On 22-sep-05, at 11:20, Tomasz Zielonka wrote:
On 9/22/05, Simon Marlow <simonmar@xxxxxxxxxxxxx> wrote:
> The error message on this is really poor: parse error on input
> `length'
>
> An addition like "expecting '('" might be more helpful.
Not easy to do - some significant work on Happy would be required (or
rewriting GHC's parser, which is even more unlikely).
However we *would* like to do some simple error recovery in the parser
at some point. The main reason for doing this is to get better
results
from checking partially complete modules in Visual Haskell or hIDE.
How about using the parser made by Arthur Baars, trading speed for
better error messages?
Maybe even the speed issue could be solved.
Speed, error messages and recovery were not my main concern when
writing the parser. I think a lot can be gained by optimizing the
combinator-based parsers. However this is no fun. I had to do some
left-factorization to gain reasonable efficiency(at first the speed was
sometimes worse than 1 token per minute :-)). For a complex language
such as Haskell, the code resulting from factorization is really messy
and the notational elegance of the combinators is lost. Maybe someone
can write a parser-generator that uses Doaitses parsing-engine as a
back-end :-)
A main feature of Doaitses parsing-combinators is that they can recover
from an error in the input. This is done by considering a number of
alternatives to "repair" the input, and taking the "best" one to
continue parsing. This proces can be improved by tweaking the insert
and delete costs of certain symbols. Furthermore the error messages can
be customized as with Parsec.
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