which closely resemble your
`Value` production above.

The key idea goes back to the 1980 paper "Prettyprinting" by Oppen and
boils down to a single decision how to insert line breaks.  Given a
delimited group

  <start><elem><line><elem><line>...<elem><end>

where <start> and <end> are usually literals, <elem> arbitrary data,
and <line> is eventually a single whitespace or a line break.
Determine the width of the group assuming that all <line> (both in the
group itself and as part of elements) are of width one.  If the
group's width fits within the page width remaining, then print each
<line> as a single whitespace, otherwise the group's <line> separators
as line breaks.

The "one line width" flows naturally from the leaves to the root.
Combine this with indentation tracking, and the result is a simple but
capable prettyprinter targeting a given page width as a soft target.
Building on this, I print colorized side-by-side diffs of lists of
classfiles.

-- mva
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