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 exectuable 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 specific big-endian parsers for primitive integer types.
• 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.

• 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 Int 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 8.10.7 -O2 -fllvm. Executed on Intel 1165G7 CPU at 28W power draw.

benchmark	runtime
sexp/fpbasic	1.625 ms
sexp/fpstateful	1.815 ms
sexp/attoparsec	21.75 ms
sexp/megaparsec	33.12 ms
sexp/parsec	98.65 ms
long keyword/fpbasic	115.9 μs
long keyword/fpstateful	117.7 μs
long keyword/attoparsec	2.955 ms
long keyword/megaparsec	2.185 ms
long keyword/parsec	29.91 ms
numeral csv/fpbasic	549.3 μs
numeral csv/fpstateful	595.5 μs
numeral csv/attoparsec	10.82 ms
numeral csv/megaparsec	6.581 ms
numeral csv/parsec	39.33 ms

Object file sizes for each module containing the s-exp, long keyword and numeral csv benchmarks.

library	object file size (bytes)
fpbasic	23752
fpstateful	25920
attoparsec	93584
megaparsec	257000
parsec	134296