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A dead simple parser package for Go

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V2

This is version 2 of Participle.

It can be installed with:

$ go get github.com/alecthomas/participle/v2@latest

The latest version from v0 can be installed via:

$ go get github.com/alecthomas/participle@latest

<a name='Introduction'></a>Introduction

The goal of this package is to provide a simple, idiomatic and elegant way of defining parsers in Go.

Participle's method of defining grammars should be familiar to any Go programmer who has used the encoding/json package: struct field tags define what and how input is mapped to those same fields. This is not unusual for Go encoders, but is unusual for a parser.

Tutorial

A tutorial is available, walking through the creation of an .ini parser.

Tag syntax

Participle supports two forms of struct tag grammar syntax.

The easiest to read is when the grammar uses the entire struct tag content, eg.

Field string `@Ident @("," Ident)*`

However, this does not coexist well with other tags such as JSON, etc. and may cause issues with linters. If this is an issue then you can use the parser:"" tag format. In this case single quotes can be used to quote literals making the tags somewhat easier to write, eg.

Field string `parser:"@ident (',' Ident)*" json:"field"`

Overview

A grammar is an annotated Go structure used to both define the parser grammar, and be the AST output by the parser. As an example, following is the final INI parser from the tutorial.

type INI struct {
  Properties []*Property `@@*`
  Sections   []*Section  `@@*`
}

type Section struct {
  Identifier string      `"[" @Ident "]"`
  Properties []*Property `@@*`
}

type Property struct {
  Key   string `@Ident "="`
  Value *Value `@@`
}

type Value struct {
  String *string  `  @String`
  Float *float64  `| @Float`
  Int    *int     `| @Int`
}

Note: Participle also supports named struct tags (eg. <code>Hello string `parser:"@Ident"`</code>).

A parser is constructed from a grammar and a lexer:

parser, err := participle.Build[INI]()

Once constructed, the parser is applied to input to produce an AST:

ast, err := parser.ParseString("", "size = 10")
// ast == &INI{
//   Properties: []*Property{
//     {Key: "size", Value: &Value{Int: &10}},
//   },
// }

Grammar syntax

Participle grammars are defined as tagged Go structures. Participle will first look for tags in the form parser:"...". It will then fall back to using the entire tag body.

The grammar format is:

The following modifiers can be used after any expression:

Notes:

Capturing

Prefixing any expression in the grammar with @ will capture matching values for that expression into the corresponding field.

For example:

// The grammar definition.
type Grammar struct {
  Hello string `@Ident`
}

// The source text to parse.
source := "world"

// After parsing, the resulting AST.
result == &Grammar{
  Hello: "world",
}

For slice and string fields, each instance of @ will accumulate into the field (including repeated patterns). Accumulation into other types is not supported.

For integer and floating point types, a successful capture will be parsed with strconv.ParseInt() and strconv.ParseFloat() respectively.

A successful capture match into a bool field will set the field to true.

Tokens can also be captured directly into fields of type lexer.Token and []lexer.Token.

Custom control of how values are captured into fields can be achieved by a field type implementing the Capture interface (Capture(values []string) error).

Additionally, any field implementing the encoding.TextUnmarshaler interface will be capturable too. One caveat is that UnmarshalText() will be called once for each captured token, so eg. @(Ident Ident Ident) will be called three times.

Capturing boolean value

By default, a boolean field is used to indicate that a match occurred, which turns out to be much more useful and common in Participle than parsing true or false literals. For example, parsing a variable declaration with a trailing optional syntax:

type Var struct {
  Name string `"var" @Ident`
  Type string `":" @Ident`
  Optional bool `@"?"?`
}

In practice this gives more useful ASTs. If bool were to be parsed literally then you'd need to have some alternate type for Optional such as string or a custom type.

To capture literal boolean values such as true or false, implement the Capture interface like so:

type Boolean bool

func (b *Boolean) Capture(values []string) error {
	*b = values[0] == "true"
	return nil
}

type Value struct {
	Float  *float64 `  @Float`
	Int    *int     `| @Int`
	String *string  `| @String`
	Bool   *Boolean `| @("true" | "false")`
}

"Union" types

A very common pattern in parsers is "union" types, an example of which is shown above in the Value type. A common way of expressing this in Go is via a sealed interface, with each member of the union implementing this interface.

eg. this is how the Value type could be expressed in this way:

type Value interface { value() }

type Float struct { Value float64 `@Float` }
func (f Float) value() {}

type Int struct { Value int `@Int` }
func (f Int) value() {}

type String struct { Value string `@String` }
func (f String) value() {}

type Bool struct { Value Boolean `@("true" | "false")` }
func (f Bool) value() {}

Thanks to the efforts of Jacob Ryan McCollum, Participle now supports this pattern. Simply construct your parser with the Union[T](member...T) option, eg.

parser := participle.MustBuild[AST](participle.Union[Value](Float{}, Int{}, String{}, Bool{}))

Custom parsers may also be defined for union types with the ParseTypeWith option.

Custom parsing

There are three ways of defining custom parsers for nodes in the grammar:

  1. Implement the Capture interface.
  2. Implement the Parseable interface.
  3. Use the ParseTypeWith option to specify a custom parser for union interface types.

Lexing

Participle relies on distinct lexing and parsing phases. The lexer takes raw bytes and produces tokens which the parser consumes. The parser transforms these tokens into Go values.

The default lexer, if one is not explicitly configured, is based on the Go text/scanner package and thus produces tokens for C/Go-like source code. This is surprisingly useful, but if you do require more control over lexing the included stateful participle/lexer lexer should cover most other cases. If that in turn is not flexible enough, you can implement your own lexer.

Configure your parser with a lexer using the participle.Lexer() option.

To use your own Lexer you will need to implement two interfaces: Definition (and optionally StringsDefinition and BytesDefinition) and Lexer.

Stateful lexer

In addition to the default lexer, Participle includes an optional stateful/modal lexer which provides powerful yet convenient construction of most lexers. (Notably, indentation based lexers cannot be expressed using the stateful lexer -- for discussion of how these lexers can be implemented, see #20).

It is sometimes the case that a simple lexer cannot fully express the tokens required by a parser. The canonical example of this is interpolated strings within a larger language. eg.

let a = "hello ${name + ", ${last + "!"}"}"

This is impossible to tokenise with a normal lexer due to the arbitrarily deep nesting of expressions. To support this case Participle's lexer is now stateful by default.

The lexer is a state machine defined by a map of rules keyed by the state name. Each rule within the state includes the name of the produced token, the regex to match, and an optional operation to apply when the rule matches.

As a convenience, any Rule starting with a lowercase letter will be elided from output, though it is recommended to use participle.Elide() instead, as it better integrates with the parser.

Lexing starts in the Root group. Each rule is matched in order, with the first successful match producing a lexeme. If the matching rule has an associated Action it will be executed.

A state change can be introduced with the Action Push(state). Pop() will return to the previous state.

To reuse rules from another state, use Include(state).

A special named rule Return() can also be used as the final rule in a state to always return to the previous state.

As a special case, regexes containing backrefs in the form \N (where N is a digit) will match the corresponding capture group from the immediate parent group. This can be used to parse, among other things, heredocs. See the tests for an example of this, among others.

Example stateful lexer

Here's a cut down example of the string interpolation described above. Refer to the stateful example for the corresponding parser.

var lexer = lexer.Must(Rules{
	"Root": {
		{`String`, `"`, Push("String")},
	},
	"String": {
		{"Escaped", `\\.`, nil},
		{"StringEnd", `"`, Pop()},
		{"Expr", `\${`, Push("Expr")},
		{"Char", `[^$"\\]+`, nil},
	},
	"Expr": {
		Include("Root"),
		{`whitespace`, `\s+`, nil},
		{`Oper`, `[-+/*%]`, nil},
		{"Ident", `\w+`, nil},
		{"ExprEnd", `}`, Pop()},
	},
})

Example simple/non-stateful lexer

Other than the default and stateful lexers, it's easy to define your own stateless lexer using the lexer.MustSimple() and lexer.NewSimple() functions. These functions accept a slice of lexer.SimpleRule{} objects consisting of a key and a regex-style pattern.

Note: The stateful lexer replaces the old regex lexer.

For example, the lexer for a form of BASIC:

var basicLexer = lexer.MustSimple([]lexer.SimpleRule{
    {"Comment", `(?i)rem[^\n]*`},
    {"String", `"(\\"|[^"])*"`},
    {"Number", `[-+]?(\d*\.)?\d+`},
    {"Ident", `[a-zA-Z_]\w*`},
    {"Punct", `[-[!@#$%^&*()+_={}\|:;"'<,>.?/]|]`},
    {"EOL", `[\n\r]+`},
    {"whitespace", `[ \t]+`},
})

Experimental - code generation

Participle v2 now has experimental support for generating code to perform lexing.

This will generally provide around a 10x improvement in lexing performance while producing O(1) garbage.

To use:

  1. Serialize the stateful lexer definition to a JSON file (pass to json.Marshal).
  2. Run the participle command (see scripts/participle) to generate go code from the lexer JSON definition. For example:
participle gen lexer <package name> [--name SomeCustomName] < mylexer.json | gofmt > mypackage/mylexer.go

(see genLexer in conformance_test.go for a more detailed example)

  1. When constructing your parser, use the generated lexer for your lexer definition, such as:
var ParserDef = participle.MustBuild[someGrammer](participle.Lexer(mylexer.SomeCustomnameLexer))

Consider contributing to the tests in conformance_test.go if they do not appear to cover the types of expressions you are using the generated lexer.

Known limitations of the code generated lexer:

Options

The Parser's behaviour can be configured via Options.

Examples

There are several examples included, some of which are linked directly here. These examples should be run from the _examples subdirectory within a cloned copy of this repository.

ExampleDescription
BASICA lexer, parser and interpreter for a rudimentary dialect of BASIC.
EBNFParser for the form of EBNF used by Go.
ExprA basic mathematical expression parser and evaluator.
GraphQLLexer+parser for GraphQL schemas
HCLA parser for the HashiCorp Configuration Language.
INIAn INI file parser.
ProtobufA full Protobuf version 2 and 3 parser.
SQLA very rudimentary SQL SELECT parser.
StatefulA basic example of a stateful lexer and corresponding parser.
ThriftA full Thrift parser.
TOMLA TOML parser.

Included below is a full GraphQL lexer and parser:

package main

import (
	"fmt"
	"os"

	"github.com/alecthomas/kong"
	"github.com/alecthomas/repr"

	"github.com/alecthomas/participle/v2"
	"github.com/alecthomas/participle/v2/lexer"
)

type File struct {
	Entries []*Entry `@@*`
}

type Entry struct {
	Type   *Type   `  @@`
	Schema *Schema `| @@`
	Enum   *Enum   `| @@`
	Scalar string  `| "scalar" @Ident`
}

type Enum struct {
	Name  string   `"enum" @Ident`
	Cases []string `"{" @Ident* "}"`
}

type Schema struct {
	Fields []*Field `"schema" "{" @@* "}"`
}

type Type struct {
	Name       string   `"type" @Ident`
	Implements string   `( "implements" @Ident )?`
	Fields     []*Field `"{" @@* "}"`
}

type Field struct {
	Name       string      `@Ident`
	Arguments  []*Argument `( "(" ( @@ ( "," @@ )* )? ")" )?`
	Type       *TypeRef    `":" @@`
	Annotation string      `( "@" @Ident )?`
}

type Argument struct {
	Name    string   `@Ident`
	Type    *TypeRef `":" @@`
	Default *Value   `( "=" @@ )`
}

type TypeRef struct {
	Array       *TypeRef `(   "[" @@ "]"`
	Type        string   `  | @Ident )`
	NonNullable bool     `( @"!" )?`
}

type Value struct {
	Symbol string `@Ident`
}

var (
	graphQLLexer = lexer.MustSimple([]lexer.SimpleRule{
		{"Comment", `(?:#|//)[^\n]*\n?`},
		{"Ident", `[a-zA-Z]\w*`},
		{"Number", `(?:\d*\.)?\d+`},
		{"Punct", `[-[!@#$%^&*()+_={}\|:;"'<,>.?/]|]`},
		{"Whitespace", `[ \t\n\r]+`},
	})
	parser = participle.MustBuild[File](
		participle.Lexer(graphQLLexer),
		participle.Elide("Comment", "Whitespace"),
		participle.UseLookahead(2),
	)
)

var cli struct {
	EBNF  bool     `help"Dump EBNF."`
	Files []string `arg:"" optional:"" type:"existingfile" help:"GraphQL schema files to parse."`
}

func main() {
	ctx := kong.Parse(&cli)
	if cli.EBNF {
		fmt.Println(parser.String())
		ctx.Exit(0)
	}
	for _, file := range cli.Files {
		r, err := os.Open(file)
		ctx.FatalIfErrorf(err)
		ast, err := parser.Parse(file, r)
		r.Close()
		repr.Println(ast)
		ctx.FatalIfErrorf(err)
	}
}

Performance

One of the included examples is a complete Thrift parser (shell-style comments are not supported). This gives a convenient baseline for comparing to the PEG based pigeon, which is the parser used by go-thrift. Additionally, the pigeon parser is utilising a generated parser, while the participle parser is built at run time.

You can run the benchmarks yourself, but here's the output on my machine:

BenchmarkParticipleThrift-12    	   5941	   201242 ns/op	 178088 B/op	   2390 allocs/op
BenchmarkGoThriftParser-12      	   3196	   379226 ns/op	 157560 B/op	   2644 allocs/op

On a real life codebase of 47K lines of Thrift, Participle takes 200ms and go- thrift takes 630ms, which aligns quite closely with the benchmarks.

Concurrency

A compiled Parser instance can be used concurrently. A LexerDefinition can be used concurrently. A Lexer instance cannot be used concurrently.

Error reporting

There are a few areas where Participle can provide useful feedback to users of your parser.

  1. Errors returned by Parser.Parse*() will be:
    1. Of type Error. This will contain positional information where available.
    2. May either be ParseError or lexer.Error
  2. Participle will make a best effort to return as much of the AST up to the error location as possible.
  3. Any node in the AST containing a field Pos lexer.Position 1 will be automatically populated from the nearest matching token.
  4. Any node in the AST containing a field EndPos lexer.Position 1 will be automatically populated from the token at the end of the node.
  5. Any node in the AST containing a field Tokens []lexer.Token will be automatically populated with all tokens captured by the node, including elided tokens.

These related pieces of information can be combined to provide fairly comprehensive error reporting.

Comments

Comments can be difficult to capture as in most languages they may appear almost anywhere. There are three ways of capturing comments, with decreasing fidelity.

The first is to elide tokens in the parser, then add Tokens []lexer.Token as a field to each AST node. Comments will be included. This has the downside that there's no straightforward way to know where the comments are relative to non-comment tokens in that node.

The second way is to not elide comment tokens, and explicitly capture them at every location in the AST where they might occur. This has the downside that unless you place these captures in every possible valid location, users might insert valid comments that then fail to parse.

The third way is to elide comment tokens and capture them where they're semantically meaningful, such as for documentation comments. Participle supports explicitly matching elided tokens for this purpose.

Limitations

Internally, Participle is a recursive descent parser with backtracking (see UseLookahead(K)).

Among other things, this means that Participle grammars do not support left recursion. Left recursion must be eliminated by restructuring your grammar.

EBNF

The old EBNF lexer was removed in a major refactoring at 362b26 -- if you have an EBNF grammar you need to implement, you can either translate it into regex-style lexer.Rule{} syntax or implement your own EBNF lexer you might be able to use the old EBNF lexer -- as a starting point.

Participle supports outputting an EBNF grammar from a Participle parser. Once the parser is constructed simply call String().

Participle also includes a parser for this form of EBNF (naturally).

eg. The GraphQL example gives in the following EBNF:

File = Entry* .
Entry = Type | Schema | Enum | "scalar" ident .
Type = "type" ident ("implements" ident)? "{" Field* "}" .
Field = ident ("(" (Argument ("," Argument)*)? ")")? ":" TypeRef ("@" ident)? .
Argument = ident ":" TypeRef ("=" Value)? .
TypeRef = "[" TypeRef "]" | ident "!"? .
Value = ident .
Schema = "schema" "{" Field* "}" .
Enum = "enum" ident "{" ident* "}" .

Syntax/Railroad Diagrams

Participle includes a command-line utility to take an EBNF representation of a Participle grammar (as returned by Parser.String()) and produce a Railroad Diagram using tabatkins/railroad-diagrams.

Here's what the GraphQL grammar looks like:

EBNF Railroad Diagram

Footnotes

  1. Either the concrete type or a type convertible to it, allowing user defined types to be used. ↩ ↩2