Updated 2016-10-02 07:45:53 by dkf

error, a built-in Tcl command, triggers an error state.

Synopsis  edit

error message ?info? ?code?

Description  edit

Generates an error with the specified message. If supplied, info is used to seed the errorInfo value, and code becomes the errorCode, which is otherwise NONE.

error is short for return -level 0 -code error, which is not the same as return -code error, the latter being the equivalent of return -level 1 -code error. When in doubt, just use error.

DKF: I find that it is best to use error (or throw) when it is an internal problem of the code, and return -code error when it is a problem caused by the caller like passing in a name when a number was specifically requested for the argument. The problem in that case is that the caller isn't obeying the interface contract, not that the implementation has gone wrong.

RS 2001-11-21: In C, errors are something you loathe and try to avoid. In Tcl, they're like "little friends" - helpful (explaining the problem), not messing up everything (as a Segmentation Fault/Bus Error/Divide by Zero would do). For instance,
set fp [open foo.bar]

leads to the error
couldn't open "foo.bar": no such file or directory

which tells it pretty well, and does not terminate the application if it has an event loop or is Interactive Tcl. In C, you would have received a NULL pointer, and without checking that, Segmentation Fault would be right around the corner. So in C, you have to add checks for everything that might go wrong; in Tcl, you only need to treat errors if you can express it better than Tcl already does.

And you can also play with errors: to fully break out of a multiply nested loop, put a catch outside and a throw within:
catch {
   ...# deep nesting ...

There is no throw command in Tcl [editors note: Of course there is one nowadays], so it is handled by unknown, which tries to load it from auto_index, and if that fails too, an error is raised - which in turn is caught by the catch. Pretty natural, no? And still somehow Zen buddhistic, if you deliberately use a non-existing command, and it does just the right thing... (Kevin Kenny introduced this trick in Tricky catch).

In some cases, stack traces look better (one less layer) if you replace
error {Something went wrong}

return -code error {Something went wrong}

See also edit

An error experiment, by RS
A quick hack to implement restarts.
documenting Tcl's warning/error messages
Useful when you're looking to figure out what went wrong. Incomplete and subject to change at short notice, but something is better than nothing. :-)