Grammar Syntax¶
竜 TatSu uses a variant of the standard EBNF syntax. Syntax
definitions for VIM and for Sublime Text can be found under the
etc/vim
and etc/sublime
directories in the source code
distribution.
Rules¶
A grammar consists of a sequence of one or more rules of the form:
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:
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 several rules.
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.
Expressions¶
The expressions, in reverse order of operator precedence, can be:
# comment
¶
Python-style comments are allowed.
e1 | e2
¶
Choice. Match either e1
or e2
.
A | may be used before the first option if desired:
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 active option and prevent other options from being considered even if what follows fails to parse.
In this example, other options won’t be considered if a parenthesis is parsed:
atom
=
| '(' ~ @:expre ')'
| int
| bool
;
There are also options in optional expressions, because [foo]
is
equivalent to (foo|())
.
There are options also in closures, because of a similar equivalency,
so the following rule will fail if expression
is not parsed after
an =
is parsed, while the version without the ~
would succeed
over a partial parse of the name '=' expression
ahead in the
input:
parameters
=
','.{name '=' ~ expression}
;
s%{ e }+
¶
Positive join. Inspired by Python’s str.join()
, it parses the
same as this expression:
e {s ~ e}
yet the result is a single list of the form:
[e, s, e, s, e, ...]
Use grouping if s is more complex than a token or a pattern:
(s t)%{ e }+
s%{ e }
or s%{ e }*
¶
Join. Parses the list of s
-separated expressions, or the empty
closure. It is equivalent to:
s%{e}+|{}
op<{ e }+
¶
Left join. Like the join expression, but the result is a
left-associative tree built with tuple()
, in wich the first
element is the separator (op
), and the other two elements are the
operands.
The expression:
'+'<{/\d+/}+
Will parse this input:
1 + 2 + 3 + 4
To this tree:
(
'+',
(
'+',
(
'+',
'1',
'2'
),
'3'
),
'4'
)
op>{ e }+
¶
Right join. Like the join expression, but the result is a
right-associative tree built with tuple()
, in wich the first
element is the separator (op
), and the other two elements are the
operands.
The expression:
'+'>{/\d+/}+
Will parse this input:
1 + 2 + 3 + 4
To this tree:
(
'+',
'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 is equivalent to:
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 竜 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.
Alternatively, the @@nameguard
or @@namechars
directives may
be specified in the grammar:
@@nameguard :: False
or to specify additional characters that should also be considered part of names:
@@namechars :: '$-.'
r'text'
or r"text"
¶
Match the token text within the quotation marks, interpreting text like Python’s raw string literals.
?"regexp"
or ?'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 are involved.
The expression is equivalent to:
{ !e /./ } e
A common form of the expression is ->&e
, which is equivalent to:
{ !e /./ } &e
This is an example of the use of the “skip to” expression for recovery:
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:
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 scaped to
\{
if they are not intended for interpolation. A constant expression
that hast type str
is evaluated using:
eval(f'{"f" + repr(text)}', {}, ast)
```constant```
¶
A multiline version of `constant`
.
^ `constant`
and ^ ```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 indicate the alert
level:
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:
includable = exp1 ;
expanded = exp0 >includable exp2 ;
Has the same effect as defining expanded as:
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:
number = /[0-9]+/ ;
Without having to write:
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:
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:
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.
Rules with Arguments¶
竜 TatSu allows rules to specify Python-style arguments:
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:
addition::Add, '+'
=
addend '+' addend
;
Semantic methods must be ready to receive any arguments declared in the corresponding rule:
def addition(self, ast, name, op=None):
...
When working with rule arguments, it is good to define a
_default()
method that is ready to take any combination of
standard and keyword arguments:
def _default(self, ast, *args, **kwargs):
...
Based Rules¶
Rules may extend previously defined rules using the <
operator.
The base rule must be defined previously in the grammar.
The following set of declarations:
base::Param = exp1 ;
extended < base = exp2 ;
Has the same effect as defining extended as:
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¶
竜 TatSu is a packrat parser. The result of parsing a rule at a given position in the input is cached, so the next time the parser visits the same input position with the same rule the same result is returned and the input advanced, without repeating the parsing. Memoization allows for grammars that are clearer and easier to write because there’s no fear that repeating subexpressions will impact performance.
There are rules that should not be memoized. For example, rules that may succeed or not depending on the associated semantic action should not be memoized if sucess depends on more than just the input.
The @nomemo
decorator turns off memoization for a particular rule:
@nomemo
INDENT = () ;
@nomemo
DEDENT = () ;
Rule Overrides¶
A grammar rule may be redefined by using the @override
decorator:
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 竜 TatSu can be passed to
the command-line tool using the -m
option:
$ tatsu -m MyLanguage mygrammar.ebnf
will generate:
class MyLanguageParser(Parser):
...
The name can also be specified within the grammar using the
@@grammar
directive:
@@grammar :: MyLanguage
Whitespace¶
By default, 竜 TatSu generated parsers skip the usual whitespace
characters with the regular expression r'\s+'
using the
re.UNICODE
flag (or with the Pattern_White_Space
property if the
regex module is available), but you can change that behavior by
passing a whitespace
parameter to your parser.
For example, the following will skip over tab (\t
) and space
characters, but not so with other typical whitespace characters such as
newline (\n
):
parser = MyParser(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 above example:
parser = MyParser(text, whitespace=re.compile(r'[\t ]+'))
Note that the regular expression must be pre-compiled to let 竜 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):
parser = MyParser(text, whitespace='')
Whitespace may also be specified within the grammar using the
@@whitespace
directive, although any of the above methods will
overwrite the setting in the grammar:
@@whitespace :: /[\t ]+/
If no whitespace
or @@whitespace
is specified, 竜 TatSu will use
r'(?m)\s+'
as a default. Use None
to have no whitespace definition.
parser = MyParser(text, whitespace=None)
or:
@@whitespace :: None
Case Sensitivity¶
If the source language is case insensitive, it can be specified in the
parser by using the ignorecase
parameter:
parser = MyParser(text, ignorecase=True)
You may also specify case insensitivity within the grammar using the
@@ignorecase
directive:
@@ignorecase :: True
The change will affect token matching, but not pattern matching. Use (?i) in patterns that should ignore case.
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
竜 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:
@@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:
@name
identifier = /(?!\d)\w+/ ;
There are situations in which a token is reserved only in a very specific context. In those cases, a negative lookahead will prevent the use of the token:
statements = {!'END' statement}+ ;
Include Directive¶
竜 TatSu grammars support file inclusion through the include directive:
#include :: "filename"
The resolution of the filename is relative to the directory/folder of
the source. Absolute paths and ../
navigations are honored.
The functionality required for implementing includes is available to all
竜 TatSu-generated parsers through the Buffer
class; see the
EBNFBuffer
class in the tatsu.parser
module for an example.
Left Recursion¶
竜 TatSu supports left recursion in PEG grammars. The algorithm used is Warth et al’s.
Sometimes, while debugging a grammar, it is useful to turn left-recursion support on or off:
parser = MyParser(
text,
left_recursion=True,
)
Left recursion can also be turned off from within the grammar using the
@@left_recursion
directive:
@@left_recursion :: False
Comments¶
Parsers will skip over comments specified as a regular expression using the
comments_re
parameter:For more complex comment handling, you can override the
Buffer.eat_comments()
method.For flexibility, it is possible to specify a pattern for end-of-line comments separately:
Both patterns may also be specified within a grammar using the
@@comments
and@@eol_comments
directives: