[kgo]

1) If CL can only sometimes enforce invariants about type sometimes, and in special cases, I'd argue that the original statement was true. You'll still be deprived in many (probably most) cases. (People have survived falls where their parachutes didn't open, but in an essay most people wouldn't say "You can of course jump out of a plane without a parachute" despite the existence proof. And saying "Although there have been cases where people have survived, you can't generally expected with any reasonable success rate to survive a fall from an airplane without a working parachute" just isn't good writing.)

2) I don't get the database analogy. When I add a column to a table, the size of the table on disk does indeed change. Not to mention the schema can be changed statically. If you add/remove/modify a Field in a FieldList, the type doesn't change. So that really isn't a static vs dynamic issue.

[tomp]

About 1): I think the point was that CL can only sometimes enforce invariants at compile time. If the compiler can't prove that something is always true or always false, it will simply emit code to do a runtime check.

[kgo]

But as a general principal, you can't reliably or consistently do this in a dynamic language in any sort of guaranteed fashion, which was implicit in the original statement being refuted.

Even with a runtime check, you still CAN pass in a bad value, and the program will just fail, either locally or globally.

(Not to mention that the paragraph immediately following the refuted statement starts with 'Now I am fully aware that “the compiler can optimize this away”, at least in some cases, but to achieve this requires one of two things (apart from unreachable levels of ingenuity that can easily, and more profitably, be expressed by the programmer in the first place)...')

[ScottBurson]

1) I think the right question to ask here is, how often is the lack of compile-time checking a problem in practice, and how big a problem is it? I've worked extensively in both static and dynamic languages, and my experience has been that in dynamic languages, while I do miss static type checking occasionally, it happens quite a bit less often than your parachute analogy would lead one to imagine.

Look at it this way. Programs have lots of important properties. Some of these we have figured out how to encode as types so that we can verify them statically. But (although research in this area is ongoing) there are still a lot of important properties we require of our programs that can't be statically verified. The upshot is, we have to test them. Yes, testing is necessarily imperfect, but we have to do it anyway. In my experience, in the course of testing a program in a dynamic language, the kinds of errors that would have been caught by static typing are relatively easy to find by testing; usually, the errors that are hard to find by testing are also well beyond the scope of static typing. Maybe that will change eventually, though I'm skeptical, but it's certainly the state of type systems in common use at the moment.

So the parachute analogy is grossly exaggerated; it's more like leaving your shoelaces untied.

When Doel says "you're depriving yourself of the ability to state and enforce the invariant...", it's not clear whether he means "always" or "at least sometimes". I concede that he could have meant the latter, in which case you're right, my counterargument fails. But as I've just argued, there are lots of other invariants that we can't statically enforce anyway, and they tend to be the more important ones.

2) A table is a bag (multiset) of tuples of some specific type (schema). When you add a column, the table now has a different type, because it's now a bag of tuples of a different type.

Imagine that instead of using a database you keep all your data in a running process of a program written in a static language. (Let's ignore the possibility that the program or the machine might crash.) How are you going to structure the data? To make use of static typing, you need to use collections of records (or "structs" or whatever you like to call them). How then would you add a field to one of these record types? There's no way to do it without recompiling your program, and there's no way to do that without either losing your data or dumping them to files and reading them back in to the new version of the program.

Now, you could store your data differently, by explicitly modelling rows as maps from keys to values. But what's the range type of your map? Why, it's the union of the types of the possible data values you might want to store; you've lost static typing for the data values, and will have to check at runtime whether each value you read out of one of these row maps is of the correct type.

[vorg]

Programs written in Python are often augmented with programs written in C. Didn't Python become popular BECAUSE it worked seamlessly with C programs? Use both types of languages together.

But I'm interested in whether type-inferring languages will ever take off, bringing the best of both worlds, i.e. the code tersity of "dynamic" and the compile-time checking of "static". I'm not sure if types can be changed dynamically in these types of languages, though.