What is a type error?

[Chris Smith]

I <cdsmith at twu.net> wrote:
> Incidentally, I'm not saying that such a feature would be a good idea.  
> It generally isn't provided in languages specifically because it gets to 
> be a big pain to maintain all of the type specifications for this kind 
> of stuff.

There are other good reasons, too, as it turns out.  I don't want to 
overstate the "possible" until it starts to sound like "easy, even if 
it's a pain".  This kind of stuff is rarely done in mainstream 
programming languages because it has serious negative consequences.

For example, I wrote that example using variables of type int.  If we 
were to suppose that we were actually working with variables of type 
Person, then things get a little more complicated.  We would need a few 
(infinite classes of) derived subtypes of Person that further constrain 
the possible values for state.  For example, we'd need types like:

    Person{age:{18..29}}

But this starts to look bad, because we used to have this nice property 
called encapsulation.  To work around that, we'd need to make one of a 
few choices: (a) give up encapsulation, which isn't too happy; (b) rely 
on type inference for this kind of stuff, and consider it okay if the 
type inference system breaks encapsulation; or (c) invent some kind of 
generic constraint language so that constraints like this could be 
expressed without exposing field names.  Choice (b) is incomplete, as 
there will often be times when I need to ascribe a type to a parameter 
or some such thing, and the lack of ability to express the complete type 
system will be rather limiting.  Choice (c), though, looks a little 
daunting.

So I'll stop there.  The point is that while it is emphatically true 
that this kind of stuff is possible, it is also very hard in Java.  
Partly, that's because Java is an imperative language, but it's also 
because there are fundamental design trade-offs involved between 
verbosity, complexity, expressive power, locality of knowledge, etc. 
that are bound to be there in all programming languages, and which make 
it harder to take one design principle to its extreme and produce a 
usable language as a result.  I don't know that it's impossible for this 
sort of thing to be done in a usable Java-like language, but in any 
case, the way to accomplish it is not obvious.

-- 
Chris Smith - Lead Software Developer / Technical Trainer
MindIQ Corporation