Why Dylan?

What earthly reason could there be for learning yet another computer language? And why should that language be Dylan?

Dylan has an interesting combination of features. It is a dynamic language, but is designed to perform nearly as well as a static language. It is a functional language – like Scheme or TCL – but uses an algebraic infix syntax similar to C’s. Dylan is object-oriented from the ground up, supports multiple inheritance and exceptions, implements multiple dispatch, and collects garbage.

Dynamic vs. Static Languages

Static languages need to know the type of every variable at compile time. Examples of static languages include C++, Java, and Go. Code written in static languages typically compiles efficiently, and strong type-checking at compile-time reduces the risk of errors.

Dynamic languages allow the programmer to create variables without explicitly specifying the type of information they contain. This simplifies prototyping and cleans up certain kinds of object oriented code. Typical dynamic languages include Common Lisp, Javascript, and Python.

Dylan provides a good balance between the advantages of static and dynamic languages. The programmer may choose to specify or omit type declarations as desired. Code using explicit variable types can be compiled to very efficient code, and type mismatch errors can be caught at compile time. Code omitting those type declarations gains the flexibility of a dynamic language.

Functional Languages

Functional languages, such as Common Lisp, Scheme and to a large extent TCL, view an entire program as one large function to be evaluated. Expressions, statements and even control structures all return values, which may in turn be used as arguments elsewhere.

Dylan is a functional language, permitting programmers to write functions like the following:

define method shoe-size (person :: <string>)
  if (person = "Larry")
    14
  else
    11
  end if
end method;

The function shoe-size has one argument, a string, and an untyped return value. (If this function didn’t link against external code, the compiler could easily infer the return type.) If person equals "Larry", then the if statement returns 14, otherwise it returns 11. Since no other statements follow the if, its return value is used as the return value of the entire function.

The same function could also have been written as follows, in a more imperative idiom:

define method shoe-size (person :: <string>)
  let the-size = 11;
  if (person = "Joe")
    the-size := 14;
  end if;
  the-size
end method;

Algebraic Infix Syntax

Languages based on LISP typically use a notation called fully-parenthesized prefix syntax (also known as s-expressions). This consists of nested parentheses, as seen in the following Scheme version of the shoe-size function:

(define (shoe-size person)
  (if (equal? person "Joe")
    14
    11))

This has a certain elegance, but takes some time to learn to read. Dylan, as shown in the previous section, uses a syntax similar to those of C and Pascal.

Object Orientation

Unlike many other object-oriented languages, Dylan uses objects for every data value. Integers and strings are objects, as are functions and classes themselves.

Dylan’s design makes this reasonably efficient. Compile-time analysis and explicit type declarations allow the compiler to optimize away most of the overhead. Other language features permit the programmer to mark certain classes as sealed, that is, ineligible for further subclassing. This allows for further compile-time optimizations.

Dylan’s object model, detailed in the following sections of this tutorial, differs from that of C++ in several important respects. Multiple inheritance may be used freely, without concern for object slicing, erroneous down-casting or a whole host of other gotchas familiar to C++ programmers. Methods are separate from class declarations, allowing a programmer to write new polymorphic functions without editing the relevant base class. Methods may also dispatch polymorphically on more than one parameter, a powerful technique known as multiple dispatch. All of these features will be explained in greater detail later on.

Garbage Collection

Languages with garbage collection have no need of a free or delete operator, because unused heap memory gets reclaimed automatically by the language runtime. This reduces the complexity of source code, eliminates the need of keeping reference counts for shared objects, and prevents most memory allocation bugs and a major source of memory leaks.

Why Not Dylan?

Dylan’s greatest weaknesses are its lack of a battle-hardened compiler and IDE, and a large user base (and hence a large set of libraries). However, the compiler and IDE themselves are written in Dylan so there are several hundred thousand lines of Dylan code. You probably want to consider very carefully before using Open Dylan for mission critical code.