1.2: Static types

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Many interpreted languages support dynamic types, but compiled languages are usually limited to static types. In a statically-typed language, you can tell by looking at the program what type each variable refers to. In a dynamically-typed language, you don’t always know the type of a variable until the program is running. In general, static refers to things that happen at compile time (while a program is being compiled), and dynamic refers to things that happen at run time (while a program is running).

For example, in Python you can write a function like this:

def add(x, y):
    return x + y

Looking at this code, you can’t tell what type x and y will refer to at run time. This function might be called several times, each time with values with different types. Any values that support the addition operator will work; any other types will cause an exception or runtime error.

In C you would write the same function like this:

int add(int x, int y) {
    return x + y;
}

The first line of the function includes type declarations for the parameters and the return value: x and y are declared to be integers, which means that we can check at compile time whether the addition operator is legal for this type (it is). The return value is also declared to be an integer.

Because of these declarations, when this function is called elsewhere in the program, the compiler can check whether the arguments provided have the right type, and whether the return value is used correctly.

These checks happen before the program starts executing, so errors can be found earlier. More importantly, errors can be found in parts of the program that have never run. Furthermore, these checks don’t have to happen at run time, which is one of the reasons compiled languages generally run faster than interpreted languages.

Declaring types at compile time also saves space. In dynamic languages, variable names are stored in memory while the program runs, and they are often accessible by the program. For example, in Python the built-in function locals returns a dictionary that contains variable names and their values. Here’s an example in a Python interpreter:

>>> x = 5
>>> print locals()
{'x': 5, '__builtins__': <module '__builtin__' (built-in)>,
'__name__': '__main__', '__doc__': None, '__package__': None}

This shows that the name of the variable is stored in memory while the program is running (along with some other values that are part of the default runtime environment).

In compiled languages, variable names exist at compile-time but not at run time. The compiler chooses a location for each variable and records these locations as part of the compiled program.¹ The location of a variable is called its address. At run time, the value of each variable is stored at its address, but the names of the variables are not stored at all (unless they are added by the compiler for purposes of debugging).

This is a simplification; we will go into more detail later.