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'Induced' Attribute Dependence Cycles


Typical Error Situations

Indirect cycles that show up after the so-called induced attribute dependences have been computed.

There are abstract trees where an attribute value depends on itself:

   ATTR a1,a2 : int;
   RULE  r1 :  root ::= a
   COMPUTE a.a1 = 1;
   END;
   RULE r2 : a ::=  b
   COMPUTE b.a1 = ADD(a.a1,b.a2);
   END;
   RULE r3 : b ::=  'a'
   COMPUTE b.a2 = 7;
   END;
   RULE r4 : b ::= 'b'
   COMPUTE b.a2 = ADD(b.a1,1);
   END;

The resulting error message is

   ERROR: INDUCED cyclic dependencies; 
          see OrdInfo, or use gorto
   ERROR: evaluation order for b.a1 not found
   ERROR: evaluation order for b.a2 not found

 

To locate the error write a .ctl file containing the line

   ORDER: GRAPH DIRECT_RULE;

add it to your .specs or .fw file and make a request for

   foo.specs:OrdInfo>

OrdInfo yields

  INDUCED GRAPH FOR RULE r2 ROW 7; COL 6
  a ::=   b
  ATNO CLASS ATNAME DEPENDS ON
  0    INH   a.a1   []
  1    INH   b.a1   CYCLIC [0 1 2]
  2    SYNT  b.a2   CYCLIC [0 1 2]
  END RULE
[this listing is slightly abbreviated and reformatted]

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