Exceptions/Catch an exception thrown in a nested call: Difference between revisions

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=={{header|XPL0}}==

The obvious solution is to simply do the catch error handling at the

point where the error is detected. However, XPL0's Restart intrinsic can

be used to do something similar to C++'s catch operation. This technique

avoids having to pass an error condition back up through several levels

of function calls. (Technically, these functions are actually procedures

because they don't return a value, but XPL0 doesn't enforce the

distinction.)

<syntaxhighlight lang "XPL0">func U0; \Exception caused by square root of negative value

real X;

X:= Sqrt(-42.);

func U1; \Exception caused by opening a non-existent file for input

int F;

F:= FOpen("unobtainium.txt", 0);

func Baz;

int CallNo;

[CallNo:= [1]; \static-like variable

if CallNo(0) = 1 then \first time Baz is called

[CallNo(0):= 2;

Text(0, "Calling U0^m^j");

Trap(false); \turn off error trapping to prevent program abort

U0;

Restart;

]

else \second time Baz is called

[Text(0, "Calling U1^m^j");

U1; \error trapping is still disabled

];

];

func Bar;

Baz;

func Foo;

Bar;

int Err;

[Err:= GetErr; \get the exception error after the program is restarted

if Err then \reading GetErr resets any error number to 0, = no error

[Text(0, "Error "); IntOut(0, Err); Text(0, " detected^m^j")];

Foo;

Text(0, "Finished^m^j");

]</syntaxhighlight>

{{out}}

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Calling U0

Error 10 detected

Calling U1

Finished

RUN-TIME ERROR 3: I/O