.. Copyright (c) 2017-2026 Juancarlo Añez (apalala@gmail.com)
.. SPDX-License-Identifier: BSD-4-Clause

.. include:: links.rst
.. highlight:: none

Grammar Syntax
--------------

|TatSu| grammars use an extension of the classic `EBNF`_ syntax. The classic
variations of EBNF_ (Tomassetti, EasyExtend, Wirth) and `ISO EBNF`_ are also
supported as input grammar format.

Rules
~~~~~

A grammar consists of a sequence of one or more rules of the form:

.. code:: apl
    :force:

    name = <expre> ;

If a *name* collides with a `Python`_ keyword, an underscore (``_``)
will be appended to it on the generated parser.

Rule names that start with an uppercase character:

.. code:: apl
    :force:

    FRAGMENT = /[a-z]+/ ;

*do not* advance over whitespace before beginning to parse. This feature
becomes handy when defining complex lexical elements, as it allows
breaking them into more than one rule.

The parser returns an `AST`_ value for each rule depending on what was
parsed:

-  A single value
-  A list of `AST`_
-  A dict-like object for rules with named elements
-  An object, when ModelBuilderSemantics is used
-  None

See the `Abstract Syntax Trees`_ and `Building Models`_ sections for more
details.

.. _Abstract Syntax Trees: ast.html
.. _Building Models: models.html

Expressions
~~~~~~~~~~~

The expressions, in reverse order of operator precedence, can be any of the following.

.. note::
   Because |TatSu| now supports EBNF_, there must not be empty lines in expressions.

``# comment ...``
^^^^^^^^^^^^^^^^^

`Python`_-style end-of-line comments are allowed.


``// comment ...``
^^^^^^^^^^^^^^^^^^

`Java`_-style end-of-line comments are allowed.


``/* ... */``
^^^^^^^^^^^^^

`EBNF`_-style multi-line comments are allowed.


``e1 | e2``
^^^^^^^^^^^

Choice. Match either ``e1`` or ``e2``.

A ``|`` may be used before the first option if desired:

.. code:: apl
    :force:

    choices
      =
      | e1
      | e2
      | e3
      ;


``e1 e2``
^^^^^^^^^

Sequence. Match ``e1`` and then match ``e2``.


``( e )``
^^^^^^^^^

Grouping. Match ``e``. For example: ``('a' | 'b')``.


``[ e ]``
^^^^^^^^^

Optionally match ``e``.


``{ e }`` or ``{ e }*``
^^^^^^^^^^^^^^^^^^^^^^^

Closure. Match ``e`` zero or more times. The `AST`_ returned for a
closure is always a ``list``.


``{ e }+``
^^^^^^^^^^

Positive closure. Match ``e`` one or more times. The `AST`_ is always
a ``list``.


``{}``
^^^^^^

Empty closure. Match nothing and produce an empty ``list`` as `AST`_.


``~``
^^^^^

The *cut* expression: commit to the current option and prevent
other options from being considered even when what follows fails to
parse.

In this example, other options won't be considered if a parenthesis is
parsed:

.. code:: apl

      atom
          =
          | '(' ~ @:expre ')'
          | int
          | bool
          ;

Cut expression may be used anywhere. The effect of ``~`` is scoped to the nearest
enclosing brackets (*group*, *optional*, *closure*), the enclosing *choice*, or
the enclosing *rule*.

On the scoping of *cut*, consider these theoretical equivalences about implicit
choices in some expressions:

.. code:: apl
    :force:

    A →  α   ≡ A → α | ⊥
    A → [x]  ≡ A → B, B → x | ε
    A → {x}  ≡ A → B, B → xB | ε
    A → {x}+ ≡ A → B, B → xB | x

This is a common use of ``~``. The *closure* doesn't allow a partial assignment
expressions to escape it:

.. code:: apl
    :force:

    parameters = ','.{name '=' ~ expression} ;


``s%{ e }+``
^^^^^^^^^^^^

Positive join. Inspired by `Python`_'s ``str.join()``, it parses the
same as this expression:

.. code:: apl
    :force:

    e {s ~ e}

yet the result is a single list of the form:

.. code:: apl
    :force:

    [e, s, e, s, e, ...]

Use grouping if `s` is more complex than a *token* or a *pattern*:

.. code:: apl
    :force:

    (s t)%{ e }+


``s%{ e }`` or ``s%{ e }*``
^^^^^^^^^^^^^^^^^^^^^^^^^^^

Join. Parses the list of ``s``-separated expressions, or the empty
closure. It's equivalent to:

.. code:: apl
    :force:

    s%{e}+|{}


``op<{ e }+``
^^^^^^^^^^^^^

Left join. Like the *join expression*, but the result is a
left-associative tree built with ``tuple()``, in which the first
element is the separator (``op``), and the other two elements are the
operands.

The expression:

.. code:: apl
    :force:

    '+'<{/\d+/}+

Will parse this input:

.. code:: apl
    :force:

    1 + 2 + 3 + 4

To this tree:

.. code:: apl
    :force:

    (
        '+',
        (
            '+',
            (
                '+',
                '1',
                '2'
            ),
            '3'
        ),
        '4'
    )


``op>{ e }+``
^^^^^^^^^^^^^

Right join. Like the *join expression*, but the result is a
right-associative tree built with ``tuple()``, in which the first
element is the separator (``op``), and the other two elements are the
operands.

The expression:

.. code:: apl
    :force:

    '+'>{/\d+/}+

Will parse this input:

.. code:: apl
    :force:

    1 + 2 + 3 + 4

To this tree:

.. code:: python

   (
      '+',
      '1',
      (
          '+',
          '2',
          (
              '+',
              '3',
              '4'
          )
      )
   )


``s.{ e }+``
^^^^^^^^^^^^

Positive *gather*. Like *positive join*, but the separator is not
included in the resulting `AST`_.

``s.{ e }`` or ``s.{ e }*``
^^^^^^^^^^^^^^^^^^^^^^^^^^^

*Gather*. Like the *join*, but the separator is not included in the
resulting `AST`_. It's equivalent to:

.. code:: apl
    :force:

    s.{e}+|{}


``&e``
^^^^^^

Positive lookahead. Succeed if ``e`` can be parsed, but do not consume
any input.


``!e``
^^^^^^

Negative lookahead. Fail if ``e`` can be parsed, and do not consume
any input.


``'text'`` or ``"text"``
^^^^^^^^^^^^^^^^^^^^^^^^

Match the token *text* within the quotation marks.

Note that if *text* is alphanumeric, then |TatSu| will check that the
character following the token is not alphanumeric. This is done to
prevent tokens like *IN* matching when the text ahead is
*INITIALIZE*. This feature can be turned off by passing
``nameguard=False`` to the ``Parser`` or the ``Buffer``, or by using a
pattern expression (see below) instead of a token expression.
The ``@@nameguard`` and ``@@namechars`` directives may be specified
in the grammar for the same effect:

.. code:: apl
    :force:

    @@nameguard :: False

or to specify additional characters that should also be considered
part of names:

.. code:: apl
    :force:

    @@namechars :: '$-.'


``r'text'`` or ``r"text"``
^^^^^^^^^^^^^^^^^^^^^^^^^^

Match the token *text* within the quotation marks, interpreting *text*
like `Python`_'s `raw string literal`_\ s.


``/regexp/``
^^^^^^^^^^^^

Also  ``?"regexp"`` or ``?'regexp'``, The *pattern* expression.
Match the `Python`_ regular expression
``regexp`` at the current text position. Unlike other expressions,
this one does not advance over whitespace or comments. For that, place
the ``regexp`` as the only term in its own rule.

The *regex* is interpreted as a Python_ `raw string literal`_ and
passed the Python_ re_ module using ``match()`` at the current
position in the text. The returned AST_ has the semantics of
``re.findall(pattern, text)[0]`` (a `tuple` if there is more than one
group), so use ``(?:)`` for groups that should not be in the resulting
AST_.

Consecutive *patterns* are concatenated to form a single one.


``/./``
^^^^^^^

The *any* expression, matches the next position in the input. It works
exactly like the ``?'.'`` pattern, but is implemented at the lexical
level, without regular expressions.


``->e``
^^^^^^^

The "*skip to*" expression, useful for writing *recovery* rules.

The parser will advance over input, one character at time, until ``e``
matches. Whitespace and comments will be skipped at each
step. Advancing over input is done efficiently, with no regular
expressions involved.

The expression is equivalent to:

.. code:: apl
    :force:

    { !e /./ } e

A common form of the expression is ``->&e``, which is equivalent to:

.. code:: apl
    :force:

    { !e /./ } &e

This is an example of the use of the "*skip to*" expression for
recovery:

.. code:: apl
    :force:

    statement =
        | if_statement
        # ...
        ;

    if_statement
        =
        | 'if' condition 'then' statement ['else' statement]
        | 'if' statement_recovery
        ;

    statement_recovery = ->&statement ;


```constant```
^^^^^^^^^^^^^^

Match nothing, but behave as if ``constant`` had been parsed.

Constants can be used to inject elements into the concrete and
abstract syntax trees, perhaps avoiding having to write a
semantic action. For example:

.. code:: apl
    :force:

    boolean_option = name ['=' (boolean|`true`) ] ;


If the text evaluates to a Python literal (with ``ast.literal_eval()``), that
will be the returned value. Otherwise, string interpolation in the style of
``str.format()`` over the names in the current `AST`_ is applied for
*constant* elements. Occurrences of the ``{`` character must be escaped to
``\{`` if they are not intended for interpolation. A *constant* expression
that hast type ``str`` is evaluated using:

.. code:: apl
    :force:

    eval(f'{"f" + repr(text)}', {}, ast)


`````constant`````
^^^^^^^^^^^^^^^^^^

A multi-line version of ```constant```.


``^`constant```
^^^^^^^^^^^^^^^

Also  ``^```constant`````. An alert. There will be no token returned by
the parser, but an alert will be registed in the parse context and added
to the current node's ``parseinfo``.

The ``^`` character may appear more than once to encode the *alert
level*:

.. code:: apl
    :force:

    assignment = identifier '=' (
        | value
        | -> &';' ^^^`could not parse value in assignment to {identifier}`
    )


``rulename``
^^^^^^^^^^^^

Invoke the rule named ``rulename``. To help with lexical aspects of
grammars, rules with names that begin with an uppercase letter will
not advance the input over whitespace or comments.


``>rulename``
^^^^^^^^^^^^^

The include operator. Include the *right hand side* of rule
``rulename`` at this point.

The following set of declarations:

.. code:: apl
    :force:

    includable = exp1 ;

    expanded = exp0 >includable exp2 ;

Has the same effect as defining *expanded* as:

.. code:: apl
    :force:

    expanded = exp0 exp1 exp2 ;

Note that the included rule must be defined before the rule that
includes it.


``()``
^^^^^^

The empty expression. Succeed without advancing over input. Its value
is ``None``.


``!()``
^^^^^^^

The *fail* expression. This is actually ``!`` applied to ``()``, which
always fails.


``name:e``
^^^^^^^^^^

Add the result of ``e`` to the `AST`_ using ``name`` as key. If
``name`` collides with any attribute or method of ``dict``, or is a
`Python`_ keyword, an underscore (``_``) will be appended to the name.

When there are no named items in a rule, the `AST`_ consists of the
elements parsed by the rule, either a single item or a ``list``. This
default behavior makes it easier to write simple rules:

.. code:: apl
    :force:

    number = /[0-9]+/ ;

Without having to write:

.. code:: apl
    :force:

    number = number:/[0-9]+/ ;

When a rule has named elements, the unnamed ones are excluded from the
`AST`_ (they are ignored).


``name+:e``
^^^^^^^^^^^

Add the result of ``e`` to the `AST`_ using ``name`` as key. Force the
entry to be a ``list`` even if only one element is added. Collisions
with ``dict`` attributes or `Python`_ keywords are resolved by
appending an underscore to ``name``.


``@:e``
^^^^^^^

The override operator. Make the `AST`_ for the complete rule be the
`AST`_ for ``e``.

The override operator is useful to recover only part of the right hand
side of a rule without the need to name it, or add a semantic
action. This is a typical use of the override operator:

.. code:: apl
    :force:

    subexp = '(' @:expre ')' ;

The `AST`_ returned for the ``subexp`` rule will be the `AST`_
recovered from invoking ``expre``.


``@+:e``
^^^^^^^^

Like ``@:e``, but make the `AST`_ always be a ``list``.

This operator is convenient in cases such as:

.. code:: apl
    :force:

    arglist = '(' @+:arg {',' @+:arg}* ')' ;

In which the delimiting tokens are of no interest.


``$``
^^^^^

The *end of text* symbol. Verify that the end of the input text has
been reached.

Deprecated Expressions
~~~~~~~~~~~~~~~~~~~~~~

The following expressions are still recognized in grammars, but they are
considered deprecated, and will be removed in a future version of
|TatSu|.


``?/regexp/?``
^^^^^^^^^^^^^^

Another form of the pattern expression that can be used when there
are slashes (``/``) in the pattern is *deprecated*. Use the ``?"regexp"``
or ``?'regexp'`` forms instead.


``+/regexp/``
^^^^^^^^^^^^^

Also  or ``+?"regexp"`` or ``+?'regexp'``, concatenate the given pattern
with the preceding one is *deprecated*. Use ``?" ..."`` string concatenations
instead.


``(* comment *)``
^^^^^^^^^^^^^^^^^

`Pascal`_-style multi-line comments are *deprecated*. Use `Java`_-style
comments instead.


Rules with Arguments
~~~~~~~~~~~~~~~~~~~~

|TatSu| allows rules to specify `Python`_-style arguments:

.. code:: apl
    :force:

    addition(Add, op='+')
      =
      addend '+' addend
      ;

The arguments values are fixed at grammar-compilation time. An
alternative syntax is available if no *keyword parameters* are
required:

.. code:: apl
    :force:

    addition::Add, '+'
      =
      addend '+' addend
      ;

Semantic methods must be ready to receive any arguments declared in
the corresponding rule:

.. code:: python

   def addition(self, ast, name, op=None):
       ...

When working with rule arguments, it's good to define a
``_default()`` method that is ready to take any combination of
standard and keyword arguments:

.. code:: python

   def _default(self, ast, *args, **kwargs):
       ...


Based Rules
~~~~~~~~~~~

Rules may extend rules defined before by using the ``<`` operator.

The following set of declarations:

.. code:: apl
    :force:

    base::Param = exp1 ;

    extended < base = exp2 ;

Has the same effect as defining *extended* as:

.. code:: apl
    :force:

    extended::Param = exp1 exp2 ;

Parameters from the *base rule* are copied to the new rule if the new
rule doesn't define its own. Repeated inheritance should be possible,
but it *hasn't been tested*.


Memoization
~~~~~~~~~~~

|TatSu| generates *packrat* parsers. The result of parsing a rule at a
given position in the input is *memoized*. The next time the parser
visits the same input position, with the same rule, the memoized result
is returned and the input advanced accordingly.

Some rules should not be memoized. For example, rules that
may succeed or not depending on the associated semantic action *should
not* be memoized.

The ``@nomemo`` decorator turns off memoization for a particular rule:

.. code:: apl
    :force:

    @nomemo
    INDENT = () ;

    @nomemo
    DEDENT = () ;


Rule Overrides
~~~~~~~~~~~~~~

A grammar rule may be redefined by using the ``@override`` decorator:

.. code:: apl
    :force:

    start = ab $;

    ab = 'xyz' ;

    @override
    ab = @:'a' {@:'b'} ;

When combined with the ``#include`` directive, rule overrides can be
used to create a modified grammar without altering the original.


Grammar Name
~~~~~~~~~~~~

The prefix to be used in classes generated by |TatSu| can be passed to
the command-line tool using the ``-m`` option:

.. code:: bash

   $ tatsu -m MyLanguage mygrammar.tatsu

will generate:

.. code:: python

   class MyLanguageParser(Parser):
       ...

The name can also be specified within the grammar using the
``@@grammar`` directive:

.. code:: apl
    :force:

    @@grammar :: MyLanguage


Whitespace
~~~~~~~~~~

By default, |TatSu| generated parsers skip the usual whitespace characters with
the regular expression ``r'\s+'``, but you can change that behavior.

Whitespace may be specified within the grammar using the
``@@whitespace`` directive in the grammar:

.. code:: apl
    :force:

    @@whitespace :: /[\t ]+/

or:

.. code:: apl
    :force:

    @@whitespace :: None

If no ``whitespace`` or ``@@whitespace`` is specified, |TatSu| will use
``r'(?m)\s+'`` as a default. Use ``None`` to have *no whitespace definition*.

You can also pass a ``whitespace`` parameter to your parser, overriding any
directive setting in the grammar. For example, the following will skip over
*tab* (``\t``) and *space* characters, but not so with other typical whitespace
characters such as *newline* (``\n``):

.. code:: python

   parser = tatsu.parse(grammar, text, whitespace='\t ')

The character string is converted into a regular expression character
set before starting to parse.

You can also provide a regular expression directly instead of a string.
The following is equivalent to the previous example:

.. code:: python

   parser = tatsu.parse(grammar, text, whitespace=re.compile(r'[\t ]+'))

Note that the regular expression must be pre-compiled to let |TatSu|
distinguish it from plain string.

If you do not define any whitespace characters, then you will have to
handle whitespace in your grammar rules (as it's often done in `PEG`_
parsers):

.. code:: python

   parser = tatsu.parse(grammar, text, whitespace='')


.. code:: python

   parser = tatsu.parse(grammar, text, whitespace=None)

Case Sensitivity
~~~~~~~~~~~~~~~~

You may specify case insensitivity within the grammar using the
``@@ignorecase`` directive:

.. code:: apl
    :force:

    @@ignorecase :: True

The change will affect token matching, but not pattern matching. Use ``(?i)``
in patterns that should ignore case.

Case sensitivity can also be specified in the parser by using the ``ignorecase``
parameter when instantiating a parser:

.. code:: python

   parser = tatsu.parse(grammar, text, ignorecase=True)

Comments
~~~~~~~~

Parsers will skip over comments specified as a regular expression using
the ``comments`` parameter:

.. code:: python

   parser = tatsu.parse(grammar, text, comments="\(\*.*?\*\)")

For more complex comment handling, you can override the
``Buffer.eat_comments()`` method.

For flexibility, it's also possible to specify a pattern for end-of-line
comments:

.. code:: python

   parser = tatsu.compile(
       grammar,
       comments="\(\*.*?\*\)",
       eol_comments="#.*?$"
   )

Both patterns may also be specified within a grammar using the
``@@comments`` and ``@@eol_comments`` directives:

.. code:: apl
    :force:

    @@comments :: /\(\*.*?\*\)/
    @@eol_comments :: /#.*?$/


Reserved Words and Keywords
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Some languages must reserve the use of certain tokens as valid
identifiers because the tokens are used to mark particular constructs in
the language. Those reserved tokens are known as `Reserved Words`_ or
`Keywords`_

|TatSu| provides support for preventing the use of `keywords`_ as
identifiers though the ``@@keyword`` directive,and the ``@name``
decorator.

A grammar may specify reserved tokens providing a list of them in one or
more ``@@keyword`` directives:

.. code:: apl
    :force:

    @@keyword :: if endif
    @@keyword :: else elseif

The ``@name`` decorator checks that the result of a grammar rule does
not match a token defined as a `keyword`_:

.. code:: apl
    :force:

    @name
    identifier = /(?!\d)\w+/ ;

Note that the rule decorated with ``@name`` must produce a single string as result
(no named expressions that will produce a dict, and no rule arguments).

In some situations a token is reserved only in a specific context. In those
cases, a negative lookahead will prevent the use of that token:

.. code:: apl
    :force:

    statements = {!'END' statement}+ ;

Include Directive
~~~~~~~~~~~~~~~~~

|TatSu| grammars support file inclusion through the include directive:

.. code:: apl
    :force:

    #include :: "filename"

The resolution of the *filename* is relative to the directory/folder of
the source. Absolute paths and ``../`` navigation are honored.

The base for implementing includes is available to |TatSu|-generated
parsers through the ``Buffer`` class. See the ``EBNFBuffer`` class in
``tatsu.parser`` module for an example.


Left Recursion
~~~~~~~~~~~~~~

|TatSu| supports left recursion in `PEG`_ grammars. The algorithm used is
`Warth et al`_'s. Left recursion support is enabled by default.

Left recursion can be turned *on* or *off* from within the grammar using the
``@@left_recursion`` directive:

.. code:: apl
    :force:

    @@left_recursion :: False

Sometimes, while debugging a grammar, it's useful to turn left-recursion
support *on* or *off* in the code:

.. code:: python

   parser = tatsu.parse(grammar, text, left_recursion=True)