This is part of my parser grammar:
expression:
multiplyingExpression
(
PLUS multiplyingExpression #plus
| MINUS multiplyingExpression #minus
)*;
I want to define plus and minus Alternative Label, apparently it doesn't allow me to do so.
at #plus and #minus it gives me the error:
missing RPAREN at '#' while look for rule element
Anybody knows how to achieve this whithout changing the structure of this rule's definition?
An alt label can only be defined on the outer edge (non-nested) of an alt. Therefore, cannot be done without changing the structure of the rule.
What can be done is to use an ordinary label to effectively achieve the desired result.
expression:
multiplyingExpression
( type+=PLUS multiplyingExpression
| type+=MINUS multiplyingExpression
)*;
The result is that the ExpressionContext will contain List<Token> type; whose successive values will, by inference, identify the alts matched.
Related
The following test grammars differ only in that the first alternative of the rule 'expr' is either specified inline or refers to another rule 'notExpression' with just the same definition. But this grammars produce different trees parsing this: '! a & b'. Why?
I really want the grammar to produce the first result (with NOT associated with identifier, not with AND expression) but still need to have 'expr' to reference 'notExpression' in my real grammar. What do I have to change?
grammar test;
s: expr ';' <EOF>;
expr:
NOT expr
| left=expr AND right=expr
| identifier
;
identifier: LETTER (LETTER)*;
WS : ' '+ ->skip;
NOT: '!';
AND: '&';
LETTER: 'A'..'z';
Tree one
grammar test;
s: expr ';' <EOF>;
expr:
notExpression
| left=expr AND right=expr
| identifier
;
notExpression: NOT expr;
identifier: LETTER (LETTER)*;
WS : ' '+ ->skip;
NOT: '!';
AND: '&';
LETTER: 'A'..'z';
Tree two
I kind of got an answer to the second part of my question, which still do not quite give me a satisfaction because using this approach in real elaborate grammar is going to be ugly. As to the first part (WHY) I still have no idea, so more answers are welcome.
Anyway, to fix precedence in presence of referenced rule the 'notExpression' rule can be modified as follows:
notExpression: NOT (identifier|expr);
Which produces the tree different from both shown in original question, but at least the NOT does acquire higher precedence.
Parse tree
I must define a rule which expresses the following statement: {x in y | x > 0}.
For the first part of that comprehension "x in y", i have the subrule:
FIRSTPART: Name "in" Name
, whereas Name can be everything.
My problem is that I do not want a greedy behaviour. So, it should parse until the "|" sign and then stop. Since I am new in ANTLR4, I do not know how to achieve that.
best regards,
Normally, the lexer/parser rules should represent the allowable syntax of the source input stream.
The evaluation (and consequences) of how the source matches any rule or subrule is a matter of semantics -- whether the input matches a particular subrule and whether that should control how the rule is finally evaluated.
Normally, semantics are implemented as part of the tree-walker analysis. You can use alternate subrule lables (#inExpr, etc) to create easily distinguishable tree nodes for analysis purposes:
comprehension : LBrace expression property? RBrace ;
expression : ....
| Name In Name #inExpr
| Name BinOp Name #binExpr
| ....
;
property : Provided expression ;
BinOp : GT | LT | GTE | .... ;
Provided : '|' ;
In : 'in' ;
I am trying to go through regular expression and language questions however, this one seems to have gotten me stuck.
Can somebody help?
I am trying to write out the set that is defined by this regular expression:
To understand this regular expression, lets consider its three parts separately:
( a | Ɛ ) abb (a | b)
\---1---- --2--- ---3---
this regular expression is defined in three groups/parts using parenthesis
Part-1: Ɛ is a null symbol in regular expression, if it appears with some other symbol (or a group of symbols) with union operator | that means that symbol(or group) is option e.g. can be appear or not appear in some strings of language ( Ɛ symbols in FA as edge label defines 'null-transition' — which allows a transformation to a new state without consuming any input symbols).
In your regular expression, first 'a' is written with Ɛ — ( a | Ɛ ) so it is option - it can appear in some string or absent in other. Hence strings generated with using this regular expression either starts with two 'a' or one 'a'.
Part-2: Sub-string 'aab' always appears in all possible string using this regular expression.
so strings can be in two possible forms:
aabb(a|b)
abb(a|b)
Part-3: (a | b) string either ends with symbol 'a' or symbol 'b'.
if both above forms ends with 'a'
aabba
abba
if both above forms ends with 'b'
aabbb
abbb
Of-course it is a finite language and its DFA does not contain any loop. Its DFA for this language { aabba, abba, aabbb, abbb } would be as following:
I am doing a parser in bison/flex.
This is part of my code:
I want to implement the assignment production, so the identifier can be both boolean_expr or expr, its type will be checked by a symbol table.
So it allows something like:
int a = 1;
boolean b = true;
if(b) ...
However, it is reduce/reduce if I include identifier in both term and boolean_expr, any solution to solve this problem?
Essentially, what you are trying to do is to inject semantic rules (type information) into your syntax. That's possible, but it is not easy. More importantly, it's rarely a good idea. It's almost always best if syntax and semantics are well delineated.
All the same, as presented your grammar is unambiguous and LALR(1). However, the latter feature is fragile, and you will have difficulty maintaining it as you complete the grammar.
For example, you don't include your assignment syntax in your question, but it would
assignment: identifier '=' expr
| identifier '=' boolean_expr
;
Unlike the rest of the part of the grammar shown, that production is ambiguous, because:
x = y
without knowing anything about y, y could be reduced to either term or boolean_expr.
A possibly more interesting example is the addition of parentheses to the grammar. The obvious way of doing that would be to add two productions:
term: '(' expr ')'
boolean_expr: '(' boolean_expr ')'
The resulting grammar is not ambiguous, but it is no longer LALR(1). Consider the two following declarations:
boolean x = (y) < 7
boolean x = (y)
In the first one, y must be an int so that (y) can be reduced to a term; in the second one y must be boolean so that (y) can be reduced to a boolean_expr. There is no ambiguity; once the < is seen (or not), it is entirely clear which reduction to choose. But < is not the lookahead token, and in fact it could be arbitrarily distant from y:
boolean x = ((((((((((((((((((((((y...
So the resulting unambiguous grammar is not LALR(k) for any k.
One way you could solve the problem would be to inject the type information at the lexical level, by giving the scanner access to the symbol table. Then the scanner could look a scanned identifier token in the symbol table and use the information in the symbol table to decide between one of three token types (or more, if you have more datatypes): undefined_variable, integer_variable, and boolean_variable. Then you would have, for example:
declaration: "int" undefined_variable '=' expr
| "boolean" undefined_variable '=' boolean_expr
;
term: integer_variable
| ...
;
boolean_expr: boolean_variable
| ...
;
That will work but it should be obvious that this is not scalable: every time you add a type, you'll have to extend both the grammar and the lexical description, because the now the semantics is not only mixed up with the syntax, it has even gotten intermingled with the lexical analysis. Once you let semantics out of its box, it tends to contaminate everything.
There are languages for which this really is the most convenient solution: C parsing, for example, is much easier if typedef names and identifier names are distinguished so that you can tell whether (t)*x is a cast or a multiplication. (But it doesn't work so easily for C++, which has much more complicated name lookup rules, and also much more need for semantic analysis in order to find the correct parse.)
But, honestly, I'd suggest that you do not use C -- and much less C++ -- as a model of how to design a language. Languages which are hard for compilers to parse are also hard for human beings to parse. The "most vexing parse" continues to be a regular source of pain for C++ newcomers, and even sometimes trips up relatively experienced programmers:
class X {
public:
X(int n = 0) : data_is_available_(n) {}
operator bool() const { return data_is_available_; }
// ...
private:
bool data_is_available_;
// ...
};
X my_x_object();
// ...
if (!x) {
// This code is unreachable. Can you see why?
}
In short, you're best off with a language which can be parsed into an AST without any semantic information at all. Once the parser has produced the AST, you can do semantic analyses in separate passes, one of which will check type constraints. That's far and away the cleanest solution. Without explicit typing, the grammar is slightly simplified, because an expr now can be any expr:
expr: conjunction | expr "or" conjunction ;
conjunction: comparison | conjunction "and" comparison ;
comparison: product | product '<' product ;
product: factor | product '*' factor ;
factor: term | factor '+' term ;
term: identifier
| constant
| '(' expr ')'
;
Each action in the above would simply create a new AST node and set $$ to the new node. At the end of the parse, the AST is walked to verify that all exprs have the correct type.
If that seems like overkill for your project, you can do the semantic checks in the reduction actions, effectively intermingling the AST walk with the parse. That might seem convenient for immediate evaluation, but it also requires including explicit type information in the parser's semantic type, which adds unnecessary overhead (and, as mentioned, the inelegance of letting semantics interfere with the parser.) In that case, every action would look something like this:
expr : expr '+' expr { CheckArithmeticCompatibility($1, $3);
$$ = NewArithmeticNode('+', $1, $3);
}
I am parsing a C++ like declaration with this scaled down grammar (many details removed to make it a fully working example). It fails to work mysteriously (at least to me). Is it related to the use of context dependent predicate? If yes, what is the proper way to implement the "counting the number of child nodes logic"?
grammar CPPProcessor;
cppCompilationUnit : decl_specifier_seq? init_declarator* ';' EOF;
init_declarator: declarator initializer?;
declarator: identifier;
initializer: '=0';
decl_specifier_seq
locals [int cnt=0]
#init { $cnt=0; }
: decl_specifier+ ;
decl_specifier : #init { System.out.println($decl_specifier_seq::cnt); }
'const'
| {$decl_specifier_seq::cnt < 1}? type_specifier {$decl_specifier_seq::cnt += 1;} ;
type_specifier: identifier ;
identifier:IDENTIFIER;
CRLF: '\r'? '\n' -> channel(2);
WS: [ \t\f]+ -> channel(1);
IDENTIFIER:[_a-zA-Z] [0-9_a-zA-Z]* ;
I need to implement the standard C++ rule that no more than 1 type_specifier is allowed under an decl_specifier_seq.
Semantic predicate before type_specifier seems to be the solution. And the count is naturally declared as a local variable in decl_specifier_seq since nested decl_specifier_seq are possible.
But it seems that a context dependent semantic predicate like the one I used will produce incorrect parsing i.e. a semantic predicate that references $attributes. First an input file with correct result (to illustrate what a normal parse tree looks like):
int t=0;
and the parse tree:
But, an input without the '=0' to aid the parsing
int t;
0
1
line 1:4 no viable alternative at input 't'
1
the parsing failed with the 'no viable alternative' error (the numbers printed in the console is debug print of the $decl_specifier_cnt::cnt value as a verification of the test condition). i.e. the semantic predicate cannot prevent the t from being parsed as type_specifier and t is no longer considered a init_declarator. What is the problem here? Is it because a context dependent predicate having $decl_specifier_seq::cnt is used?
Does it mean context dependent predicate cannot be used to implement "counting the number of child nodes" logic?
EDIT
I tried new versions whose predicate uses member variable instead of the $decl_specifier_seq::cnt and surprisingly the grammar now works proving that the Context Dependent predicate did cause the previous grammar to fail:
....
#parser::members {
public int cnt=0;
}
decl_specifier
#init {System.out.println("cnt:"+cnt); }
:
'const'
| {cnt<1 }? type_specifier {cnt++;} ;
A normal parse tree is resulted:
This gives rise to the question of how to support nested rule if we must use member variables to replace the local variables to avoid context sensitive predicates?
And a weird result is that if I add a /*$ctx*/ after the predicate, it fails again:
decl_specifier
#init {System.out.println("cnt:"+cnt); }
:
'const'
| {cnt<1 /*$ctx*/ }? type_specifier {cnt++;} ;
line 1:4 no viable alternative at input 't'
The parsing failed with no viable alternative. Why the /*$ctx*/ causes the parsing to fail like when $decl_specifier_seq::cnt is used although the actual logic uses a member variable only?
And, without the /*$ctx*/, another issue related to the predicate called before #init block appears(described here)
ANTLR 4 evaluates semantic predicates in two cases.
The generated code evaluates a semantic predicate during parsing, and throws an exception of the evaluation returns false. All predicates traversed during parsing are evaluated in this way, including context-dependent predicates and predicates which do not appear at the left side of a decision.
The prediction method evaluates predicates in order to make correct decisions during parsing. In this case, predicates which appear anywhere other than the left edge of the decision being evaluated are assumed to return true (i.e. they are ignored). In addition, context-dependent predicates are only evaluated if the context data is available. The prediction algorithm will not create context structures that were not already provided by the parsing code. If a context-dependent predicate is encountered during prediction and no context is available, the predicate is assumed to return true (i.e. it is ignored for that decision).
The code generator does not evaluate the semantics of the target language, so it has no way to know that $ctx is semantically irrelevant when it appears in /*$ctx*/. Both cases result in the predicate being treated as context-dependent.