flatparse
flatparse
is a high-performance parsing library, supporting parsing for programming languages, human-readable data and machine-readable data. The “flat” in the name refers to the ByteString
parsing input, which has pinned contiguous data, and also to the library internals, which avoids indirections and heap allocations whenever possible. flatparse
is generally lower-level than parsec
-style libraries, but it is possible to build higher-level features (such as source spans, hints, indentation parsing) on top of it, without making any compromises in performance.
LLVM
It is advised to build with -fllvm
option
when using this package, since that can result in significant speedups (20-40%
from what I’ve seen). Additionally, you can enable -fllvm
for flatparse
specifically by enabling the llvm
package flag. However, this has minor
impact, since almost all parser code will be typically inlined into modules
outside flatparse
, and compiled there.
Features and non-features
- Excellent performance. On microbenchmarks,
flatparse
is around 10 times faster than attoparsec
or megaparsec
. On larger examples with heavier use of source positions and spans and/or indentation parsing, the performance difference grows to 20-30 times. Compile times and executable sizes are also significantly better with flatparse
than with megaparsec
or attoparsec
. flatparse
internals make liberal use of unboxed tuples and GHC primops. As a result, pure validators (parsers returning ()
) in flatparse
are not difficult to implement with zero heap allocation.
- No incremental parsing, and only strict
ByteString
is supported as input. However, it can be still useful to convert from Text
, String
or other types to ByteString
, and then use flatparse
for parsing, since flatparse
performance usually more than makes up for the conversion costs.
- Only little-endian 64 bit systems are currently supported as the host machine. This may change in the future. Getting good performance requires architecture-specific optimizations; I’ve only considered the most common setting at this point. However,
flatparse
does include primitive integer parsers with specific endianness.
- Support for fast source location handling, indentation parsing and informative error messages.
flatparse
provides a low-level interface to these. Batteries are not included, but it should be possible for users to build custom solutions, which are more sophisticated, but still as fast as possible. In my experience, the included batteries in other libraries often come with major unavoidable overheads, and often we still have to extend existing machinery in order to scale to production features.
- The backtracking model of
flatparse
is different to parsec libraries, and is more close to the nom library in Rust. The idea is that parser failure is distinguished from parsing error. The former is used for control flow, and we can backtrack from it. The latter is used for unrecoverable errors, and by default it’s propagated to the top. flatparse
does not track whether parsers have consumed inputs. In my experience, what we really care about is the failure/error distinction, and in parsec
or megaparsec
the consumed/non-consumed separation is often muddled and discarded in larger parser implementations. By default, basic flatparse
parsers can fail but can not throw errors, with the exception of the specifically error-throwing operations. Hence, flatparse
users have to be mindful about grammar, and explicitly insert errors where it is known that the input can’t be valid.
flatparse
comes in two flavors: FlatParse.Basic
and FlatParse.Stateful
. Both support a custom error type. Also, both come in three modes, where we can respectively run IO
actions, ST
actions, or no side effects. The modes are selected by a state token type parameter on the parser types.
FlatParse.Basic
only supports the above features. If you don’t need indentation
parsing, this is sufficient.
FlatParse.Stateful
additionally supports a built-in Int
worth of internal state
and an additional custom reader environment. This can support a wide range of indentation parsing
features. There is a slight overhead in performance and code size compared to Basic
. However, in
small parsers and microbenchmarks the difference between Basic
and Stateful
is often reduced
to near zero by GHC and/or LLVM optimization.
Tutorial
Informative tutorials are work in progress. See src/FlatParse/Examples
for a lexer/parser example with acceptably good error messages.
Contribution
Pull requests are welcome. I’m fairly quick to add PR authors as collaborators.
Some benchmarks
Execution times below. See source code in bench. Compiled with GHC
9.4.4 -O2 -fllvm
. Executed on Intel 1165G7 CPU at 28W power draw. Uses
nightly-2023-02-06
Stackage snapshot for the involved packages.
benchmark |
runtime |
sexp/fpbasic |
1.93 ms |
sexp/fpstateful |
2.00 ms |
sexp/attoparsec |
21.82 ms |
sexp/megaparsec |
59.60 ms |
sexp/parsec |
79.81 ms |
long keyword/fpbasic |
96.95 μs |
long keyword/fpstateful |
95.30 μs |
long keyword/attoparsec |
2.43 ms |
long keyword/megaparsec |
5.196 ms |
long keyword/parsec |
10.02 ms |
numeral csv/fpbasic |
715.2 μs |
numeral csv/fpstateful |
555.0 μs |
numeral csv/attoparsec |
10.52 ms |
numeral csv/megaparsec |
19.77 ms |
numeral csv/parsec |
26.46 ms |
Object file sizes for each module containing the s-exp
, long keyword
and numeral csv
benchmarks.
library |
object file size (bytes) |
fpbasic |
20656 |
fpstateful |
26664 |
attoparsec |
69384 |
megaparsec |
226232 |
parsec |
117696 |