[Haskell-cafe] [ANN] faster-megaparsec

[Daneel Yaitskov]

Hi,

Recently I was improving performance of simple-sql-parser library which
sits on top of megaparsec.
Main issue was memory consumption, not speed. Machine generated SQL query
might weigh megabytes.
I got success with a deduplicating cache for lexer tokens.

A complex parser produces very chunky lazy text.
To avoid traversing these linked lists (cache locality) and copying to
strict one
- offsets in the input Text  are analyzed before and after success return
from parser => shallow substring which is deduplicated with high
probability and no extra memcpy.

Another interesting idea, I would like to try, is to use ByteString for
text input and mmap input byte string.
SQL query is 99% ASCII only and all keywords are ASCII bytes.
So knowing that constraint parser doesn't spend time on reconstructing
characters out of impossible multibyte input,
when it expects keywords.

Megaparsec doesn't have digital search tree, which help a lot with avoiding
back tracking.

Thanks

On Tue, Nov 8, 2022 at 3:32 PM Olaf Klinke <olf at aatal-apotheke.de> wrote:

> Dear Haskellers,
>
> megaparsec [1] is a parsing library with excellent error
> reporting. Being able to emit decent parse error messages is traded off
> against speed, however. Wouldn't it be nice to have a faster parser for
> your daily parsing needs, but obtain megaparsec's informative error
> messages for the rare cases when things go wrong?
>
> Thanks to megaparsec's parser type class, MonadParsec, you can have
> both: I wrote a MonadParsec instance for StateT s Maybe (s being the
> input stream type such as Text or ByteString) which does not do all the
> bookkeeping ParsecT needs to do. If you manage to write your parser
> only using the type class primitives, then you can specialize your
> parser to StateT s Maybe. Only if that returns Nothing specialize again
> to ParsecT Void s and parse the input again, thus obtaining a decent
> error message. The package faster-megaparsec [1,2] provides a
> convenient function to do just that.
>
> Of course StateT s Maybe can not provide all the features ParsecT has:
> * StateT s Maybe is always backtracking.
> * It does not know the offset in the stream.
> * It must hold the entire input stream in memory for a second pass,
>   so no garbage collecting the consumed input while parsing.
> So if your parsing relies on non-backtracking choice, stream offset or
> garbage-collecting input while parsing, you can not use faster-
> megaparsec as a drop-in replacement.
>
> Happy parsing!
> Olaf
>
> [1] https://hackage.haskell.org/package/megaparsec
> [2] https://hackage.haskell.org/package/faster-megaparsec
> [3] https://hub.darcs.net/olf/faster-megaparsec
>
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-- 

Best regards,
Daniil Iaitskov
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