Syntactic analysis¶
-
@
production
(prod, priority=None)¶ Use this decorator to declare a grammar production:
class MyParser(LRParser, ReLexer): @production('E -> E "+" E') def sum(self): pass
See the Production syntax section.
The priority argument may be specified to declare that the production has the same priority as an existing token type. Typical use for unary minus:
class MyParser(LRParser, ReLexer): # omitting productions for binary +, -, * and / @production('E -> "-" E', priority='*') def minus(self): pass
You can also use a token type that has not been declared to the lexer as long as you have declared an explicit priority for it, using one of the associativity decorators:
@leftAssoc('+', '-') @leftAssoc('*', '/') @nonAssoc('UNARYMINUS') # Non associative, higher priority than anything else class MyParser(LRParser, ReLexer): @production('E -> "-" E', priority='UNARYMINUS') def minus(self): pass
-
exception
ptk.parser.
ParseError
(grammar, tok, state)[source]¶ Syntax error when parsing.
Variables: token – The unexpected token.
-
ptk.parser.
leftAssoc
(*operators)[source]¶ Class decorator for left associative operators. Use this to decorate your
Parser
class. Operators passed as argument are assumed to have the same priority. The later you declare associativity, the higher the priority; so the following code@leftAssoc('+', '-') @leftAssoc('*', '/') class MyParser(LRParser): # ...
declares ‘+’ and ‘-‘ to be left associative, with the same priority. ‘*’ and ‘/’ are also left associative, with a higher priority than ‘+’ and ‘-‘.
See also the priority argument to
production()
.
-
ptk.parser.
rightAssoc
(*operators)[source]¶ Class decorator for right associative operators. Same remarks as
leftAssoc()
.
-
ptk.parser.
nonAssoc
(*operators)[source]¶ Class decorator for non associative operators. Same remarks as
leftAssoc()
.
-
class
ptk.parser.
LRParser
[source]¶ LR(1) parser. This class is intended to be used with a lexer class derived from
LexerBase
, using inheritance; it overridesLexerBase.newToken()
so you must inherit from the parser first, then the lexer:class MyParser(LRParser, ReLexer): # ...
Production syntax¶
Basics¶
The productions specified through the production()
decorator must be specified in a variant of BNF; for example
class Parser(LRParser, ReLexer):
@production('E -> E plus E')
def binaryop(self):
pass
@production('E -> E minus E')
def binaryop(self):
pass
Here non terminal symbols are uppercase and terminals (token types) are lowercase, but this is only a convention.
Note
Yes, you can use the same method name for different semantic actions. Don’t abuse it.
When you don’t need separate semantic actions you can group several productions by using either the ‘|’ symbol:
class Parser(LRParser, ReLexer):
@production('E -> E plus E | E minus E')
def binaryop(self):
pass
Or decorating the same method several times:
class Parser(LRParser, ReLexer):
@production('E -> E plus E')
@production('E -> E minus E')
def binaryop(self):
pass
Semantic values¶
The semantic value associated with a production is the return value of the decorated method. Values for items on the right side of the production are not passed to the method by default; you have to use a specific syntax to associate each item with a name, which will then be used as the name of a keyword argument passed to the method. The name must be specified between brackets after the item, for instance:
class Parser(LRParser, ReLexer):
@production('E -> E<left> plus E<right>')
def sum(self, left, right):
return left + right
You can thus use alternatives and default argument values to slightly change the action’s behavior depending on the actual matched production:
class Parser(LRParser, ReLexer):
@production('SYMNAME -> identifier<value> | identifier<value> left_bracket identifier<name> right_bracket')
def symname(self, value, name=None):
if name is None:
# First form, name not specified
else:
# Second form
Litteral tokens¶
A litteral token name may appear in a production, between double quotes. This allows you to skip declaring “simple” tokens at the lexer level.
class Parser(LRParser, ReLexer):
@production('E -> E "+" E')
def sum(self):
pass
Note
Those tokens are considered “declared” after the ones explicitely declared through the token()
decorator. This may be important because of the disambiguation rules; see the notes for the token()
decorator.
Litteral tokens may be named as well.
Repeat operators¶
A nonterminal in the right side of a production may be immediately followed by a repeat operator among “*”, “+” and ”?”, which have the same meaning as in regular expressions. Note that this is only syntactic sugar; under the hood additional productions are generated.
A -> B?
is equivalent to
A ->
A -> B
The semantic value is None if the empty production was applied, or the semantic value of B if the ‘L_B -> B’ production was applied.
A -> B*
is equivalent to
A ->
A -> L_B
L_B -> B
L_B -> L_B B
The semantic value is a list of semantic values for B. ‘+’ works the same way except for the empty production, so the list cannot be empty.
Additionally, the ‘*’ and ‘+’ operators may include a separator specification, which is a symbol name or litteral token between parens:
A -> B+("|")
is equivalent to
A -> L_B
L_B -> B
L_B -> L_B "|" B
The semantic value is still a list of B values; there is no way to get the values for the separators.
Wrapping it up¶
Fully functional parser for a four-operations integer calculator:
@leftAssoc('+', '-')
@leftAssoc('*', '/')
class Parser(LRParser, ReLexer):
@token('[1-9][0-9]*')
def number(self, tok):
tok.value = int(tok.value)
@production('E -> number<n>')
def litteral(self, n):
return n
@production('E -> "-" E<val>', priority='*')
def minus(self, val):
return -val
@production('E -> "(" E<val> ")"')
def paren(self, val):
return val
@production('E -> E<left> "+"<op> E<right>')
@production('E -> E<left> "-"<op> E<right>')
@production('E -> E<left> "*"<op> E<right>')
@production('E -> E<left> "/"<op> E<right>')
def binaryop(self, left, op, right):
return {
'+': operator.add,
'-': operator.sub,
'*': operator.mul,
'/': operator.floordiv
}[op](left, right)
Parsing lists of integers separated by commas:
class Parser(LRParser, ReLexer):
@token('[1-9][0-9]*')
def number(self, tok):
tok.value = int(tok.value)
@production('LIST -> number*(",")<values>')
def integer_list(self, values):
print('Values are: %s' % values)
Conflict resolution rules¶
Conflict resolution rules are the same as those used by Yacc/Bison. A shift/reduce conflict is resolved by choosing to shift. A reduce/reduce conflict is resolved by choosing the reduction associated with the first declared production. leftAssoc()
, rightAssoc()
, nonAssoc()
and the priority argument to production()
allows you to explicitely disambiguate.