What is a semantic predicate in ANTLR?
For predicates in ANTLR 4, checkout these stackoverflow Q&A's:
A semantic predicate is a way to enforce extra (semantic) rules upon grammar actions using plain code.
There are 3 types of semantic predicates:
Let's say you have a block of text consisting of only numbers separated by
comma's, ignoring any white spaces. You would like to parse this input making
sure that the numbers are at most 3 digits "long" (at most 999). The following
grammar (Numbers.g
) would do such a thing:
grammar Numbers;
// entry point of this parser: it parses an input string consisting of at least
// one number, optionally followed by zero or more comma's and numbers
parse
: number (',' number)* EOF
;
// matches a number that is between 1 and 3 digits long
number
: Digit Digit Digit
| Digit Digit
| Digit
;
// matches a single digit
Digit
: '0'..'9'
;
// ignore spaces
WhiteSpace
: (' ' | '\t' | '\r' | '\n') {skip();}
;
The grammar can be tested with the following class:
import org.antlr.runtime.*;
public class Main {
public static void main(String[] args) throws Exception {
ANTLRStringStream in = new ANTLRStringStream("123, 456, 7 , 89");
NumbersLexer lexer = new NumbersLexer(in);
CommonTokenStream tokens = new CommonTokenStream(lexer);
NumbersParser parser = new NumbersParser(tokens);
parser.parse();
}
}
Test it by generating the lexer and parser, compiling all .java
files and
running the Main
class:
java -cp antlr-3.2.jar org.antlr.Tool Numbers.g javac -cp antlr-3.2.jar *.java java -cp .:antlr-3.2.jar Main
When doing so, nothing is printed to the console, which indicates that nothing went wrong. Try changing:
ANTLRStringStream in = new ANTLRStringStream("123, 456, 7 , 89");
into:
ANTLRStringStream in = new ANTLRStringStream("123, 456, 7777 , 89");
and do the test again: you will see an error appearing on the console right after the string 777
.
This brings us to the semantic predicates. Let's say you want to parse numbers between 1 and 10 digits long. A rule like:
number
: Digit Digit Digit Digit Digit Digit Digit Digit Digit Digit
| Digit Digit Digit Digit Digit Digit Digit Digit Digit
/* ... */
| Digit Digit Digit
| Digit Digit
| Digit
;
would become cumbersome. Semantic predicates can help simplify this type of rule.
A validating semantic predicate is nothing more than a block of code followed by a question mark:
RULE { /* a boolean expression in here */ }?
To solve the problem above using a validating
semantic predicate, change the number
rule in the grammar into:
number
@init { int N = 0; }
: (Digit { N++; } )+ { N <= 10 }?
;
The parts { int N = 0; }
and { N++; }
are plain Java statements of which
the first is initialized when the parser "enters" the number
rule. The actual
predicate is: { N <= 10 }?
, which causes the parser to throw a
FailedPredicateException
whenever a number is more than 10 digits long.
Test it by using the following ANTLRStringStream
:
// all equal or less than 10 digits
ANTLRStringStream in = new ANTLRStringStream("1,23,1234567890");
which produces no exception, while the following does thow an exception:
// '12345678901' is more than 10 digits
ANTLRStringStream in = new ANTLRStringStream("1,23,12345678901");
A gated semantic predicate is similar to a validating semantic predicate,
only the gated version produces a syntax error instead of a FailedPredicateException
.
The syntax of a gated semantic predicate is:
{ /* a boolean expression in here */ }?=> RULE
To instead solve the above problem using gated predicates to match numbers up to 10 digits long you would write:
number
@init { int N = 1; }
: ( { N <= 10 }?=> Digit { N++; } )+
;
Test it again with both:
// all equal or less than 10 digits
ANTLRStringStream in = new ANTLRStringStream("1,23,1234567890");
and:
// '12345678901' is more than 10 digits
ANTLRStringStream in = new ANTLRStringStream("1,23,12345678901");
and you will see the last on will throw an error.
The final type of predicate is a disambiguating semantic predicate, which looks a bit like a validating predicate ({boolean-expression}?
), but acts more like a gated semantic predicate (no exception is thrown when the boolean expression evaluates to false
). You can use it at the start of a rule to check some property of a rule and let the parser match said rule or not.
Let's say the example grammar creates Number
tokens (a lexer rule instead of a parser rule) that will match numbers in the range of 0..999. Now in the parser, you'd like to make a distinction between low- and hight numbers (low: 0..500, high: 501..999). This could be done using a disambiguating semantic predicate where you inspect the token next in the stream (input.LT(1)
) to check if it's either low or high.
A demo:
grammar Numbers;
parse
: atom (',' atom)* EOF
;
atom
: low {System.out.println("low = " + $low.text);}
| high {System.out.println("high = " + $high.text);}
;
low
: {Integer.valueOf(input.LT(1).getText()) <= 500}? Number
;
high
: Number
;
Number
: Digit Digit Digit
| Digit Digit
| Digit
;
fragment Digit
: '0'..'9'
;
WhiteSpace
: (' ' | '\t' | '\r' | '\n') {skip();}
;
If you now parse the string "123, 999, 456, 700, 89, 0"
, you'd see the following output:
low = 123
high = 999
low = 456
high = 700
low = 89
low = 0