CSC/ECE 517 Fall 2009/wiki1b 2 SD

Advantages of statically typed vs. dynamically typed languages

Type System

The formal definition of a type system is[1]:

“A type system is a tractable syntactic method for proving the absence of certain program behaviors by classifying phrases 
according to the kinds of values they compute.”

Type system is the study of types which defines the interaction between various types, the way in which a programming language assigns types. Type systems help detecting errors in the program, provide abstraction, improve the readability of the program, and improve the efficiency of the program [1],[2]. Detecting errors related to types in a program is called type checking.

Type Checking

In layman terms type checking is the process of determining errors related to types in the expressions and calculations of a program. There are a few programming languages which are typed and there are a few which are untyped. Among the typed programming languages, based on when the type checking happens, they are either dynamically typed or statically typed. There is another factor used to classify programming languages, that is, the level to which type checking is enforced. Based on the extent to which type checking is enforced, programming languages are either strongly typed or weakly typed.

Therefore, broadly we classify programming languages as:

• Statically typed: Statically typed languages are those languages in which type checking is done at the compile time as opposed to run time. A few examples of statically typed languages are C, C++, C#, Java, FORTRAN, and Pascal etc.
• Dynamically typed: Dynamically typed languages are those set of languages where type checking is done, unlike static typing, at run time. Examples of dynamically typed languages are Ruby, Prolog, Python, SNOBOL, and Smalltalk etc.
• Strongly typed: Those set of languages where type checking is strictly enforced are called strongly typed languages. Strongly typed languages guarantee type-safety [5]. Examples of strong typing are Lisp dialects, Python, Haskell etc.
• Weakly typed: Weak typing means that the language does implicit type casting. For e.g. consider the following operations:

var a = 5;
var b = "string";
var c = a + b;
print c;

We know that 5 is an integer and the type of ‘a’ is integer, which is different from the type of ‘b’ which is a string. Therefore, the operation a + b, should not have any value. In a weakly typed programming language, the result might be ‘5string’; this is because ‘a’ is implicitly converted into a string. Examples of weakly typed languages are JavaScript, PHP, Perl etc.

Statically typed languages

Static typed programming languages are those in which the variables and their values have types. A variable of type number cannot hold anything other than a number. Types are determined and enforced at compile-time, or declaration-time. This implies that in statically typed programming languages the variables should be explicitly declaration (or initialization) before they're employed. A few examples of statically typed languages are C, C++, C#, Java, FORTRAN, and Pascal etc.

Consider the following example:

 /*Sample C Code*/

 int variable, sum;
 variable = 5;
 sum = 10;
 sum = sum + num;

The above code fragment is an example of how variable declaration in static typed languages generally appears.

There are two main types of static type checking:

• Explicit type decoration: In this kind of type checking the variables, parameters have to be explicitly declared that they are of a given type in the program.
• Implicit type inference: In this kind of type checking the variables are not explicitly decorated with types, but type the programming language automatically decides the type.

Consider the following expression:

 (a + string-length(b)) 

In explicit type decoration it is checked, before the evaluation of the expression, that a is a number and b is a string. Whereas, in type inference by studying the expression it is decided that string-length can only be used on strings therefore b has to be a string and ‘+’ is only used on numbers therefore a has to be a number and string-length(b) also has to be a number.

In the present time Information technology has become a part and parcel of everyone’s life. We cannot imagine a day without a computer. So much are we dependent on the computer that the reliability of software programs is a huge thing. There are many factors which make a program reliable, functional and efficient; one of these factors is static typing. In a statically typed system all the function calls and variables are checked at the compile time and any error detected here is treated as a serious error and the compilation stops and the system does not allow the error to propagate to run-time. With static typing, it is impossible to build a program that would assign a string value to an integer variable. Whereas, in a dynamically typed program, the compiler wouldn’t check for this possibility, and the error would happen at run-time.

To Err is human and humans write computer programs, therefore, static typing helps us write bug free programs. It is nice to know that the compiler is there to point out mistakes in the program. Static typing also helps us make modifications to existing code. If a program does not compile, we know there is a problem in the code and we need to modify it. It also helps us use the code of other programmers, because there is a protocol on how functions should be used [4].

In the statically typed programming language communities, there is a saying that “if it compiles, it works”, this the level of reliability that static typing provides.

The advantages of a statically-typed programming language are that they allow errors to be detected earlier at the compile time, enforce disciplined programming styles, better documentation in the form of type signatures, provides more opportunities for compiler optimizations, generate an efficient object code, and a better design time developer experience [3][6].

There are also few disadvantages about static typing, according John Ousterhout [6]:

“Statically typed systems programming languages make code less reusable, more verbose, not safer, and less expressive than dynamically typed scripting languages”.

References:

[1] Pierce, B. C., & NetLibrary, I. (2002). Types and programming languages. Cambridge, Mass.: MIT Press.

[2] http://www.ljosa.com/~ljosa/teaching/cs162/lectures/L7%20-%20Types.pdf

[3] Martin Abadi et. al, Dynamic Typing in a Statically Typed Language, ACM Transactions on Programming Languages and Systems, Vol. 13, No, 2, April 1991.

[4] http://gnuvince.wordpress.com/2008/02/15/static-typing-%E2%88%A7-dynamic-typing/

[5] http://www.ljosa.com/~ljosa/teaching/cs162/lectures/L7%20-%20Types.pdf

[6] Erik Meijer and Peter Drayton, Static Typing Where Possible, Dynamic Typing When Needed: The End of the Cold War Between Programming Languages

[7] http://articles.sitepoint.com/article/typing-versus-dynamic-typing