type2/readme.md

## Description

`type2` is a slow, but versatile reader for arbitrary context free languages (Chomsky hierarchy type 2).

Being fed with a JSON file, which specifies the language, and a string via `stdin`, it analyzes the input, decomposes it and writes its corresponding abstract syntax tree to `stdout`.

A typical call looks like this: `cat input.txt | type2 language.tp2.json`


## Working principle

```mermaid
flowchart LR
	input_string[input string]
	terminal_symbol_chain[terminal symbol chain]
	abstract_syntax_tree[abstract syntax tree]
	output_string[output string]
	
	input_string -- lexical analysis --> terminal_symbol_chain
	terminal_symbol_chain -- parsing --> abstract_syntax_tree
	abstract_syntax_tree -- JSON encoding --> output_string
```


## Language specification

A language is specified by a JSON file, which represents a value of the following type:

```
record<
	version:string,
	lexer_rules:list<
		record<
			type:string,
			parameters:map<string,any>
		>
	>,
	parser_rules:list<
		record<
			label:string,
			premise:string,
			conclusion:list<
				record<
					type:string,
					parameters:map<string,any>
				>
			>
		>
	>,
	parser_start:string
>
```

where:
- `parser_start` is one of the parser rule `label` values.
- `lexer_rules.*.type` is one of `ignore`, `void`, `boolean`, `int`, `float`, `string` (see section _Lexer rule types_)
- `parser_rules.*.conclusion.*.type` is one of `terminal`, `variable` (see section _Conclusion element types_)

The recommended extension for such files is `.tp2.json`.


### Lexer rule types

| type | meaning | parameters |
|-- |-- |-- |
| `ignore` | disregard matching strings | `pattern` |
| `void` | do not assign a value to matching strings | `pattern`, `id` |
| `boolean` | interpret matching strings as boolean values | `pattern`, `id`, `value` |
| `int` | interpret matching strings as integer number values | `pattern`, `id` |
| `float` | interpret matching strings as floating point number values | `pattern`, `id` |
| `string` | interpret matching strings as string values | `pattern`, `id` |


### Conclusion element types

| type | meaning | parameters |
|-- |-- |-- |
| `terminal` | the part is a terminal symbol | `id` |
| `variable` | the part is a syntactic variable | `id` |