Seeking clarification on apparent contradictions regarding weakly typed languages

Question

I think I understand strong typing, but every time I look for examples for what is weak typing I end up finding examples of programming languages that simply coerce/convert types automatically.

For instance, in this article named Typing: Strong vs. Weak, Static vs. Dynamic says that Python is strongly typed because you get an exception if you try to:

Python

1 + "1"
Traceback (most recent call last):
File "", line 1, in ? 
TypeError: unsupported operand type(s) for +: 'int' and 'str'

However, such thing is possible in Java and in C#, and we do not consider them weakly typed just for that.

Java

  int a = 10;
  String b = "b";
  String result = a + b;
  System.out.println(result);

C#

int a = 10;
string b = "b";
string c = a + b;
Console.WriteLine(c);

In this another article named Weakly Type Languages the author says that Perl is weakly typed simply because I can concatenate a string to a number and viceversa without any explicit conversion.

Perl

$a=10;
$b="a";
$c=$a.$b;
print $c; #10a

So the same example makes Perl weakly typed, but not Java and C#?.

Gee, this is confusing

The authors seem to imply that a language that prevents the application of certain operations on values of different types is strongly typed and the contrary means weakly typed.

Therefore, at some point I have felt prompted to believe that if a language provides a lot of automatic conversions or coercion between types (as perl) may end up being considered weakly typed, whereas other languages that provide only a few conversions may end up being considered strongly typed.

I am inclined to believe, though, that I must be wrong in this interepretation, I just do not know why or how to explain it.

So, my questions are:

• What does it really mean for a language to be truly weakly typed?
• Could you mention any good examples of weakly typing that are not related to automatic conversion/automatic coercion done by the language?
• Can a language be weakly typed and strongly typed at the same time?

Answer 1

UPDATE: This question was the subject of my blog on the 15th of October, 2012. Thanks for the great question!

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What does it really mean for a language to be "weakly typed"?

It means "this language uses a type system that I find distasteful". A "strongly typed" language by contrast is a language with a type system that I find pleasant.

The terms are essentially meaningless and you should avoid them. Wikipedia lists eleven different meanings for "strongly typed", several of which are contradictory. This indicates that the odds of confusion being created are high in any conversation involving the term "strongly typed" or "weakly typed".

All that you can really say with any certainty is that a "strongly typed" language under discussion has some additional restriction in the type system, either at runtime or compile time, that a "weakly typed" language under discussion lacks. What that restriction might be cannot be determined without further context.

Instead of using "strongly typed" and "weakly typed", you should describe in detail what kind of type safety you mean. For example, C# is a statically typed language and a type safe language and a memory safe language, for the most part. C# allows all three of those forms of "strong" typing to be violated. The cast operator violates static typing; it says to the compiler "I know more about the runtime type of this expression than you do". If the developer is wrong, then the runtime will throw an exception in order to protect type safety. If the developer wishes to break type safety or memory safety, they can do so by turning off the type safety system by making an "unsafe" block. In an unsafe block you can use pointer magic to treat an int as a float (violating type safety) or to write to memory you do not own. (Violating memory safety.)

C# imposes type restrictions that are checked at both compile-time and at runtime, thereby making it a "strongly typed" language compared to languages that do less compile-time checking or less runtime checking. C# also allows you to in special circumstances do an end-run around those restrictions, making it a "weakly typed" language compared with languages which do not allow you to do such an end-run.

Which is it really? It is impossible to say; it depends on the point of view of the speaker and their attitude towards the various language features.