What is the difference between a strongly typed language and a statically typed language?

Question

Also, does one imply the other?

Answer 1

What is the difference between a strongly typed language and a statically typed language?

A statically typed language has a type system that is checked at compile time by the implementation (a compiler or interpreter). The type check rejects some programs, and programs that pass the check usually come with some guarantees; for example, the compiler guarantees not to use integer arithmetic instructions on floating-point numbers.

There is no real agreement on what "strongly typed" means, although the most widely used definition in the professional literature is that in a "strongly typed" language, it is not possible for the programmer to work around the restrictions imposed by the type system. This term is almost always used to describe statically typed languages.

Static vs dynamic

The opposite of statically typed is "dynamically typed", which means that

1. Values used at run time are classified into types.
2. There are restrictions on how such values can be used.
3. When those restrictions are violated, the violation is reported as a (dynamic) type error.

For example, Lua, a dynamically typed language, has a string type, a number type, and a Boolean type, among others. In Lua every value belongs to exactly one type, but this is not a requirement for all dynamically typed languages. In Lua, it is permissible to concatenate two strings, but it is not permissible to concatenate a string and a Boolean.

Strong vs weak

The opposite of "strongly typed" is "weakly typed", which means you can work around the type system. C is notoriously weakly typed because any pointer type is convertible to any other pointer type simply by casting. Pascal was intended to be strongly typed, but an oversight in the design (untagged variant records) introduced a loophole into the type system, so technically it is weakly typed. Examples of truly strongly typed languages include CLU, Standard ML, and Haskell. Standard ML has in fact undergone several revisions to remove loopholes in the type system that were discovered after the language was widely deployed.

What's really going on here?

Overall, it turns out to be not that useful to talk about "strong" and "weak". Whether a type system has a loophole is less important than the exact number and nature of the loopholes, how likely they are to come up in practice, and what are the consequences of exploiting a loophole. In practice, it's best to avoid the terms "strong" and "weak" altogether, because

• Amateurs often conflate them with "static" and "dynamic".

• Apparently "weak typing" is used by some persons to talk about the relative prevalance or absence of implicit conversions.

• Professionals can't agree on exactly what the terms mean.

• Overall you are unlikely to inform or enlighten your audience.

The sad truth is that when it comes to type systems, "strong" and "weak" don't have a universally agreed on technical meaning. If you want to discuss the relative strength of type systems, it is better to discuss exactly what guarantees are and are not provided. For example, a good question to ask is this: "is every value of a given type (or class) guaranteed to have been created by calling one of that type's constructors?" In C the answer is no. In CLU, F#, and Haskell it is yes. For C++ I am not sure—I would like to know.

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".

Does one imply the other?

On a pedantic level, no, because the word "strong" doesn't really mean anything. But in practice, people almost always do one of two things:

• They (incorrectly) use "strong" and "weak" to mean "static" and "dynamic", in which case they (incorrectly) are using "strongly typed" and "statically typed" interchangeably.

• They use "strong" and "weak" to compare properties of static type systems. It is very rare to hear someone talk about a "strong" or "weak" dynamic type system. Except for FORTH, which doesn't really have any sort of a type system, I can't think of a dynamically typed language where the type system can be subverted. Sort of by definition, those checks are bulit into the execution engine, and every operation gets checked for sanity before being executed.

Either way, if a person calls a language "strongly typed", that person is very likely to be talking about a statically typed language.

Answer 2

This is often misunderstood so let me clear it up.

Static/Dynamic Typing

Static typing is where the type is bound to the variable. Types are checked at compile time.

Dynamic typing is where the type is bound to the value. Types are checked at run time.

So in Java for example:

String s = "abcd"; 

s will "forever" be a String. During its life it may point to different Strings (since s is a reference in Java). It may have a null value but it will never refer to an Integer or a List. That's static typing.

In PHP:

$s = "abcd";          // $s is a string $s = 123;             // $s is now an integer $s = array(1, 2, 3);  // $s is now an array $s = new DOMDocument; // $s is an instance of the DOMDocument class 

That's dynamic typing.

Strong/Weak Typing

(Edit alert!)

Strong typing is a phrase with no widely agreed upon meaning. Most programmers who use this term to mean something other than static typing use it to imply that there is a type discipline that is enforced by the compiler. For example, CLU has a strong type system that does not allow client code to create a value of abstract type except by using the constructors provided by the type. C has a somewhat strong type system, but it can be "subverted" to a degree because a program can always cast a value of one pointer type to a value of another pointer type. So for example, in C you can take a value returned by malloc() and cheerfully cast it to FILE*, and the compiler won't try to stop you—or even warn you that you are doing anything dodgy.

(The original answer said something about a value "not changing type at run time". I have known many language designers and compiler writers and have not known one that talked about values changing type at run time, except possibly some very advanced research in type systems, where this is known as the "strong update problem".)

Weak typing implies that the compiler does not enforce a typing discpline, or perhaps that enforcement can easily be subverted.

The original of this answer conflated weak typing with implicit conversion (sometimes also called "implicit promotion"). For example, in Java:

String s = "abc" + 123; // "abc123"; 

This is code is an example of implicit promotion: 123 is implicitly converted to a string before being concatenated with "abc". It can be argued the Java compiler rewrites that code as:

String s = "abc" + new Integer(123).toString(); 

Consider a classic PHP "starts with" problem:

if (strpos('abcdef', 'abc') == false) {   // not found } 

The error here is that strpos() returns the index of the match, being 0. 0 is coerced into boolean false and thus the condition is actually true. The solution is to use === instead of == to avoid implicit conversion.

This example illustrates how a combination of implicit conversion and dynamic typing can lead programmers astray.

Compare that to Ruby:

val = "abc" + 123 

which is a runtime error because in Ruby the object 123 is not implicitly converted just because it happens to be passed to a + method. In Ruby the programmer must make the conversion explicit:

val = "abc" + 123.to_s 

Comparing PHP and Ruby is a good illustration here. Both are dynamically typed languages but PHP has lots of implicit conversions and Ruby (perhaps surprisingly if you're unfamiliar with it) doesn't.

Static/Dynamic vs Strong/Weak

The point here is that the static/dynamic axis is independent of the strong/weak axis. People confuse them probably in part because strong vs weak typing is not only less clearly defined, there is no real consensus on exactly what is meant by strong and weak. For this reason strong/weak typing is far more of a shade of grey rather than black or white.

So to answer your question: another way to look at this that's mostly correct is to say that static typing is compile-time type safety and strong typing is runtime type safety.

The reason for this is that variables in a statically typed language have a type that must be declared and can be checked at compile time. A strongly-typed language has values that have a type at run time, and it's difficult for the programmer to subvert the type system without a dynamic check.

But it's important to understand that a language can be Static/Strong, Static/Weak, Dynamic/Strong or Dynamic/Weak.