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1 # -----------------------------------------------------------------------------
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2 # yacc_unused_rule.py
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3 #
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4 # Grammar with an unused rule
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5 # -----------------------------------------------------------------------------
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6 import sys
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7
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8 if ".." not in sys.path: sys.path.insert(0,"..")
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9 import ply.yacc as yacc
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10
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11 from calclex import tokens
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12
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13 # Parsing rules
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14 precedence = (
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15 ('left','PLUS','MINUS'),
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16 ('left','TIMES','DIVIDE'),
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17 ('right','UMINUS'),
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18 )
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19
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20 # dictionary of names
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21 names = { }
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22
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23 def p_statement_assign(t):
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24 'statement : NAME EQUALS expression'
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25 names[t[1]] = t[3]
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26
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27 def p_statement_expr(t):
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28 'statement : expression'
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29 print(t[1])
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30
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31 def p_expression_binop(t):
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32 '''expression : expression PLUS expression
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33 | expression MINUS expression
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34 | expression TIMES expression
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35 | expression DIVIDE expression'''
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36 if t[2] == '+' : t[0] = t[1] + t[3]
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37 elif t[2] == '-': t[0] = t[1] - t[3]
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38 elif t[2] == '*': t[0] = t[1] * t[3]
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39 elif t[2] == '/': t[0] = t[1] / t[3]
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40
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41 def p_expression_uminus(t):
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42 'expression : MINUS expression %prec UMINUS'
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43 t[0] = -t[2]
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44
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45 def p_expression_group(t):
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46 'expression : LPAREN expression RPAREN'
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47 t[0] = t[2]
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48
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49 def p_expression_number(t):
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50 'expression : NUMBER'
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51 t[0] = t[1]
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52
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53 def p_expression_name(t):
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54 'expression : NAME'
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55 try:
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56 t[0] = names[t[1]]
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57 except LookupError:
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58 print("Undefined name '%s'" % t[1])
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59 t[0] = 0
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60
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61 def p_integer(t):
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62 'integer : NUMBER'
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63 t[0] = t[1]
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64
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65 def p_error(t):
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66 print("Syntax error at '%s'" % t.value)
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67
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68 yacc.yacc()
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69
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70
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71
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72
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