CS 330 Lecture 10 – Basecalc Interpreter
Agenda
- what ?s
- whitespace
- expr productions
- parse trees
- a REPL
- an interpreter
TODO
- Read up on the origin of a programming language of your choosing, and share some interesting factoids on a 1/4 sheet.
Review
- Name one reason why early Fortran compilers were so good.
- Why is it harder to debug C code than, say, JavaScript or Python?
- Name some language rule that can be checked statically. Dynamically?
Process of Interpretation
- Write your grammar in a *.g file.
- Run antlr on *.g file to produce the Java source for your lexer and parser.
- Compile all your Java code.
- Test the parsing with
grun GrammarName startingRule -(gui|tree) < input.txt. - Write a parse tree walk listener. Label productions as necessary to generate specific events.
- In your callbacks, generate an abstract syntax tree or directly execute the interpreted code.
Code
Basecalc.g
grammar Basecalc;
line
: expr
| expr CAST DIGITS
;
expr
: LEFT_PARENTHESIS expr RIGHT_PARENTHESIS # Grouped
| NOT expr # Negate
| expr POWER expr # Power
| expr MULTIPLICATIVE_OPERATOR expr # Multiply
| expr ADDITIVE_OPERATOR expr # Add
| expr SHIFT expr # Shift
| expr AND expr # And
| expr XOR expr # Xor
| expr OR expr # Or
| IDENTIFIER ASSIGNMENT expr # Assignment
| DIGITS # Digits
| DIGITS LESS DIGITS GREATER # DigitsWithBase
| IDENTIFIER # Identifier
;
LEFT_PARENTHESIS: '(';
RIGHT_PARENTHESIS: ')';
POWER: '^^';
MULTIPLICATIVE_OPERATOR: [*/%];
ADDITIVE_OPERATOR: [-+];
NOT: '~';
SHIFT: '<<' | '>>';
AND: '&';
XOR: '^';
OR: '|';
ASSIGNMENT: ':=';
CAST: '->';
LESS: '<';
GREATER: '>';
IDENTIFIER: '$' [a-z]+ [a-z_]*;
DIGITS: [0-9a-zA-Z]+;
WHITESPACE: [ \n\r\t]+ -> skip;
Repl.java
import org.antlr.v4.runtime.ANTLRInputStream;
import org.antlr.v4.runtime.tree.ParseTreeWalker;
import org.antlr.v4.runtime.tree.ParseTree;
import org.antlr.v4.runtime.CommonTokenStream;
import java.util.Scanner;
public class Repl {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
System.out.print("> ");
while (in.hasNextLine()) {
String line = in.nextLine();
ANTLRInputStream ais = new ANTLRInputStream(line);
BasecalcLexer lexer = new BasecalcLexer(ais);
CommonTokenStream tokens = new CommonTokenStream(lexer);
BasecalcParser parser = new BasecalcParser(tokens);
ParseTree tree = parser.line();
System.out.println(parser.getNumberOfSyntaxErrors());
ParseTreeWalker walker = new ParseTreeWalker();
Interpreter interpreter = new Interpreter();
walker.walk(interpreter, tree);
System.out.print("> ");
}
}
}
Interpreter.java
public class Interpreter extends BasecalcBaseListener {
@Override
public void exitDigits(BasecalcParser.DigitsContext ctx) {
String digitsAsString = ctx.DIGITS().getText();
int digits = Integer.parseInt(digitsAsString);
System.out.println(digits);
}
}
Haiku
I learned English, sure
But it wasn’t till Spanish
That I learned language
But it wasn’t till Spanish
That I learned language