XParsec

XParsec is a parser combinator library for F#, with several important differences from FParsec:

• Generalization over collection and token types

With XParsec all common contiguous collections string 'T array ResizeArray<'T> ImmutableArray<'T> and ReadOnlyMemory<'T> can be parsed with essentially the same code.

• Pure F# implementation

F# is a great .NET language, and with the Fable compiler, a powerful JavaScript language too. XParsec's pure F# implementation provides a robust, easy to use parsing library for Fable target languages.

• More Performant

XParsec uses newer F# & .NET features like [<InlineIfLambda>], Span<'T>, and struct unions to compete with imperative parsing libraries while remaining terse and easy to reason about. Parsing a single large JSON file takes roughly half the time of FParsec with ~1/6 the allocations.

Method	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated
XParsecJson	30.61 ms	0.196 ms	0.164 ms	718.7500	625.0000	-	35.61 MB
FParsecJson	61.09 ms	0.267 ms	0.208 ms	4555.5556	777.7778	222.2222	208.37 MB

• Simplified operator precedence parsing
• No line number tracking by default. A separate line ending parser is available for generating detailed error messages.
• <|>, choice, and choiceL always backtrack. There is no attempt combinator because every alternative already behaves like one. See Migrating from FParsec below.

Detailed Error Messages

The quick brown fox jumps over the lazy dog.
    ^ At index 4 (Ln 1, Col 5)
All choices failed.
├───Expected 'a'
└───All choices failed.
    ├───Unexpected 'q'
    └───Expected 'c'

Real-world usage

XParsec is capable of parsing extremely complex grammars, including F# itself. I'm documenting building a complete F# language parser with XParsec in an ongoing blog series, starting with the prologue.

Migrating from FParsec

XParsec is API-shaped to be familiar to FParsec users, but a few semantics are deliberately different. Two are worth calling out:

• <|>, choice, and choiceL always backtrack. FParsec's alternative combinators only continue to the next branch if the previous branch failed without consuming input; you wrap branches in attempt to force backtracking after partial consumption. XParsec's alternatives save the reader position before running each branch and restore it on any failure. There is no attempt combinator — drop attempt calls when migrating; every alternative already behaves like one. The save is a struct copy, so the always-backtrack default is cheap.
• pzero produces a structural Empty error. Aggregating combinators (<|>, choice, manyTill, …) filter Empty siblings before constructing nested errors, so pzero <|> p propagates only p's error rather than wrapping a blank stub. The default formatter renders nothing for Empty.

If you need "fail in place when input was consumed" behaviour for a specific alternative, use notFollowedBy/<?> to gate or relabel rather than reaching for attempt.

Running Tests

.NET

dotnet test

Fable JS

npm run test