Catch

Catch

catch an error

Syntax


Catch C EndCatch;
Catch C In E EndCatch;

where C is a sequence of commands and E is a variable identifier.

Description

Usually, when an error occurs during the execution of a command, the error is automatically propagated out of the nesting of the evaluation. This can be prevented with the use of Catch.

If an error occurs during the execution of C, then it is captured by the command Catch and (in the second form) assigned to the variable E. If no error occurs, then E will contain the value Null. Note the use of the function GetErrMesg in the example below.

Example

  Deg(0);
ERROR: Deg of zero is not defined
CONTEXT: Error("Deg of zero is not defined")
-------------------------------
  Define MyDeg(F)
    Catch  D := Deg(F);  In E EndCatch;
    If Type(E) = ERROR Then
      If IsIn("Deg of zero", GetErrMesg(E)) Then
        Return -1234;
      Else
        Return E;
      EndIf; 
    EndIf;
    Return D;
  EndDefine;

  MyDeg(x);
1
-------------------------------
  MyDeg(0);
-1234
-------------------------------
  MyDeg(1/x);
ERROR: Deg: first argument must be POLY or VECTOR
CONTEXT: Return(E)
-------------------------------

IMPORTANT NOTE: There is a bug in Catch. Any Return command used inside Catch must return some value. If not, the Return command will just return from the Catch-EndCatch statement; it will not return from the function within which the statement is embedded. There is an example below.

Example

  Define Test2()
    Catch Print "Hello "; Return; EndCatch;
    PrintLn "world.";
  EndDefine;
  Test2();
Hello world.

-------------------------------
  Define Test3()
    Catch Print "Hello "; Return 3; EndCatch;
    PrintLn "world.";
  EndDefine;
  Test3();
Hello 3
-------------------------------

See Also