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catch an error
Catch C EndCatch;
Catch C In E EndCatch;
where C is a sequence of commands and E is a variable identifier.
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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.
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.
Define Test(N)
Catch
PrintLn(1/N);
In E EndCatch;
If Type(E) = ERROR Then Print("An error occurred: ", GetErrMesg(E)) EndIf;
EndDefine;
Test(3);
1/3
-------------------------------
Test(0);
An error occurred: Division by zero
-------------------------------
--Illustration of the BUG --
Define Test2()
Catch
Print("Hello ");
Return; -- incorrect: no value is returned
EndCatch;
PrintLn("world.");
EndDefine;
Test2();
Hello world.
-------------------------------
Define Test3()
Catch
Print("Hello ");
Return 3; -- correct a value is returned
EndCatch;
PrintLn("world.");
EndDefine;
Test3();
Hello 3
-------------------------------
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