Re: object(x) rethink
Robert Craig wrote:
>
> All types in Euphoria, whether they are
> built-in, like integer(), or user_defined,
> can be used to either declare a variable,
> or test if a value belongs to that type.
> e.g.
> }}}
<eucode>
> atom a
> sequence s
> my_special_type month
>
> if atom(9.9) then ...
> if sequence(x) then ...
> if my_special_type(11) then ...
> </eucode>
{{{
>
> So, just for completeness, if you can
> declare something as:
> object y
> it seems reasonable that you should
> be able to say:
> if object(z) then ...
> even if that is rather pointless.
>
> As for detecting uninitialized variables,
> this would be trivial to implement in the
> interpreter as it is currently implemented.
>
> It would be harder to implement in the translator,
> since the translator currently does no run-time
> checking at all for uninitialized variables.
> You would have to initialize variables to the
> magic "NOVALUE", something that is currently not done.
> This might waste a bit of time, in the case of
> private variables that would have to be set to NOVALUE
> each time a routine was called. (In some cases I
> currently set certain variables to 0 to achieve
> an optimization later on.)
>
> All future implementations of Euphoria,
> interpreted, compiled, etc. would
> have to support this, even if it proved to
> be inconvenient for the implementor.
>
> Currently Euphoria can often detect *at compile-time*
> that a variable is never assigned a value.
>
> I'd like to see some good examples of where
> this would be useful. It seems to me you
> could have code that detected that a variable
> was not initialized, but then what would you
> typically do? Probably either issue an error
> message "Variable not initialized!", or quietly
> set the variable to some default value.
>
> If the program was composed entirely of your own
> code, it would be silly to add *error-checking* code
> to test if a particular variable is initialized.
> It would be easier just to initialize it.
> If you wanted to test that it "hasn't really
> been set to a meaningful value yet", e.g. a file handle,
> in almost all cases you can just set it to some special
> value that indicates that fact, like -1, or whatever.
>
This is correct, but certainlly doesn't apply to all Eu code.
> If you wrote a library used by others, they might
> fail to initialize one of your global variables, maybe because they
> made a silly mistake, or just didn't realize it was
> required. So in that case you could either issue an error message
> telling them they made a mistake, or quietly set the
> variable to some default value.
How to detect this condition without crashing? This is the whole point.
> Your message might be
> slightly more meaningful to them, than a generic
> Euphoria-issued message, but probably not much.
> You could also initialize the variable yourself to some
> special value that means "not really initialized properly yet"
> and issue the same kind of message.
>
The practice of using special values to mark routine error status, not properly
initialised yet variables and other equivalent situations is a source of
countless bugs, because a value is hardly ever special enough.
This is why I, and some other, have pushed for a nil builtin "type". Returning
nil or testing a var for nil are convenient phrases, but the idea is that nil is
not a value, and hardly a type. It's a condition which can conveniently be
assessed as if it were a value or type. And if nil is a type, it can have no
legal value. Such a non type would have to be builtin, and the type system cannot
be simply adjustd as it stands now.
As I mentioned in an earlier post, you can work around a bad sequence index by
checking it prior, but not around an unitialised variable. If Eu programs can use
code plugins some day, they will certainly need this.
CChris
> I don't think this is a good idea,
> but I would not oppose it if a majority
> wanted it, and someone were willing to
> implement it.
>
> Regards,
> Rob Craig
> Rapid Deployment Software
> <a href="http://www.RapidEuphoria.com">http://www.RapidEuphoria.com</a>
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