# Running multiple parsers on an input file

Universal Ctags provides parser developers two ways, guest parser (guest/host) and subparser (sub/base), to run multiple parsers for an input file.

This section shows concepts behind the running multiple parsers and real examples.

## Guest parser: Applying a parser to specified areas of input file

Guest parser (guest/host) considers the case that an input file has areas written in languages different from the language for the input file.

A host parser parses the input file and detects the areas. The host parser schedules guest parsers parsing the areas. The guest parsers parse the areas.

### Command line interface

Running guest parser can be controlled with guest (`g`) extras flag. By default it is disabled. To turning on the feature running guest parser, specify `--extras=+g`.

If `--fields=+E` is given, all tags generated by a guest parser is marked `guest` in their `extras:` fields.

### Examples of guest parser

#### {CSS,JavaScript}/HTML parser combination

For an HTML file, you may want to run HTML parser, of course. The HTML file may have CSS areas and JavaScript areas. In other hand Universal Ctags has both CSS and JavaScript parsers. Don’t you think it is useful if you can apply these parsers to the areas?

In this case, HTML has responsible to detect the CSS and JavaScript areas and record the positions of the areas. The HTML parser schedules delayed invocations of CSS and JavaScript parsers on the area with promise API.

Here HTML parser is a host parser. CSS and JavaScript parsers are guest parsers.

See “The new HTML parser” and “parsers/html.c”.

#### C/Yacc parser combination

A yacc file has some areas written in C. Universal Ctags has both YACC and C parsers. You may want to run C parser for the areas from YACC parser.

Here YACC parser is a host parser. C parser is a guest parser. See “optlib/yacc.ctags”.

#### Pod/Perl parser combination

Pod (Plain Old Documentation) is a language for documentation. The language can be used not only in a stand alone file but also it can be used inside a Perl script.

Universal Ctags has both parsers for Perl and Pod. The Perl parser recognizes the area where Pod document is embedded in a Perl script and schedules applying pod parser as a guest parser on the area.

## Subparser: Tagging definitions of higher (upper) level language

### Background

Consider an application written in language X. The application has its domain own concepts. Developers of the application may try to express the concepts in the syntax of language X.

In language X level, the developer can define functions, variables, types, and so on. Further more, if the syntax of X allows, the developers want to define higher level (= application level) things for implementing the domain own concepts.

Let me show the part of source code of SPY-WARS, an imaginary game application. It is written in scheme language, a dialect of lisp. (Here gauche is considered as the implementation of scheme interpreter).

    (define agent-tables (make-hash-table))
    (define-class <agent> ()
      ((rights :init-keyword :rights)
       (responsibilities :init-keyword :responsibilities)))

    (define-macro (define-agent name rights responsibilities)
      `(hash-table-put! agent-tables ',name
                        (make <agent>
                          :rights ',rights
                          :responsibilities ',responsibilities)))

    (define-agent Bond (kill ...) ...)
    (define-agent Bourne ...)

    ...

`define`, `define-class`, and `define-macro` are keywords of scheme for defining a variable, class and macro. Therefore scheme parser of ctags should make tags for `agent-tables` with variable kind, `<agent>` with class kind, and `define-agent` with macro kind. There is no discussion here.

> NOTE: To be exactly `define-class` and `define-macro` are not the part of scheme language. They are part of gauche. That means three parsers are stacked: scheme, gosh, and SPY-WARS.

The interesting things here are `Bond` and `Bourne`.

    (define-agent Bond (kill ...) ...)
    (define-agent Bourne ...)

In scheme parser level, the two expressions define nothing; the two expressions are just macro (`define-agent`) expansions.

However, in the application level, they define agents as the macro name shown. In this level Universal Ctags should capture `Bond` and `Bourne`. The question is which parser should capture them? scheme parser should not; define-agent is not part of scheme language. Newly defined SPY-WARS parser is the answer.

Though `define-agent` is just a macro in scheme parser level, it is keyword in SPY-WARS parser. SPY-WARS parser makes a tag for a token next to `define-agent`.

The above example illustrates levels of language in an input file. scheme is used as the base language. With the base language we can assume an imaginary higher level language named SPY-WARS is used to write the application. To parse the source code of the application written in two stacked language, ctags uses the two stacked parsers.

Making higher level language is very popular technique in the languages of lisp family (see “On Lisp” for more details). However, it is not special to lisp.

Following code is taken from linux kernel written in C:

    DEFINE_EVENT(mac80211_msg_event, mac80211_info,
            TP_PROTO(struct va_format *vaf),
            TP_ARGS(vaf)
    );

There is no concept EVENT in C language, however it make sense in the source tree of linux kernel. So we can consider linux parser, based on C parser, which tags `mac80211_msg_event` as `event` kind.

### Terms

#### Base parser and subparser

In the context of the SPY-WARS example, scheme parser is called a base parser. The SPY-WARS is called a subparser. A base parser tags definitions found in lower level view. A subparser on the base parser tags definitions found in higher level view. This relationship can be nested. A subparser can be a base parser for another subparser.

At a glance the relationship between two parsers are similar to the relationship guest parser and host parser description in “Guest parser: Applying a parser to specified areas of input file”. However, they are different. Though a guest parser can run stand-alone, a subparser cannot; a subparser needs help from base parser to work.

#### Top down parser choice and bottom up parser choice

There are two ways to run a subparser: top down or bottom up parser choices.

Universal Ctags can chose a subparser automatically. Matching file name patterns and extensions are the typical ways for choosing. A user can choose a subparser with `--language-force=` option. Choosing a parser in these deterministic way is called top down. When a parser is chosen as a subparser in the top down way, the subparser must call its base parser. The base parser may call methods defined in the subparser.

Universal Ctags uses bottom up choice when the top down way doesn’t work; a given file name doesn’t match any patterns and extensions of subparsers and the user doesn’t specify `--language-force=` explicitly. In choosing a subparser bottom up way it is assumed that a base parser for the subparser can be chosen by top down way. During a base parser running, the base parser tries to detect use of higher level languages in the input file. As shown later in this section, the base parser utilizes methods defined in its subparsers for the detection. If the base parser detects the use of a higher level language, a subparser for the higher level language is chosen. Choosing a parser in this non-deterministic way (dynamic way) is called bottom up.

input file | subparser choices
--- | ---
<SUB_LANG> (`input.sub`) | top down
<BASE_LANG> (`input.base`) | bottom up

Here is an example. Universal Ctags has both m4 parser and Autoconf parser. The m4 parser is a base parser. The Autoconf parser is a subparser based on the m4 parser. If `configure.ac` is given as an input file, Autoconf parser is chosen automatically because the Autoconf parser has `configure.ac` in its patterns list. Based on the pattern matching, Universal Ctags chooses the Autoconf parser automatically (top down choice).

If `input.m4` is given as an input file, the Autoconf parser is not chosen. Instead the m4 parser is chosen automatically because the m4 parser has `.m4` in its extension list. The m4 parser passes every token finding in the input file to the Autoconf parser. The Autoconf parser gets the chance to probe whether the Autoconf parser itself can handle the input or not; if a token name is started with `AC_`, the Autoconf parser reports “this is Autoconf input though its file extension is `.m4`” to the m4 parser. As the result the Autoconf parser is chosen (bottom up choice).

Some subparsers can be chosen both top down and bottom up ways. Some subparser can be chosen only top down way or bottom up ways.

#### Direction flags

Direction flags specify how a base parser and a subparser work together. You can choose directions by putting a long flag after `--langdef=<SUB_LANG>{base=<BASE_LANG>}`.

The following three direction flags are available;

`shared` (default):

  For an input file of <BASE_LANG> (ex. `input.base`), tags captured by both <BASE_LANG> and <SUB_LANG> parser are recorded to tags file.

  For an input file of <SUB_LANG> (ex. `input.sub`), tags captured by only <SUB_LANG> parser are recorded to tags file.

  input file | tags of <BASE_LANG> | tags of <SUB_LANG>
  --- | --- | ---
  `input.base` | recorded | recorded
  `input.sub` | --- | recorded

`dedicated`:

  For an input file of <BASE_LANG> (ex. `input.base`), tags captured by only <BASE_LANG> parser are recorded to tags file.

  For an input file of <SUB_LANG> (ex. `input.sub`), tags captured by both <BASE_LANG> and <SUB_LANG> parser are recorded to tags file.

  input file | tags of <BASE_LANG> | tags of <SUB_LANG>
  --- | --- | ---
  `input.base` | recorded | ---
  `input.sub` | recorded | recorded

`bidirectional`:

  For an input file of both <BASE_LANG> (ex. `input.base`) and <SUB_LANG> (ex. `input.sub`), tags captured by both <BASE_LANG> and <SUB_LANG> parser are recorded to tags file.

  input file | tags of <BASE_LANG> | tags of <SUB_LANG>
  --- | --- | ---
  `input.base` | recorded | recorded
  `input.sub` | recorded | recorded

If no direction flag is specified, it implies `{shared}`.

See “Direction flags” in “Extending ctags with Regex parser (optlib)” for examples of using the direction flags.

### Listing subparsers

Subparsers can be listed with `--list-subparser`:

    $ ctags --options=./linux.ctags --list-subparsers=C
    #NAME                          BASEPARSER           DIRECTION
    linux                          C                    base => sub {shared}

### Command line interface

Running subparser can be controlled with subparser (`s`) extras flag. By default it is enabled. To turning off the feature running subparser, specify `--extras=-s`.

### Examples of subparser

#### Automake/Make parser combination

Simply to say the syntax of Automake is the subset of Make. However, the Automake parser has interests in Make macros having special suffixes: `_PROGRAMS`, `_LTLIBRARIES`, and `_SCRIPTS` so on.

Here is an example of input for Automake:

    bin_PROGRAMS = ctags
    ctags_CPPFLAGS =    \
            -I.         \
            -I$(srcdir) \
            -I$(srcdir)/main

From the point of the view of the Make parser, `bin_PROGRAMS` is a just a macro; the Make parser tags `bin_PROGRAMS` as a macro. The Make parser doesn’t tag `ctags` being right side of ‘`=`’ because it is not a new name: just a value assigned to bin_PROGRAMS. However, for the Automake parser `ctags` is a new name; the Automake parser tags `ctags` with kind `Program`. The Automake parser can tag it with getting help from the Make parser.

The Automake parser is an exclusive subparser. It is chosen in top down way; an input file name `Makefile.am` gives enough information for choosing the Automake parser.

To give chances to the Automake parser to capture Automake own definitions, The Make parser provides following interface in `parsers/make.h`:

    struct sMakeSubparser {
            subparser subparser;

            void (* valueNotify) (makeSubparser *s, char* name);
            void (* directiveNotify) (makeSubparser *s, char* name);
            void (* newMacroNotify) (makeSubparser *s,
                                     char* name,
                                     bool withDefineDirective,
                                     bool appending);
    };

The Automake parser defines methods for tagging Automake own definitions in a `struct sMakeSubparser` type variable, and runs the Make parser by calling `scheduleRunningBaseparser` function.

The Make parser tags Make own definitions in an input file. In addition Make parser calls the methods during parsing the input file.

    $ ctags --fields=+lK  --extras=+r -o - Makefile.am
    bin  Makefile.am     /^bin_PROGRAMS = ctags$/;"      directory       language:Automake
    bin_PROGRAMS Makefile.am     /^bin_PROGRAMS = ctags$/;"      macro   language:Make
    ctags        Makefile.am     /^bin_PROGRAMS = ctags$/;"      program language:Automake       directory:bin
    ctags_CPPFLAGS       Makefile.am     /^ctags_CPPFLAGS =    \\$/;"    macro   language:Make

`bin_PROGRAMS` and `ctags_CPPFLAGS` are tagged as macros of Make. In addition `bin` is tagged as directory, and `ctags` as program of Automake.

`bin` is tagged in a callback function assigned to `newMacroFound` method. `ctags` is tagged in a callback function assigned to `valuesFound` method.

`--extras=+r` is used in the example. Reference (`r`) extra is needed to tag `bin`. `bin` is not defined in the line, `bin_PROGRAMS =`. `bin` is referenced as a name of directory where programs are stored. Therefore `r` is needed.

For tagging `ctags`, the Automake parser must recognize `bin` in `bin_PROGRAMS` first. `ctags` is tagged because it is specified as a value for `bin_PROGRAMS`. As the result `r` is also needed to tag `ctags`.

Only Automake related tags are emitted if Make parser is disabled.

    $ ctags --languages=-Make --fields=+lKr --extras=+r -o - Makefile.am
    bin     Makefile.am     /^bin_PROGRAMS = ctags$/;"      directory       language:Automake       roles:program
    ctags   Makefile.am     /^bin_PROGRAMS = ctags$/;"      program language:Automake       directory:bin

#### Autoconf/M4 parser combination

Universal Ctags uses m4 parser as a base parser and Autoconf parse as a subparser for `configure.ac` input file.

    AC_DEFUN([PRETTY_VAR_EXPAND],
              [$(eval "$as_echo_n" $(eval "$as_echo_n" "${$1}"))])

The m4 parser finds no definition here. However, Autoconf parser finds `PRETTY_VAR_EXPAND` as a macro definition. Syntax like `(...)` is part of M4 language. So Autoconf parser is implemented as a subparser of m4 parser. The most parts of tokens in input files are handled by M4. Autoconf parser gives hints for parsing `configure.ac` and registers callback functions to Autoconf parser.