Initial revision

diff --git a/mad b/mad
new file mode 100755 (executable)
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--- /dev/null
+++ b/mad
@@ -0,0 +1,109 @@
+#!/usr/bin/env python
+# -----------------------------------------------------------------------------
+# calc.py
+#
+# A simple calculator with variables.   This is from O'Reilly's
+# "Lex and Yacc", p. 63.
+# -----------------------------------------------------------------------------
+
+tokens = (
+    'NAME','NUMBER',
+    'PLUS','MINUS','TIMES','DIVIDE','EQUALS',
+    'LPAREN','RPAREN',
+    )
+
+# Tokens
+
+t_PLUS    = r'\+'
+t_MINUS   = r'-'
+t_TIMES   = r'\*'
+t_DIVIDE  = r'/'
+t_EQUALS  = r'='
+t_LPAREN  = r'\('
+t_RPAREN  = r'\)'
+t_NAME    = r'[a-zA-Z_][a-zA-Z0-9_]*'
+
+def t_NUMBER(t):
+    r'\d+'
+    try:
+        t.value = int(t.value)
+    except ValueError:
+        print "Integer value too large", t.value
+        t.value = 0
+    return t
+
+t_ignore = " \t"
+
+def t_newline(t):
+    r'\n+'
+    t.lineno += t.value.count("\n")
+    
+def t_error(t):
+    print "Illegal character '%s'" % t.value[0]
+    t.skip(1)
+    
+# Build the lexer
+import lex
+lex.lex()
+
+# Parsing rules
+
+precedence = (
+    ('left','PLUS','MINUS'),
+    ('left','TIMES','DIVIDE'),
+    ('right','UMINUS'),
+    )
+
+# dictionary of names
+names = { }
+
+def p_statement_assign(t):
+    'statement : NAME EQUALS expression'
+    names[t[1]] = t[3]
+
+def p_statement_expr(t):
+    'statement : expression'
+    print t[1]
+
+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]
+
+def p_expression_uminus(t):
+    'expression : MINUS expression %prec UMINUS'
+    t[0] = -t[2]
+
+def p_expression_group(t):
+    'expression : LPAREN expression RPAREN'
+    t[0] = t[2]
+
+def p_expression_number(t):
+    'expression : NUMBER'
+    t[0] = t[1]
+
+def p_expression_name(t):
+    'expression : NAME'
+    try:
+        t[0] = names[t[1]]
+    except LookupError:
+        print "Undefined name '%s'" % t[1]
+        t[0] = 0
+
+def p_error(t):
+    print "Syntax error at '%s'" % t.value
+
+import yacc
+yacc.yacc()
+
+while 1:
+    try:
+        s = raw_input('calc > ')
+    except EOFError:
+        break
+    yacc.parse(s)