[Parrot-users] lua: bugfixes, PIR best practices, Unicode, doc questions

[Aaron Faanes]

This email is specific to lua, but the questions are pretty basic, so
please answer if you know about precedent in other HLLs.

I've forked fperrad's lua implementation on github, so that I can
publish commits and issue pull requests, etc. It can be found here:

https://github.com/dafrito/lua

I have a few questions/comments/etc. regarding some things I've ran
into. Sorry for the digest-style format, but I didn't want to spam the
list with lots of little emails. ;)

## Bugfix regarding naive optimization

https://github.com/dafrito/lua/commit/8ea8e825c4a932af83b87a08e1d47cd858d5634f

There's an optimization that breaks some specially crafted tables. For
a quick example, consider the following snippet:

function Foo()
	return 42
end

return { Foo() }

The optimization removes a temporary variable on returned function
invocations, but this variable is used if the returned value is a
slurped table literal. As a result, the optimization's removal means
the resulting PIR code refers to a null register.

I fixed it in the following commit by removing the optimization. I
wasn't sure how to make it more specific.

As a corollary, is this sort of optimization necessary or expected in
PIR code? i would expect Parrot's optimizer to catch it.

## Unicode problems

I ran into a "Lossy conversion to single byte encoding" when trying to
parse a lua file containing some special characters (such as
'←','↑','→','↓', etc.). Changing the encoding to utf8 fixed this
issue. My patches can be found here:

https://github.com/dafrito/lua/commit/ee17af1c560171e3f0fc0264e86f73c070416212
https://github.com/dafrito/lua/commit/fcf40eed4f939c018e1b435ba85a3ce1acb7e908

This fixes the issue for compliation and for installable_luap.
installable_lua exits normally, but incorrectly displays the
characters. I imagine there's another "encoding" somewhere to find.

I don't know the performance/portability ramifications of ASCII versus
utf8 for compiling code. The official Lua implementation isn't that
clear, either, on what functionality is available.

## Refactorings in POSTGrammar

https://github.com/dafrito/lua/commits/calling_refactor

As I worked on the preceding bug, I started writing up some refactors
in POSTGrammar.tg. I started putting them in a separate
"calling_refactor" branch. The commits usually fall in one of the
following categories:

* Adding some pod and comments
* Using .local instead of $P0, $S0, etc. for long-lived variables
* Using logical names for labels: using FINISH instead of L5, for example
* Adding some noop labels for readability or consistency purposes

Please review the commit history if you're interested.

## PIR Best Practices

A few assorted best-practice questions for PIR:

* Is it preferred to use L0, L1, etc. for label names?
* Is there consensus regarding how labels should be capitalized? I
capitalized them, to distinguish them from variables.
* Is it preferred to use .local over $P0? My rule-of-thumb was if the
variable lived beyond a few lines, it should have a name.
* Is it common/frowned-upon to use labels solely for readability
purposes? I added them in a few places to group a chunk of code in a
long function together, or for symmetry reasons (to explicitly note a
branch that's accessed via fall-through)

## Documentation questions

I noticed there's ~140 places where the copyright's referenced year is
not current. Should these be updated to refer to 2011? (even if they
haven't been modified since then?) My preference is to update them all
at once, rather than updating on modification since this adds noise to
commits.

The documentation assumes, in several places, that lua resides in
$PARROT/languages/lua. Parrot's fetch_languages script does place the
git repo there, but the repository is able to stand alone. Should
lua's documentation assume it's installed in the languages
subdirectory?

--
Aaron Faanes <dafrito at gmail.com>