[PL]: I'm just starting up this page.  It's intended to explain the low-level details of terminating scripts, as regards termination categories (ok, error, return, break, continue), result values (valid data vs error messages), and handling exceptional termination.

======
proc caller {} {
    set result [callee]
}
======


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control { ''script'' } ;# -> ''result of control might be result of script''
===



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''commandA'' ;# -> ''resultA''
''commandB'' ;# -> ''resultB''
''commandC'' ;# -> ''resultC''
===

As long as none of these commands are terminating commands and no error occurs, the evaluation of this script goes like this: `''commandA''` is invoked, its side-effects (if any) happen, and its result (`''resultA''`) is produced and discarded.  The same thing happens to `''commandB''` and `''commandC''`, but as the script terminates after `''commandC''`, `''resultC''` isn't discarded: instead it becomes the result of the evaluation of the script, and gets passed back to the caller.  This is a ''normal termination'', where the result is valid data.

Now, if `''commandB''` is a terminating command, evaluation goes differently.  Commands `''commandA''` and `''commandB''` are invoked and their side-effects happen, but `''commandC''` doesn't get invoked at all and its side-effect doesn't happen.  The result of `''commandB''` becomes the result of the evaluation of the script.  This can still be a normal termination, but it can also be some kind of ''exceptional termination''.  In the latter case, `''resultB''` isn't valid data but an error message.

If an error occurs during execution of `''commandB''`, the situation is similar to the last paragraph.  Commands `''commandA''` and `''commandB''` are invoked, but only `''commandA''` is completed successfully.  The side-effect of `''commandB''` may or may not happen, or happen partially.  The normal result of `''commandB''` is replaced by an error message, which becomes the result of the evaluation of the script.  This is ''exceptional termination''.

In the case of normal termination, control is passed back to the caller.  In the case of exceptional termination, control is passed to a ''handler''.  The handler needs to know which kind of termination happened, so it can know whether to treat the result value as valid data or as an error message.


(I'll be back soon -- [PL])


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