What is Expressiveness in a Computer Language

Chris Smith cdsmith at twu.net
Tue Jun 20 05:26:54 CEST 2006


Joe Marshall <eval.apply at gmail.com> wrote:
> They *do* have a related meaning.  Consider this code fragment:
> (car "a string")

My feeling is that this code is obviously wrong.  It is so obviously 
wrong, in fact, that the majority of automated error detection systems, 
if written for Lisp, would probably manage to identify it as wrong at 
some point.  This includes the Lisp runtime.  So far, I haven't 
mentioned types.

> A string is *not* a valid argument to CAR.  Ask anyone why and
> they will tell you `It's the wrong type.'

Indeed, they will.  We're assuming, of course that they know a little 
Lisp... otherwise, it may be entirely reasonable for someone to expect 
that (car "a string") is 'a' and (cdr "a string") is " string"... but 
I'll ignore that, even though I haven't yet convinced myself that it's 
not relevant to why this is colloquially considered a type error.

I believe that in this sense, the 'anyone' actually means "type" in the 
sense that you mean "type".  The fact that a static type system detects 
this error is somewhat coincidental (except for the fact that, again, 
any general error-detection scheme worth its salt probably detects this 
error), and orthogonal to whether it is considered a type error by our 
hypothetical 'anyone'.

> Both `static typing' and `dynamic typing' (in the colloquial sense) are
> strategies to detect this sort of error.

I will repeat that static typing is a strategy for detecting errors in 
general, on the basis of tractable syntactic methods.  There are some 
types of errors that are easier to detect in such a system than 
others... but several examples have been given of problems solved by 
static type systems that are not of the colloquial "It's the wrong 
type" variety that you mention here.  The examples so far have included 
detecting division by zero, or array bounds checking.  Other type 
systems can check dimensionality (correct units).

Another particularly interesting example may be the following from 
Ocaml:

  let my_sqrt x = if x < 0.0 then None else Some(sqrt(x));;

Then, if I attempt to use my_sqrt in a context that requires a float, 
the compiler will complain about a type violation, since the type of the 
expression is "float option".  So this is a type error *in Ocaml*, but 
it's not the kind of thing that gets intuitively classified as a type 
error.  In fact, it's roughly equivalent to a NullPointerException at 
runtime in Java, and few Java programmers would consider a 
NullPointerException to be somehow "actually a type error" that the 
compiler just doesn't catch.  In this case, when the error appears in 
Ocaml, it appears to be "obviously" a type error, but that's only 
because the type system was designed to catch some class of program 
errors, of which this is a member.

> It's hardly mythical.  (car "a string") is obviously an error and you
> don't need a static type system to know that.

Sure.  The question is whether it means much to say that it's a "type 
error".  So far, I'd agree with either of two statements, depending on 
the usage of the word "type":

a) Yes, it means something, but Torben's definition of a static type 
system was wrong, because static type systems are not specifically 
looking for type errors.

    or

b) No, "type error" just means "error that can be caught by the type 
system", so it is circular and meaningless to use the phrase in defining 
a kind of type system.

> I mean that this has been argued time and time again in comp.lang.lisp
> and probably the other groups as well.

My apologies, then.  It has not been discussed so often in any newsgroup 
that I followed up until now, though Marshall has now convinced me to 
read comp.lang.functional, so I might see these endless discussions from 
now on.

> In fact, we become rather confused when you say `a
> correctly typed program cannot go wrong at runtime' because we've seen
> plenty of runtime errors from code that is `correctly typed'.

Actually, I become a little confused by that as well.  I suppose it 
would be true of a "perfect" static type system, but I haven't seen one 
of those yet.  (Someone did email me earlier today to point out that the 
type system of a specification language called LOTOS supposedly is 
perfect in that sense, that every correctly typed program is also 
correct, but I've yet to verify this for myself.  It seems rather 
difficult to believe.)

Unless I suddenly have some kind of realization in the future about the 
feasibility of a perfect type system, I probably won't make that 
statement that you say confuses you.

> >     An attempt to generalize the definition of "type" from programming
> >     language type theory to eliminate the requirement that they are
> >     syntactic in nature yields something meaningless.  Any concept of
> >     "type" that is not syntactic is a completely different thing from
> >     static types.
> 
> Agreed.  That is why there is the qualifier `dynamic'.  This indicates
> that it is a completely different thing from static types.

If we agree about this, then there is no need to continue this 
discussion.  I'm not sure we do agree, though, because I doubt we'd be 
right here in this conversation if we did.

This aspect of being a "completely different thing" is why I objected to 
Torben's statement of the form: static type systems detect type 
violations at compile time, whereas dynamic type systems detect type 
violations at runtime.  The problem with that statement is that "type 
violations" means different things in the first and second half, and 
that's obscured by the form of the statement.  It would perhaps be 
clearer to say something like:

    Static type systems detect some bugs at compile time, whereas
    dynamic type systems detect type violations at runtime.

Here's one interesting consequence of the change.  It is easy to 
recognize that static and dynamic type systems are largely orthogonal to 
each other in the sense above.  Java, for example (since that's one of 
the newsgroups on the distribution list for this thread), restricting 
the field of view to reference types for simplicity's sake, clearly has 
both a very well-developed static type system, and a somewhat well-
developed dynamic type system.  There are dynamic "type" errors that 
pass the compiler and are caught by the runtime; and there are errors 
that are caught by the static type system.  There is indeed considerable 
overlap involved, but nevertheless, neither is made redundant.  In fact, 
one way of understanding the headaches of Java 1.5 generics is to note 
that the two different meanings of "type errors" are no longer in 
agreement with each other!

> We're all rubes here, so don't try to educate us with your
> high-falutin' technical terms.

That's not my intention.

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



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