#*

  This script is an attempt to create a pl/0 parser/compiler
  in the parse script language. It just recognises pl/0 constant
  and variable declarations.

  * the pl/0 grammar in wsn/ebnf form
  -------
   program = block "." .
   block = [ "const" ident "=" number {"," ident "=" number} ";"]
        [ "var" ident {"," ident} ";"]
        { "procedure" ident ";" block ";" } statement .

   statement = [ ident ":=" expression | "call" ident 
              | "?" ident | "!" expression 
              | "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 ")".
  ,,,

*#

 # a constant declaration rule in Wirths ebnf grammar
 # constdec =  "const" ident "=" number {"," ident "=" number} ";" .
 # the script below factors out the {} repetition construct.

  read;

  [:alpha:] { 
    while [:alpha:]; 
    # keywords in pl/0
    "const","var","if","then","while","do","begin","end" { 
      put; add "*"; push; .reparse
    }
    put; clear; add "ident*"; 
    push; .reparse 
  }

  [0-9] { 
    while [0-9]; put; clear; add "number*"; 
    push; .reparse 
  }
  # literal tokens
  ",","=",";" { put; add "*"; push; }

  # catch illegal characters here.
  clear;

parse>

# ----------------------
# 2 tokens

  pop; pop;

  "var*ident*" {
    clear; add "varset*"; push; .reparse
  }

  "varset*;*" {
     clear; add "vardec*"; push; .reparse
  }
  
  "conset*;*" {
     clear; add "constdec*"; push; .reparse
  }

# ----------------------
# 3 tokens
  pop;

  "varset*,*ident*" {
    clear; add "varset*"; push; .reparse
  }

# ----------------------
# 4 tokens

  pop;

  "const*ident*=*number*" {
    clear; add "conset*"; push; .reparse
  }

# ----------------------
# 5 tokens
  pop;
  "conset*,*ident*=*number*" {
    clear; add "conset*"; push; .reparse
  }

  push; push; push; push; push;

  <eof> {
    pop;
    "constdec*","vardec*" { 
      add " declaration \n"; print; clear; quit; 
    }
  }