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Compiler

This is the implementation of the so-called compiler-compiler based on the basic capabilities of Hoa\Compiler.

The library is needed to create parsers from grammar files and is not used during the parsing itself, this is only required for development.

Before you begin to work with custom implementations of parsers, it is recommended that you review the EBNF

Grammar

Each language consists of words that are added to sentences. And for the correct construction of the proposal, some rules are needed. Such rules are called grammar.

Let's try to create the corresponding grammar for the calculator, which can add two numbers. If you are familiar with alternative grammars (Antlr, BNF, EBNF, Hoa, etc.), then it will not be difficult for you.

The grammar of Railt is partly different from the original EBNF. In this way, let's restructure the same rule into the grammar of the Railt.

In order to test the performance simply use the reading and playing grammar on the fly!

On the output you will take an AST, which will be serialized in XML by the echo operator and which will look like this:

The naming register does not matter, but it is recommended that you name the tokens in upper case ("TOKEN_NAME"), and the rules with a capital letter ("RuleName"). Such recommendations will help you in the future easier to navigate in the existing grammar.

Definitions

In the Railt grammar there are 5 types of definitions:

• %token name regex - Definition of a name and value of a token.
• %skip name regex - Definition of a name and value of a skipped token. Such tokens will be ignored and allowed anywhere in the grammar.
• %pragma name value - Rules for the configuration of a lexer and a parser.
• %include path/to/file - Link to another grammar file.
• rule or #rule - The grammar rule.

Comments

In the Railt grammar, there are two types of C-like commentaries:

1) // Inline comment - This comment type begins with two slashes and ends with an end of the line. 2) /* Multiline comment */ - This comment type begins with /* symbols and ends with a */ symbol.

Output Control

You probably already noticed that in grammar, the definitions of tokens look a little different: <TOKEN> and ::TOKEN::.

This way of determining the tokens inside the grammar tells the compiler whether to print the ordered token as a result or not. It is for this reason that the token "plus" was ignored, because We do not need information about this token, but the values of "digit" tokens are important to us.

1) <TOKEN> - Keep token in AST. 2) ::TOKEN:: - Hide token from AST.

Declaring rules

Each rule starts with the name of this rule. In addition, each rule can be marked with a # symbol that indicates that the rule should be kept in the AST.

1) #Rule - The defined rule must be present in the AST. 2) Rule - The defined rule should be hide from AST.

After the name there is a production (body) of this rule, which are separated by one of the valid characters: = or :. The separator character does not matter and is present as compatibility with other grammars. In addition, the rule can end with an optional ; char.

The constructions of the PP2 language are the following:

• rule() to call a rule,
• <token> and ::token:: to declare a lexeme.
• | for a disjunction (an "alternation").
• (…) for a group.
• e? to say that e is optional (0 or 1 times).
• e+ to say that e can be present 1 or more times.
• e* to say that e can be present 0 or more times.
• e{x,y} (e{,y}, e{x,} or e{x}) to say that e can be present between x and y times.
• #rule to create a rule node in the resulting tree.

Finally, the grammar of the PP2 language is written with the PP2 language.

Let's try to add support for the remaining symbols of the calculator: Moderation, Division and Subtraction; and at the same time slightly improve the rules of the lexer.

Simple expression 4 + 8 - 15 * 16 / 23 + -42 will be parsed into the followed tree:

Note that the grammar is quite trivial and does not contain the priorities of the operators.

Delegation

You can tell the compiler which php class to include the desired grammar rule using keyword -> after name of rule definition. In this case, each processed rule will create an instance of target class.

For more information about delegates, use the Parser documentation.

Parser compilation

Reading a grammar is quite simple operation, but it still takes time to execute. After the grammar rules have been formulated, you can "fix" the version in a separate parser class that will contain all the logic and no longer require reading the source code. After you compile it into a class, this package (railt/compiler) can be excluded from composer dependencies.

This code example will create a parser class in the current directory with the required class and namespace names. An example of the result of generation can be found in an existing project here. As a source, this grammar file.