[antlr-interest] C# Error Recovery
lgcraymer at yahoo.com
Thu Apr 17 13:49:39 PDT 2008
----- Original Message ----
From: Gavin Lambert <antlr at mirality.co.nz>
To: Loring Craymer <lgcraymer at yahoo.com>; Johannes Luber <jaluber at gmx.de>
Cc: antlr-interest at antlr.org
Sent: Thursday, April 17, 2008 2:07:29 AM
Subject: Re: [antlr-interest] C# Error Recovery
At 10:12 17/04/2008, Loring Craymer wrote:
>Most of the cases call "getTokenErrorDisplay() which cannot be
>pushed into the exceptions, and none of the error messages are
>localized (minimalist assumption: StringTemplate is not
>available for the target language). I suggest adding an "msg"
>field to RecognitionException so that Gavin's first suggestion
>can be implemented.
There's definitely no need to go adding extra fields -- they're
already there. That was basically my point -- all C# exceptions
already have a settable message, and RecognitionException is no
different (and it even already includes constructors to set that
message). The only missing piece of the puzzle is that
GetErrorMessage completely ignores the message set on the
I would have thought that Java exceptions were the
same? (Googles.) Yes, they are.
The Java RecognitionException does not have a msg field--most of its children do.
So for C#, changing the first line of
string msg = null;
string msg = e.Message;
is all that's required. Similarly for Java's
String msg = e.getMessage();
(or even getLocalizedMessage(), if you prefer.)
It would most likely be similarly minor for any other target
language that supports exceptions, and only slightly more complex
for others. (C++ actually falls halfway between the two. The
standard doesn't actually mandate that std::exception have a
constructor which can take a string and store it, so some
implementations do and some don't. But there isn't a C++ target
at the moment anyway, so the point is moot.)
But at the end of the day, having some way to raise user-specified
errors which are treated similarly to "standard" errors (as far as
error recovery and reporting go) seems like an essential thing to
What would be even cooler would be a way to link a
target-code-block-that-returns-a-string to a validating predicate,
basically cutting out the middle-man, since it seems to me like
validating predicates are the most likely source of user-specified
errors. But that's a separate issue; being able to catch the
FailedPredicateException and rethrow it as a custom one is
>I would argue against implementing Gavin's second suggestion at
>this time: the token error display adds clarity for the user
>(yeah, you can look at line 13, figure out that "message" is in
>column 2, and then try to understand what just happened; but it
>is easier to look for "message" on line 13), and the current
>approach does allow the user to override getErrorMessage() to
>support localization via StringTemplate.
The second suggestion was simply a logical follow-through from the
first -- if you're using the text in the exceptions, and that text
can usefully be constructed by the exception itself (as with
FailedPredicateException), then why not do it? Sure, still have
the central method which can be used for localisation or whatever
other overriding you want (Ter's right, it's a good idea), but why
not make the exception classes self sufficient?
I'm not really sure what you're saying about the "token error
display" though. I never suggested removing that; I was
suggesting that enough information be given to the exception
constructors so that they can generate it themselves.
You can't do that, though--the token types and display formats (since ANTLR3 defaults to Objects for tokens) are defined at the application level, while the exception classes are library items and know nothing about the application. The application's recognizer(s), though, are subclasses of BaseRecognizer and can know about application features.
But, whatever. The second idea isn't really all that important,
it's just a stylistic thing. The first, however, I think is
Actually, I thought that it was good that you raised the point--I just responded that it was not a good solution at this time, since localization is an important factor for (commercial) apps. The ANTLR 3 exception management (tool, not runtime) is pretty decent, but the runtime support is still in need of work, largely because the current implementation was designed for transportability across target language implementations and Ter could not assume the existence of StringTemplate support when he wrote it.
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