Static/Dynamic vs Strong/Weak

Question

I see these terms banded around all over the place in programming and I have a vague notion of what they mean. A search shows me that such things have been asked all over stack overflow in fact. As far as I'm aware Static/Dynamic typing in languages is subtly different to Strong/Weak typing but what that difference is eludes me. Different sources seem to use different different meanings or even use the terms interchangeably. I can't find somewhere that talks about both and actually spells out the difference. What would be nice is if someone could please spell this out clearly here for me and the rest of the world.

Answer 1

• Static/Dynamic typing is about when type information is aquired (Either at compile- or at runtime)

• Strong/Weak typing ist about how strictly types are distinguished (e.g. whether the language tries to do implicit conversion from strings to numbers).

See the wiki-page for more detailed information.

Answer 2

Weak typing means that the type of an object can change depending on context. For example in a weakly typed language the string "123" may be treated as the number 123 if you add another number to it. Examples of languages with weak typing are bash, awk and PHP.

Another kind of weakly typed language is C, where the data at a memory address can be treated as a different type by casting.

In a strongly typed language the type of an object does not change - an int is always an int and trying to use it as a string will result in an error. Both Java and Python are strongly typed.

The difference between dynamic and static typing is when the type rules are enforced. In a statically typed language the type of every variable and parameter must be declared in the source and is enforced at compile time. In a dynamically typed language the types are only checked when they are used at runtime. So Java is statically typed and Python is dynamically typed.

However the boundaries can be a little blurry at times. For example although Java is statically typed, every time you use reflection or a cast (e.g. when using containers of Objects) they you are deferring the type check to runtime.

Similarly most strongly typed languages will still automatically convert between integers and floats (and in some languages abitrary precision BigInts).

Answer 3

You have discovered a soft spot in the terminology that amateurs use to talk about programming languages. Don't use the terms "strong" and "weak" typing, because they don't have a universally agreed on technical meaning. By contrast, static typing means that programs are checked before being executed, and a program might be rejected before it starts. Dynamic typing means that the types of values are checked during execution, and a poorly typed operation might cause the program to halt or otherwise signal an error at run time. A primary reason for static typing is to rule out programs that might have such "dynamic type errors".

Strong typing generally means that there are no loopholes in the type system, whereas weak typing means the type system can be subverted (invalidating any guarantees). The terms are often used incorrectly to mean static and dynamic typing. To see the difference, think of C: the language is type-checked at compile time (static typing), but there are plenty of loopholes; you can pretty much cast a value of any type to another type of the same size---in particular, you can cast pointer types freely. Pascal was a language that was intended to be strongly typed but famously had an unforeseen loophole: a variant record with no tag.

Implementations of strongly typed languages often acquire loopholes over time, usually so that part of the run-time system can be implemented in the high-level language. For example, Objective Caml has a function called Obj.magic which has the run-time effect of simply returning its argument, but at compile time it converts a value of any type to one of any other type. My favorite example is Modula-3, whose designers called their type-casting construct LOOPHOLE.

Having said that, you can't count on any two people using the words "strong" and "weak" in exactly the same way. So avoid them.