From 8183119b0f60573d7615dcd2b58a58ef9a6b310c Mon Sep 17 00:00:00 2001
From: "Eric S. Raymond" <esr@thyrsus,com>
Date: Wed, 14 Jan 2004 03:39:20 +0000
Subject: [PATCH] Fix the calculator to return parse trees.

---
 mad | 21 ++++++---------------
 1 file changed, 6 insertions(+), 15 deletions(-)

diff --git a/mad b/mad
index efa391e..d892624 100755
--- a/mad
+++ b/mad
@@ -54,30 +54,25 @@ precedence = (
     ('right','UMINUS'),
     )
 
-# dictionary of names
-names = { }
-
 def p_statement_assign(t):
     'statement : NAME EQUALS expression'
-    names[t[1]] = t[3]
+    t[0] = ('substitution', t[1], t[3])
 
 def p_statement_expr(t):
     'statement : expression'
-    print t[1]
+    t[0] = t[1]
+    print t[0]
 
 def p_expression_binop(t):
     '''expression : expression PLUS expression
                   | expression MINUS expression
                   | expression TIMES expression
                   | expression DIVIDE expression'''
-    if t[2] == '+'  : t[0] = t[1] + t[3]
-    elif t[2] == '-': t[0] = t[1] - t[3]
-    elif t[2] == '*': t[0] = t[1] * t[3]
-    elif t[2] == '/': t[0] = t[1] / t[3]
+    t[0] = (t[2], t[1], t[3])
 
 def p_expression_uminus(t):
     'expression : MINUS expression %prec UMINUS'
-    t[0] = -t[2]
+    t[0] = ('-', t[2])
 
 def p_expression_group(t):
     'expression : LPAREN expression RPAREN'
@@ -89,11 +84,7 @@ def p_expression_number(t):
 
 def p_expression_name(t):
     'expression : NAME'
-    try:
-        t[0] = names[t[1]]
-    except LookupError:
-        print "Undefined name '%s'" % t[1]
-        t[0] = 0
+    t[0] = t[1]
 
 def p_error(t):
     print "Syntax error at '%s'" % t.value
-- 
2.43.0