| 1 | <p align="center"> |
| 2 | <img src="https://raw.githubusercontent.com/mermaid-js/mermaid/develop/docs/public/favicon.svg" height="150"> |
| 3 | |
| 4 | </p> |
| 5 | <h1 align="center"> |
| 6 | Mermaid Parser |
| 7 | </h1> |
| 8 | |
| 9 | <p align="center"> |
| 10 | Mermaid parser package |
| 11 | <p> |
| 12 | |
| 13 | [](https://www.npmjs.com/package/@mermaid-js/parser) |
| 14 | |
| 15 | ## How the package works |
| 16 | |
| 17 | The package exports a `parse` function that has two parameters: |
| 18 | |
| 19 | ```ts |
| 20 | declare function parse<T extends DiagramAST>( |
| 21 | diagramType: keyof typeof initializers, |
| 22 | text: string |
| 23 | ): T; |
| 24 | ``` |
| 25 | |
| 26 | ## How does a Langium-based parser work? |
| 27 | |
| 28 | ```mermaid |
| 29 | sequenceDiagram |
| 30 | actor Package |
| 31 | participant Module |
| 32 | participant TokenBuilder |
| 33 | participant Lexer |
| 34 | participant Parser |
| 35 | participant ValueConverter |
| 36 | |
| 37 | |
| 38 | Package ->> Module: Create services |
| 39 | Module ->> TokenBuilder: Override or/and<br>reorder rules |
| 40 | TokenBuilder ->> Lexer: Read the string and transform<br>it into a token stream |
| 41 | Lexer ->> Parser: Parse token<br>stream into AST |
| 42 | Parser ->> ValueConverter: Clean/modify tokenized<br>rules returned value |
| 43 | ValueConverter -->> Package: Return AST |
| 44 | ``` |
| 45 | |
| 46 | - When to override `TokenBuilder`? |
| 47 | - To override keyword rules. |
| 48 | - To override terminal rules that need a custom function. |
| 49 | - To manually reorder the list of rules. |
| 50 | |
| 51 | - When to override `Lexer`? |
| 52 | - To modify input before tokenizing. |
| 53 | - To insert/modify tokens that cannot or have not been parsed. |
| 54 | |
| 55 | - When to override `LangiumParser`? |
| 56 | - To insert or modify attributes that can't be parsed. |
| 57 | |
| 58 | - When to override `ValueConverter`? |
| 59 | - To modify the returned value from the parser. |
| 60 | |