Dynamic typing offers us the possibility of writing programs that are partially defined and may fail due to certain inputs. Let's see an example (using Javascript syntax):
function example(b, x) {
if (b) {
return length(x);
} else {
return x * 2;
}
}
The function above is well defined if b is true and x is applicable to length() or if x is applicable to *2. The programs example(true, "abcdef") and example(false, 3) give both the same result and should not fail at runtime, while example(false, "abcdef"), example(true, 3), example(3, false), etc., have undefined behavior. The essence of dynamic typing is to move this undefined behavior to run time, instead of trying to disallow it at compile time, allowing more programs to be expressed.
It's also possible to express and try to run an infinite amount of programs that only evaluate to undefined behavior, without hope of useful work being computed. Most, if not all, in the static typing camp find this downside to be greater than the added expressiveness.
Going back to our example I think it's safe to say that there's no value in the programs exhibiting only undefined behavior (i.e. all paths lead to crashes) and there's value in finding as early as possible if a program goes in that direction. OTOH dynamically typed programs usually have many valuable paths even if there exist undefined paths. Actually the whole dynamic typing argument can be boiled down to:
The value of executable paths is greater than the value of unexecutable paths. Undefined behavior in unexecutable paths can not be observed.In this point of view trying to avoid the execution of paths that a program don't execute is a wasteful tautology: "avoid doing what you won't do" is a meaningless advice.
Aside, it's interesting to notice that such language can have an erasure model of compilation and don't require runtime tags. Of course it will be unsafe if the unexecutable paths suddenly are executed but this is another issue.
The example program can also be written in OO style, but the idea is the same.
Fundamentally dynamic typing relies on undefined behavior for some code paths which, surprisingly, exist in almost all statically typed languages. In Haskell, for example, it's easy to add undefined behavior by using the aptly named undefined value. What dynamically typed languages offer is a single static type (i.e. any or object or stuff) for all expressions so you can write down any kind of expressions and they all type check. One could do the same in Haskell using a modified Prelude and not using data declarations, essentially having something like this:
data Any = B Bool | N Int | C Any Any | Nil | F (Any -> Any)
-- we support booleans, numbers, cons cells, nil and functions
length (Nil ) = N 0
length (C car cdr) = case (length cdr) of
N n -> 1 + n
-- let it crash on other paths
example (B True) x = length x
example (B False) (N n) = N (n * 2)
This is the has the same behavior and static guarantees. For this small example the static typing won't get in the way of writing the same programs we would in a dynamically typed language. Of course this breaks down when we want to declare our own data types, because we need to extend Any to support structures. We can do it using other features of Haskell or extend the language, but this exercise is pointless. Any Turing-complete language can encode another Turing-complete language and any static type system can express a dynamic type system, because it is statically unityped.
If Haskell had no undefined value (or any equivalent) we could still encode the undefined behavior like this, using U as the undefined value:
data Any = B Bool | N Int | C Any Any | Nil | F (Any -> Any) | U
length (Nil ) = N 0
length (C car cdr) = case (length cdr) ofN n -> 1 + n
U -> U
-- we propagate the undefined case
length _ = U
-- all other cases return the undefined primitive
example (B True) x = length x
example (B False) (N n) = N (n * 2)
example _ _ = U
This is less convenient, but it is a syntax issue not one of semantics.
Fundamentally dynamically typed languages rely on the existence of undefined behavior, either via failed tag matches, doesNotUnderstand: messages or plain crashes. This same feature exist in most statically typed languages, so it's possible to encode dynamic typing in them, albeit with less syntactic sugar but it adds no value to them. Most dynamic languages reify this undefined behavior and allow it to be inspected at runtime, so a program can work around a undefined behavior which adds a meta-programming feature to the language (e.g. using method_missing in Ruby) but this is orthogonal to the basic idea of allowing programs with undefined paths to be expressed because of the value found in the defined/executable paths.
Theoretically it's possible to make all behavior defined, you just need to consider all cases in the primitives. For example, if statements/expressions require a boolean but many dynamic languages have the concept of truthiness so one can use anything in the test part of the if and it'll never fail at runtime with some sort of undefined behavior error. You can extend this to all primitives (e.g. arithmetic operators return their unit when presented with invalid values, attribute accessing returns Nil for undefined attributes) and end up with a program that won't crash but may do very unexpected things. This language is equivalent to using on error resume next in classic VB.
There's research on gradual typing which tries to provide a single type system with static and dynamic parts with minimal overhead. The results from this research is very encouraging as it would allow a program to have static guarantees over some areas and not over others, which can be very useful for some forms of programming.
