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 5-10 times faster than attoparsec or megaparsec. On larger examples with heavier use of source positions and spans and/or indentation parsing, the performance difference is greater. 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 systems are currently supported as the host machine. This may change in the future. 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 moderate overhead in performance and code size compared to Basic. In microbenchmarks and small parsers, the performance difference between Basic and Stateful is more up to the whims of GHC and LLVM, and is a bit more “random”.

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 9.8.3. -O2 -fllvm with flatparse-0.5.2.1. Executed on Intel 1345U CPU. Uses nightly-2024-11-11 Stackage snapshot for the involved packages.

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