! Copyright (C) 2007 Chris Double.
! See http://factorcode.org/license.txt for BSD license.
USING: kernel arrays strings math.parser sequences
peg peg.ebnf peg.parsers memoize namespaces math ;
IN: peg.pl0

#! Grammar for PL/0 based on http://en.wikipedia.org/wiki/PL/0

EBNF: pl0 
block = ( "CONST" ident "=" number ( "," ident "=" number )* ";" )?
        ( "VAR" ident ( "," ident )* ";" )?
        ( "PROCEDURE" ident ";" ( block ";" )? )* statement 
statement = ( ident ":=" expression | "CALL" ident |
              "BEGIN" statement (";" statement )* "END" |
              "IF" condition "THEN" statement |
              "WHILE" condition "DO" statement )?
condition = "ODD" expression |
            expression ("=" | "#" | "<=" | "<" | ">=" | ">") expression 
expression = ("+" | "-")? term (("+" | "-") term )* 
term = factor (("*" | "/") factor )* 
factor = ident | number | "(" expression ")"
ident = (([a-zA-Z])+) [[ >string ]]
digit = ([0-9]) [[ digit> ]]
number = ((digit)+) [[ unclip [ swap 10 * + ] reduce ]]
program = block "."
;EBNF