up previous next
3.7.9 Describing a Tagged Object
If the object E is tagged with the string S, then when the user enters the command Describe E, CoCoA first looks for a user-defined function with name Describe_S and executes it; if not found, the output depends on the type of Untagged(E).

  Use R ::= QQ[x,y,z];
  I := Ideal(x-y^2,x-z^3);
  I := Tagged(I,"MyIdeals");  -- I is now tagged with "MyIdeals"
  Describe I;  -- the default description of an ideal
Record[Type = IDEAL, Value = Record[Gens = [-y^2 + x, -z^3 + x]]]
  Define Describe_MyIdeals(X)
    Y := Untagged(X);
    PrintLn "The generators are:";
    Foreach G In Y.Gens Do
      PrintLn"  ", G;
  Describe I;  -- Any object tagged with "MyIdeals" is now described
               -- using "Describe_MyIdeals".
The generators are:
  -y^2 + x
  -z^3 + x