Examples

Example: Subtraction language

This end-to-end example implements a small expression language that evaluates subtraction expressions. It walks through all three grammar sections and shows the output of each pipeline stage.

Sample programs

Create a file named samples:

3

-(3,2)

-(-(4,1), -(3,2))

Grammar file

Create subtract.plcc:

PythonJava

# Subtraction language
skip WHITESPACE '\s+'
token WHOLE     '\d+'
token MINUS     '\-'
token LP        '\('
token RP        '\)'
token COMMA     ','
%
<Prog>         ::= <Exp:exp>
<Exp:WholeExp> ::= <WHOLE:whole>
<Exp:SubExp>   ::= MINUS LP <Exp:exp1> COMMA <Exp:exp2> RP
%
Python

Prog
%%%
def _run(self):
    print(self.exp.eval())
%%%

WholeExp
%%%
def eval(self):
    return int(self.whole.lexeme)
%%%

SubExp
%%%
def eval(self):
    return self.exp1.eval() - self.exp2.eval()
%%%

# Subtraction language
skip WHITESPACE '\s+'
token WHOLE     '\d+'
token MINUS     '\-'
token LP        '\('
token RP        '\)'
token COMMA     ','
%
<Prog>         ::= <Exp:exp>
<Exp:WholeExp> ::= <WHOLE:whole>
<Exp:SubExp>   ::= MINUS LP <Exp:exp1> COMMA <Exp:exp2> RP
%
Java

Prog
%%%
public void _run() {
    System.out.println(exp.eval());
}
%%%

WholeExp
%%%
public int eval() {
    return Integer.parseInt(whole.lexeme);
}
%%%

SubExp
%%%
public int eval() {
    return exp1.eval() - exp2.eval();
}
%%%

Scanner

plcc-scan -s subtract.plcc samples

Expected output:

samples:1:1 WHOLE '3'
samples:3:1 MINUS '-'
samples:3:2 LP '('
samples:3:3 WHOLE '3'
samples:3:4 COMMA ','
samples:3:5 WHOLE '2'
samples:3:6 RP ')'
samples:5:1 MINUS '-'
samples:5:2 LP '('
samples:5:3 MINUS '-'
samples:5:4 LP '('
samples:5:5 WHOLE '4'
samples:5:6 COMMA ','
samples:5:7 WHOLE '1'
samples:5:8 RP ')'
samples:5:9 COMMA ','
samples:5:11 MINUS '-'
samples:5:12 LP '('
samples:5:13 WHOLE '3'
samples:5:14 COMMA ','
samples:5:15 WHOLE '2'
samples:5:16 RP ')'
samples:5:17 RP ')'

Parser

plcc-parse -s subtract.plcc samples

Expected output:

Prog
  WholeExp
    WHOLE '3' [-:1:1]
Prog
  SubExp
    WholeExp
      WHOLE '3' [-:3:3]
    WholeExp
      WHOLE '2' [-:3:5]
Prog
  SubExp
    SubExp
      WholeExp
        WHOLE '4' [-:5:5]
      WholeExp
        WHOLE '1' [-:5:7]
    SubExp
      WholeExp
        WHOLE '3' [-:5:13]
      WholeExp
        WHOLE '2' [-:5:15]

Interpreter

plcc-rep -s subtract.plcc samples

Expected output:

3
1
2