How Far to Go with a Strongly Typed Language

How far to go with a strongly typed language?

If you wanted this simpler approach generalize it so you can get more use out of it, instead of tailor it to a specific thing. Then the question is not "should I make a entire new class for this specific thing?" but "should I use my utilities?"; the latter is always yes. And utilities are always helpful.

So make something like:

template <typename T>
void check_range(const T& pX, const T& pMin, const T& pMax)
{
    if (pX < pMin || pX > pMax)
        throw std::out_of_range("check_range failed"); // or something else
}

Now you've already got this nice utility for checking ranges. Your code, even without the channel type, can already be made cleaner by using it. You can go further:

template <typename T, T Min, T Max>
class ranged_value
{
public:
    typedef T value_type;

    static const value_type minimum = Min;
    static const value_type maximum = Max;

    ranged_value(const value_type& pValue = value_type()) :
    mValue(pValue)
    {
        check_range(mValue, minimum, maximum);
    }

    const value_type& value(void) const
    {
        return mValue;
    }

    // arguably dangerous
    operator const value_type&(void) const
    {
        return mValue;
    }

private:
    value_type mValue;
};

Now you've got a nice utility, and can just do:

typedef ranged_value<unsigned char, 0, 15> channel;

void foo(const channel& pChannel);

And it's re-usable in other scenarios. Just stick it all in a "checked_ranges.hpp" file and use it whenever you need. It's never bad to make abstractions, and having utilities around isn't harmful.

Also, never worry about overhead. Creating a class simply consists of running the same code you would do anyway. Additionally, clean code is to be preferred over anything else; performance is a last concern. Once you're done, then you can get a profiler to measure (not guess) where the slow parts are.

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

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.

What strongly typed languages should I consider for my next development language?

Do you mean statically typed languages (checked at compile time)? If so, C# or Java really are probably your best bets for widely used server-side languages. Languages such as Python and Ruby are strongly typed, but they are dynamic like PHP.

http://en.wikipedia.org/wiki/Type_system

http://en.wikipedia.org/wiki/Strongly_typed_programming_language

Is Python strongly typed?

Python is strongly, dynamically typed.

• Strong typing means that the type of a value doesn't change in unexpected ways. A string containing only digits doesn't magically become a number, as may happen in Perl. Every change of type requires an explicit conversion.
• Dynamic typing means that runtime objects (values) have a type, as opposed to static typing where variables have a type.

As for your example

bob = 1
bob = "bob"

This works because the variable does not have a type; it can name any object. After bob=1, you'll find that type(bob) returns int, but after bob="bob", it returns str. (Note that type is a regular function, so it evaluates its argument, then returns the type of the value.)

Contrast this with older dialects of C, which were weakly, statically typed, so that pointers and integers were pretty much interchangeable. (Modern ISO C requires conversions in many cases, but my compiler is still lenient about this by default.)

I must add that the strong vs. weak typing is more of a continuum than a boolean choice. C++ has stronger typing than C (more conversions required), but the type system can be subverted by using pointer casts.

The strength of the type system in a dynamic language such as Python is really determined by how its primitives and library functions respond to different types. E.g., + is overloaded so that it works on two numbers or two strings, but not a string and an number. This is a design choice made when + was implemented, but not really a necessity following from the language's semantics. In fact, when you overload + on a custom type, you can make it implicitly convert anything to a number:

def to_number(x):
    """Try to convert function argument to float-type object."""
    try: 
        return float(x) 
    except (TypeError, ValueError): 
        return 0 

class Foo:
    def __init__(self, number): 
        self.number = number

    def __add__(self, other):
        return self.number + to_number(other)

Instance of class Foo can be added to other objects:

>>> a = Foo(42)
>>> a + "1"
43.0
>>> a + Foo
42
>>> a + 1
43.0
>>> a + None
42

Observe that even though strongly typed Python is completely fine with adding objects of type int and float and returns an object of type float (e.g., int(42) + float(1) returns 43.0). On the other hand, due to the mismatch between types Haskell would complain if one tries the following (42 :: Integer) + (1 :: Float). This makes Haskell a strictly typed language, where types are entirely disjoint and only a controlled form of overloading is possible via type classes.

Is C strongly typed?

"Strongly typed" and "weakly typed" are terms that have no widely agreed-upon technical meaning. Terms that do have a well-defined meaning are

• Dynamically typed means that types are attached to values at run time, and an attempt to mix values of different types may cause a "run-time type error". For example, if in Scheme you attempt to add one to true by writing (+ 1 #t) this will cause an error. You encounter the error only if you attempt to execute the offending code.

• Statically typed means that types are checked at compile time, and a program that does not have a static type is rejected by the compiler. For example, if in ML you attempt to add one to true by writing 1 + true, the program will be rejected with a (probably cryptic) error message. You always get the error even if the code might never be executed.

Different people prefer different systems according in part to how much they value flexibility and how much they worry about run-time errors.

Sometimes "strongly typed" is used loosely to mean "statically typed", and "weakly typed" is used incorrectly to mean "dynamically typed". A better use for the term "strongly typed" is that "you cannot work around or subvert the type system", whereas "weakly typed" means "there are loopholes in the type system". Perversely, most languages with static type systems have loopholes, while many languages with dynamic type systems have no loopholes.

None of these terms are connected in any way with the number of implicit conversions available in a language.

If you want to talk precisely about programming languages, it is best to avoid the terms "strongly typed" and "weakly typed". I would say that C is a language that is statically typed but that has a lot of loopholes. One loophole is that you can freely cast any pointer type to any other pointer type. You can also create a loophole between any two types of your choice by declaring a C union that has two members, one for each of the types in question.

I have written more about static and dynamic typing at why-interpreted-langs-are-mostly-ducktyped-while-compiled-have-strong-typing.

Static/Dynamic vs Strong/Weak

• Static/Dynamic Typing is about when type information is acquired (Either at compile time or at runtime)

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

See the wiki-page for more detailed information.

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