Export EBNF

[alecthomas]

It would be very handy to be able to export an EBNF representation of the parsed grammar, for documentation purposes.

[mewmew]

Similarly it would be interesting to generate participle code from an EBNF grammar. Making these two round-tripable would be really cool.

[budleigh]

+1 on having some kind of facility for building w/some arbitrary EBNF string.

[alecthomas]

Example from the GraphQL parser:

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* "}" .

From the following:

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`
}