Development/KDevelop-PG-Qt Introduction

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Development/KDevelop-PG-Qt Introduction


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Preface

KDevelop-PG-Qt is the parser-generator from KDevplatform. It is used for some KDevelop-languagesupport-plugins (Ruby, PHP, Java...).

It uses Qt classes internally. There's also the original KDevelop-PG parser, which used types from the STL, but has since been superceeded by KDevelop-PG-Qt. Most of the features are the same, though it could be that the ...-Qt parser generator is more up to date and feature rich than the plain STL style generator. The ...-Qt version should be used to write parsers for KDevelop language plugins.

In-Depth information

This document is not supposed to be a full-fledged and in-depth resource for all parts of KDevelop-PG. Instead it is intended to be a short introduction and, more importantly, a reference for developers.

To get an in-depth introduction, read Jakob Petsovits' excellent Bachelor thesis. You find it in the Weblinks section at the bottom of this page.

The Application

Usage

You can find KDevelop-PG-Qt in git. Four example packages are also included in the sources.
To download it try: 

git clone git://anongit.kde.org/kdevelop-pg-qt.git

or: 

git clone kde:kdevelop-pg-qt

(when having setup git with kde:-prefix)

The program itself requests a .g file, a so called grammar, as input: 

./kdev-pg-qt --output=prefix syntax.g

The value of the --output switch decides the prefix of the output files and additionally the namespace for the generated code. Kate provides elementary highlighting for KDevelop-PG-Qt's grammar-files.

Output Format

While evaluating the grammar and generating its parser files, the application will output information about so called conflicts to STDOUT. As said above, the following files will actually be prefixed.

ast.h

AST stands for Abstract Syntax Tree. It defines the data structure in which the parse tree is saved. Each node is a struct with the postfix Ast, which contains members that point to any possible sub elements.

parser.h and parser.cpp

One important part of parser.h is the definition of the parser tokens, the TokenType enum. The TokenStream of your lexer should to use this. You have to write your own lexer or let one generate by Flex. See also the part about Tokenizers/Lexers below.

Having the token stream available, you create your root item and call the parser on the parse method for the top-level AST item, e.g. DocumentAst* => parseDocument(&root). On success, root will contain the AST.

The parser will have one parse method for each possible node of the AST. This is nice for e.g. an expression parser or parsers that should only parse a sub-element of a full document.

visitor.h and visitor.cpp

The Visitor class provides an abstract interface to walk the AST. Most of the time you don't need to use this directly, the DefaultVisitor takes some work off your shoulders.

defaultvisitor.h and defaultvisitor.cpp

The DefaultVisitor is an implementation of the abstract Visitor interface and automatically visits each node in the AST. Hence, this is probably the best candidate for a base class for your personal visitors. Most language plugins use these in their Builder classes to create the DUChain.

Command-Line-Options

  • --namespace=namespace - sets the C++ namespace for the generated sources independently from the file prefix. When this option is set, you can also use / in the --ouput option
  • --no-ast - don't create the ast.h file, more to that below
  • --debug-visitor - generates a debug visitor that prints the AST
  • --serialize-visitor - generates code for serialization via a QIODevice
  • --terminals - all tokens will be written into the file kdev-pg-terminals
  • --symbols - all possible nodes from the AST (not the leafs) will be written into the file kdev-pg-symbols
  • --rules - all grammar rules with informationen about their syntactic correlations will be written into a file called kdev-pg-rules. useful for debugging and solving conflicts
  • --token-text - generates a function to map token-numbers onto token-names
  • --help - print usage information

Tokenizers/Lexers

As mentioned, KDevelop-PG-Qt requires a Tokenizer. You can either let KDevelop-PG-Qt generate one for you, write one per hand, as it has been done for C++ and PHP, or you can use external tools like Flex.

The tokenizer's job, in principle, boils down to:

  • converting keywords and chars with special meanings to tokens
  • converting literals and identifier to tokens
  • clean out anything that doesn't change the semantics, e.g. comments or whitespace (the latter of course not in Python)
  • while doing the above, handling character encoding (we recommend using UTF8 as much as possible)

The rest, e.g. actually building the tree and evaluating the semantics, is part of the parser and the AST visitors.

Using KDevelop-PG-Qt

KDevelop-PG-Qt can generate lexers being well integrated into its architecture (you do not have to create a token-stream-class invoking lex or something like that). See examples/foolisp in the code for a simplistic example.

TODO

Using Flex

With the existing examples, it shouldn't be too hard to write such a lexer. Between most languages, especially those "inheriting" C, there are many common syntactic elements. Especially comments and literals can be handled just the same way over and over again. Adding a simple token is trivial: 

"special-command"    return Parser::Token_SPECIAL_COMMAND;

That's pretty much it, take a look at eg. java.ll for an excellent example.

How to write Grammar-Files

Chomsky Type-2 Grammars

KDevelop-PG-Qt uses so called Type-2-grammars use a concept of non-terminals (nodes) and terminals(tokens). While writing the grammar for the basic structure of your language, you should try to mimic the semantics of the language. Lets take a look at an example:

C++-document consists of lots of declarations and definitions, a class definition could be handled e.g. in the following way:

  1. CLASS-token
  2. a identifier
  3. the {-token
  4. a member-declarations-list
  5. the }-token
  6. and finally the ;-token

The member-declarations-list is of course not a part of any C++ description, it is just a helper to explain the structure of a given semantic part of your language. The grammar could then define how exactly such helper might look like.

Basic Syntax

Now let us have a look at a basic example, a declaration in C++, as described in grammar syntax: 

 struct_declaration

This is called a rule definition. Every lower-case string in the grammar file references such a rule. Our case above defines what a declaration looks like. The |-char stands for a logical or, all rules have to end on two semicolons.

In the example we reference other rules which also have to be defined. Here's for example the class_declaration, note the tokens in all-upper-case: 

 CLASS IDENTIFIER LBRACE class_declaration* RBRACE SEMICOLON
-> class_declaration ;;

There is a new char in there: The asterisk has the same meaning as in regular expressions, i.e. that the previous rule can occur arbitrarily often or not at all.

In a grammar 0 stands for an empty token. Using it in addition with parenthesizing and the logical or from above, you can express optional elements: 

some_required_rule SOME_TOKEN
    ( some_optional_stuff | some_other_stuff | 0 )
-> my_rule ;;

All symbols never occuring on the left side of a rule are start-symbols. You can use one of them to start parsing.

Making matched rules available to Visitors

The simple rule above could be used to parse the token stream, yet no elements would be saved in the parsetree. This can be easily done though: 

class_declaration=class_declaration

The DeclarationAst struct now contains pointers to each of these elements. During the parse process the pointer for each found element gets set, all others become NULL. To store lists of elements, prepend the identifier with a hash # : 

CLASS IDENTIFIER SEMICOLON

TODO: internal structure of the list, important for Visitors

Identifier and targets can be used in more than one place: 

#one=one (#one=one)*
-> one_or_more ;;

In the example above, all matches to the rule one will be stored in one and the same list one.

Defining available Tokens

Somewhere in the grammar (you should probably put it near the head) you'll have to define a list of available Tokens. From this list, the TokenType enum in parser.h will be created. Additionally to the enum value names you should define an explanation name which will e.g. be used in error messages. Note that the representation of a Token inside the source code is not required for the grammar/parser as it operates on a TokenStream, see Lexer/Tokenizer section above. 

%token T1 ("T1-Name"), T2 ("T2-Name"), COMMA (";"), SEMICOLON (";") ;;

It is possible to use %token multiple times to group tokens in the grammar. Though all tokens will still be put into the same TokenType enum.

TODO: explain process of writing Lexer/Tokenizer and using the parser Tokens

Special Syntax...

...to use inside Rules

List of one or more elements

Alternatively to the asterisk (*) you can use a plus sign (+) to mark lists of one-or-more elements: 

(#one=one)+
-> one_or_more ;;

Separated lists

Using the #rule @ TOKEN syntax you can mark a list of rule, separated by TOKEN: 

#item=item @ COMMA
-> comma_separated_list ;;

Optional items

Alternatively to the above mentioned (item=item | 0) syntax you can use the following to mark optional items: 

?item=item
-> optional_item ;;
Attention
(0|x) is equivalent to 0


Local variables for the parse-process

Using a colon  : instead of the equal sign = you can store the sub-AST in a local variable that will only be available during parsing, and only in the current rule.

TODO: need example

Inlining

Instead of a name you can also place a dot . before the equal sign = . Then the AST will inherit all class-members from the sub-AST and the parsing-method for the sub-AST will be merged into the parent-AST. An example: 

op=PLUS

In SimpleArithmeticsAst there will be the fields val1, val2 (storing the operands) and op (storing the operator-token). parseSimpleArithmetics will not have to call parseOperator. Obviously recursive inlining is not allowed.

...to add Hand-Written Code

Sometimes it is required to integrate hand-written code into the generated parser. Instead of editing the end-result (**never** do that!) you should put this code into the grammar at the correct places. Here are a few examples when you'd need this:

  • custom error handling / error recovery, i.e. to prevent the parser to stop at the first error
  • creating tokens, if you don't want to do that externally
  • setting custom variables, especially for state tracking. e.g. in C++ you could save whether you are inside a private, protected oder public section. then you could save this information inside each node of the class elements.
  • additional verifications, lookaheads etc.

General Syntax

[:
// here be dragons^W code ;-)
:]

The code will be put into the generated parser.cpp file. If you use it inside a grammar rule, it will be put into the correct position during the parse process. You can access the current node via the variable yynode, it will have the type 'XYZAst**'.

Global Code

In KDevelop language plugins, you'll see that most grammars start with something like: 

[:

#include <QtCore/QString>
#include <kdebug.h>
#include <tokenstream.h>
#include <language/interfaces/iproblem.h>
#include "phplexer.h"

namespace KDevelop
{
    class DUContext;
}

:]

This is a code section, that will be put at the beginning of ast.h, i.e. into the global context.

But there are also some newer statements available to include header-files: 

%ast_header "header.h"
%parser_declaration_header "header.h"
%parser_bits_header "header.h"

The include-statement will be inserted into ast.h, parser.h respectively parser.cpp.

Namespace Code

Also it's very common to define a set of enumerations e.g. for operators, modifiers, etc. pp. Here's an stripped example from PHP, note that the code will again be put into the generated parser.h file: 

%namespace
[:
    enum ModifierFlags {
        ModifierPrivate      = 1,
        ModifierPublic       = 1 << 1,
        ModifierProtected    = 1 << 2,
        ModifierStatic       = 1 << 3,
        ModifierFinal        = 1 << 4,
        ModifierAbstract     = 1 << 5
    };
...
    enum OperationType {
        OperationPlus = 1,
        OperationMinus,
        OperationConcat,
        OperationMul,
        OperationDiv,
        OperationMod,
        OperationAnd,
        OperationOr,
        OperationXor,
        OperationSl,
        OperationSr
    };
:]

When you write code at the end of the grammar-file simply between [: and :], it will be added to parser.cpp.

Additional AST member

To add additional members to _every_ AST variable, use the following syntax: 

%ast_extra_members
[:
  KDevelop::DUContext* ducontext;
:]

You can also specify the base-class for the nodes: 

%ast_base symbol_name "BaseClass"

BaseClass has to inherit from AstNode.

Additional parser class members

Instead of polluting the global context with state tracker variables, and hence destroying the whole advantages of OOP, you can add additional members to the parser class. It's also very convenient to define functions for error reporting etc. pp. Again a stripped example from PHP: 

%parserclass (public declaration)
[:
  enum ProblemType {
      Error,
      Warning,
      Info
  };
  void reportProblem( Parser::ProblemType type, const QString& message );
  QList<KDevelop::ProblemPointer> problems() {
      return m_problems;
  }
  ...
  enum InitialLexerState {
      HtmlState = 0,
      DefaultState = 1
  };
:]

Note, that we used %parserclass (public declaration), we could instead have used private or protected declaration. 

protected

There is also a statement to specify a base-class: 

%parser_base "ClassName"

Initializing additional parser class members

When you add member variables to the class, you'll have to initialize and or destroy them as well. Here's how (either use ctor or dtor, of course): 

desctructor] )
[:
// Code
:]

Boolean Checks

?[:
// some bool expression
:]

The following rule will only apply if the boolean expression evaluates to true. Here's an advanced example, which also shows that you can use the pipe symbol ('|') as logical or, i.e. essentially this is a if... else...' conditional: 

?[: someCondition :] SOMETOKEN ifrule=myVar

This is especially convenient together with lookaheads (see below).

Defining local variables inside rules

You can set up the grammar to define local variables whenever a rule gets applied: 

...
-> class_member [:
   enum { A_PUBLIC, A_PROTECTED, A_PRIVATE } access;
:];;

This variable is local to the rule class_member.

Defining additional variables for the parse tree

Similar to the syntax above, you can define members whenever a rule gets applied: 

temporary] variable yourName: yourType
]

For example: 

...
-> class_member [
      member variable access : AccessType;
];;

Of course AccessType has to be defined somewhere else, see e.g. the Additional parser class members section above.

Using temporary or member is equivalent.

Conflicts

The first time you write a grammar, you'll potentially fail: Since KDevelop-PG-Qt is a LL(1) parser generator, conflicts can occur and have to be solved by hand. Here's an example for a FIRST/FIRST conflict: 

 class_definition
-> class_expression ;;

KDevelop-PG-Qt will output: 

** WARNING found FIRST/FIRST conflict in  "class_expression"

Sometime's it's these warnings can be ignored, but most of the time it will lead to problems in parsing. In our example the class_expression rule won't be evaluated properly: When you try to parse a class_definition, the parser will see that the first part (CLASS) of class_declaration matches, and jumps think's that this rule is to be applied. Since the next part of the rule does not match, an error will be reported. It does _not_ try to evaluate class_definition automatically, but there are different ways to solve this problem:

Backtracking

In theory such behavior might be unexpected: In BNF e.g. the above syntax would be enough and the parser would automatically jump back and retry with the next rule, here class_definition. But in practice such behaviour is - most of the time - not necessary and would slow down the parser. If however such behavior is explicitly what you want, you can use an explicit backtracking syntax in your grammar: 

try/rollback(class_declaration)
      catch(class_definition)
-> class_expression ;;

In theory, every FIRST/FIRST could be solved this way, but keep in mind that the more you use this feature, the slower your parser gets. If you use it, sort the rules in order of likeliness. Also, there could be cases where the sort order could make the difference between correct and wrong parsing, especially with rules that "extend" others.

Look ahead

KDevelop-PG-Qt has an alternative to the - potentially slow - backtracking mechanism: Look ahead. You can use the LA(qint64) function in embedded C++ code (see sections above). LA(1) will return the current token, you most probably never need to use that. Accordingly, LA(2) returns the next and LA(0) the previous token. (If you wonder where these somewhat uncommon indexes come from: Read the thesis, or make yourself acquainted with LA(k) parser theory.) 

 class_declaration
-> class_expression

Note: The conflict will still be output, but it is manually resolved. You should always document this somewhere with a comment, for future contributors to know that this is a false-positive.

Elegant Solutions

Often you can solve the conflict all-together with an elegant solution. Like in our example: 

LBRACE class_content RBRACE
-> class_definition ;;

   CLASS IDENTIFIER ?class_definition SEMICOLON
-> class_expression ;;

No conflicts, fast: everybody is happy!

FIRST/FOLLOW-Conflicts

A FIRST/FOLLOW conflict says, that it is undefined where a symbol ends and the parent symbol continues. A pretty stupid example is: 

item*
-> item_list ;;

   item_list item*
-> items ;;

You probably see the glaring error. try/rollback helps with serious problems (e.g. parser doesn't work), though often you can ignore these conflicts. But if you do so, be aware that the parser is greedy: In our example item_list will always contain all item elements, and items will never have an item member. If this is an issue that leads to later conflicts, only try/rollback, custom manual code to check which rule should be used, or a refactoring of your grammar structure.

Changing the Greedy Behaviour

Alternatively it is sometimes helpful/required to change the greedy behaviour. In lists you can use manual code to force a break at a given position. Here's an example that shows this on a declaration of an array with fixed size: 

typed_identifier=typed_identifier LBRACKET UNSIGNED_INTEGER
   [: count = static_cast<MyTokenStream*>(token)->strForCurrentToken().toUInt(); :]
   RBRACKET EQUAL LBRACE
   (#expression=expression [: if(--count == 0) break; :] @ COMMA) ?[: count == 0 :]
   RBRACE SEMICOLON
-> initialized_fixed_array [ temporary variable count: uint; ];;

TODO: return true/false or break??

Via return false you can enforce a premature stop (== error) of the evaluation of the current rule. Using return true you stop the evaluation prematurely, but signalize that the parse process was successful.

try/recover

   try/recover(expression)
-> symbol ;;

This is approximately the same as: 

   [: ParserState *state = copyCurrentState(); :]
   try/rollback(expression)
   catch( [: restoreState(state); :] )
-> symbol ;;

Hence you have to implement the member-functions copyCurrentState and restoreState and you have to define a type called ParseState. You do not have to write the declaration of those functions in the header-file, it is generated automatically if you use try/recover. This concept seems to be useful if there are additional states used while parsing. The Java-parser takes usage from it very often. But I do not know a lot about this feature and it seems unimportant for me. (I guess, it is not) I would be happy when somebody could explain it to me.

Operator Expression Parsing

TODO

Weblinks

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