> On 14 Jan 2020, at 18:42, Guido van Rossum <gu...@python.org> wrote:
>
> On the subject of replacing the current parser, I am actively working on
> that. See GitHub.com/gvanrossum/pegen.
Will that allow me to write this?
if a > 3
and x < 6:
doit()
Barry
>
> On Tue, Jan 14, 2020 at 10:32 Andrew Barnert via Python-ideas
> <python-ideas@python.org <mailto:python-ideas@python.org>> wrote:
> On Jan 14, 2020, at 05:22, Σταύρος Ντέντος <stde...@gmail.com
> <mailto:stde...@gmail.com>> wrote:
> >
> > Hello there,
> >
> > If I have simply missed a double colon starting a for loop
> >
> > File "./bbq.py", line 160
> > for config_file in config_files
> > ^
> > SyntaxError: invalid syntax
> >
> > the message is not as straightforward.
>
> I think almost everyone would prefer it if the compiler could say
> “SyntaxError: missing colon at end of a compound statement header” or
> something more useful.
>
> And that probably goes even more for this case:
>
> spam = eggs(cheese, (foo, bar)
> cheese = spam*2
>
> The problem is to come up with a rule that could be applied to detect these
> cases given the information the simple LR(1) parser has available at the time
> of failure. I suspect there’s no way to do that without radically changing
> the parser architecture, keeping track of a lot more state, or partially
> re-parsing things in the error handler. (If it were easy, Guido would have
> done it back in 1.x.)
>
> But maybe there’s a way to heuristically detect that these problems are
> _likely_ causes of the error (without having to be as ridiculously
> complicated as what Clang does with C++ code)? If you could find a way to
> make the error say “SyntaxError: invalid syntax (possibly missing colon at
> end of compound statement header)” in most simple “forgot the colon” cases
> and very few other cases, without massively disrupting everything, I think
> people would be happy with that.
>
> You might even be able to take advantage of re-parsing without having to
> solve all the problems that go with that. For example, technically, you can’t
> even access the last logical line to reparse; practically, you can get it in
> the same cases the traceback can print it, and those are probably the only
> cases you need to heuristically improve the error handling. You could even
> maybe do a quick & dirty proof of concept in Python in an import hook, if you
> don’t want to dive into the middle of the C compiler code.
>
> As an alternative, there are lots of projects to use more powerful parser
> algorithms on Python. There’s not much call to replace CPython’s parser,
> because there aren’t any benefits to offset the costs. (At least assuming
> that the language is going to stay LR(1), to make it easy to parse in your
> head.) But if you could improve most of the most annoying error handling
> cases, that might be a different story. And these might also be easier to
> play with. (Some have pure Python implementations, and even the ones in C
> aren’t embedded in the middle of the compiler code.) IIRC, early Java did
> something clever with a GLR parser that has LR(1) performance on all valid
> code and strictly bounded complexity on error recovery (so it may get as bad
> as worst-case cubic, but cubic on N<=5 so who cares) so they could usually
> produce error messages as good as most C compilers without the horrible mess
> of parsing that most C compilers need.
>
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