programming language
A programming language is a system of notation for writing computer programs.
Programming languages are described in terms of their Syntax (programming languages), syntax (form) and semantics (computer science), semantics (meaning), usually def ...
object-oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of '' objects''. Objects can contain data (called fields, attributes or properties) and have actions they can perform (called procedures or methods and impl ...
programming model
A programming model is an execution model coupled to an API or a particular pattern of code. In this style, there are actually two execution models in play: the execution model of the base programming language and the execution model of the p ...
designed to support generating efficient
machine code
In computer programming, machine code is computer code consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). For conventional binary computers, machine code is the binaryOn nonb ...
, including fine-grained control over dynamic and static behaviors. It was created in the early 1990s by a group led by
Apple Computer
Apple Inc. is an American multinational corporation and technology company headquartered in Cupertino, California, in Silicon Valley. It is best known for its consumer electronics, software, and services. Founded in 1976 as Apple Computer Co ...
Common Lisp
Common Lisp (CL) is a dialect of the Lisp programming language, published in American National Standards Institute (ANSI) standard document ''ANSI INCITS 226-1994 (S2018)'' (formerly ''X3.226-1994 (R1999)''). The Common Lisp HyperSpec, a hyperli ...
and adds an integrated object system derived from the Common Lisp Object System (CLOS). In Dylan, all values (including numbers, characters, functions, and classes) are first-class objects. Dylan supports
multiple inheritance
Multiple inheritance is a feature of some object-oriented computer programming languages in which an object or class can inherit features from more than one parent object or parent class. It is distinct from single inheritance, where an object ...
multiple dispatch
Multiple dispatch or multimethods is a feature of some programming languages in which a Subroutine, function or Method (computer programming), method can be dynamic dispatch, dynamically dispatched based on the run time (program lifecycle phase), ...
pattern
A pattern is a regularity in the world, in human-made design, or in abstract ideas. As such, the elements of a pattern repeat in a predictable manner. A geometric pattern is a kind of pattern formed of geometric shapes and typically repeated l ...
-based syntax extension macros, and many other advanced features. Programs can express fine-grained control over dynamism, admitting programs that occupy a continuum between dynamic and static programming and supporting evolutionary development (allowing for rapid prototyping followed by incremental refinement and optimization).
Dylan's main design goal is to be a dynamic language well-suited for developing
commercial software
Commercial software,
or, seldom, payware, is a computer software that is produced for sale or that serves commercial purposes. Commercial software can be proprietary software or free and open-source software.
Background and challenge
While ...
. Dylan attempts to address potential performance issues by introducing "natural" limits to the full flexibility of
Lisp
Lisp (historically LISP, an abbreviation of "list processing") is a family of programming languages with a long history and a distinctive, fully parenthesized Polish notation#Explanation, prefix notation.
Originally specified in the late 1950s, ...
systems, allowing the
compiler
In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primaril ...
to clearly understand compilable units, such as
libraries
A library is a collection of Book, books, and possibly other Document, materials and Media (communication), media, that is accessible for use by its members and members of allied institutions. Libraries provide physical (hard copies) or electron ...
.
Dylan derives much of its semantics from Scheme and other Lisps; some Dylan implementations were initially built within extant Lisp systems. However, Dylan has an
ALGOL
ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
-like syntax instead of a Lisp-like prefix syntax.
History
Dylan was created in the early 1990s by a group led by
Apple Computer
Apple Inc. is an American multinational corporation and technology company headquartered in Cupertino, California, in Silicon Valley. It is best known for its consumer electronics, software, and services. Founded in 1976 as Apple Computer Co ...
. At one time in its development, it was intended for use with the
Apple Newton
The Newton is a specified standard and series of personal digital assistants (PDAs) developed and marketed by Apple Inc., Apple Computer, Inc. from 1993 to 1998. An early device in the PDA categorythe term itself originating with the Newtonit w ...
computer, but the Dylan implementation did not reach sufficient maturity in time, and Newton instead used a mix of C and the NewtonScript developed by Walter Smith. Apple ended their Dylan development effort in 1995, though they made a "technology release" version available (Apple Dylan TR1) that included an advanced
integrated development environment
An integrated development environment (IDE) is a Application software, software application that provides comprehensive facilities for software development. An IDE normally consists of at least a source-code editor, build automation tools, an ...
(IDE).
Two other groups contributed to the design of the language and developed implementations: Harlequin released a commercial IDE for
Microsoft Windows
Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...
and
Carnegie Mellon University
Carnegie Mellon University (CMU) is a private research university in Pittsburgh, Pennsylvania, United States. The institution was established in 1900 by Andrew Carnegie as the Carnegie Technical Schools. In 1912, it became the Carnegie Institu ...
released an
open source
Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use and view the source code, design documents, or content of the product. The open source model is a decentrali ...
compiler for
Unix
Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
systems called Gwydion Dylan. Both of these implementations are now open source. The Harlequin implementation is now named Open Dylan and is maintained by a group of volunteers, the Dylan Hackers.
The Dylan language was code-named Ralph. James Joaquin chose the name Dylan for "DYnamic LANguage."
Syntax
Many of Dylan's syntax features come from its Lisp heritage. Originally, Dylan used a Lisp-like prefix syntax, which was based on
s-expression
In computer programming, an S-expression (or symbolic expression, abbreviated as sexpr or sexp) is an expression in a like-named notation for nested List (computing), list (Tree (data structure), tree-structured) data. S-expressions were invented ...
s. By the time the language design was completed, the syntax was changed to an ALGOL-like syntax, with the expectation that it would be more familiar to a wider audience of programmers. The syntax was designed by Michael Kahl. It is described in great detail in the Dylan Reference Manual.
Lexical syntax
Dylan is not case sensitive. Dylan's lexical syntax allows the use of a naming convention where hyphen (minus) signs are used to connect the parts of multiple-word identifiers (sometimes called " lisp-case" or " kebab case"). This convention is common in Lisp languages.
Besides
alphanumeric
Alphanumericals or alphanumeric characters are any collection of number characters and letters in a certain language. Sometimes such characters may be mistaken one for the other.
Merriam-Webster suggests that the term "alphanumeric" may often ...
characters and hyphen-minus signs, Dylan allows a variety of non-alphanumeric characters as part of identifiers. Identifiers may not consist of these non-alphanumeric characters alone. If there is any ambiguity, whitespace is used.
Example code
A simple class with several slots:
define class (
By convention, classes are named with less-than and greater-than signs used as
angle bracket
A bracket is either of two tall fore- or back-facing punctuation marks commonly used to isolate a segment of text or data from its surroundings. They come in four main pairs of shapes, as given in the box to the right, which also gives their n ...
s, e.g. the class named <point> in the code example.
In end class <point> both class and <point> are optional. This is true for all end clauses. For example, you may write end if or just end to terminate an if statement.
To make an instance of <point>:
make(, x: 100, y: 200)
The same class, rewritten in the most minimal way possible:
define class (
The slots are now both typed as <object>. The slots must be initialized manually:
let p = make();
point-x(p) := 100; // or p.point-x := 100;
point-y(p) := 200; // or p.point-y := 200;
By convention, constant names begin with "$":
define constant $pi :: = 3.1415927d0;
A factorial function:
define function factorial (n :: ) => (n! :: )
case
n < 0 => error("Can't take factorial of negative integer: %d\n", n);
n = 0 => 1;
otherwise => n * factorial(n - 1);
end
end;
Here, n! and <integer> are just normal identifiers.
There is no explicit return statement. The result of a method or function is the last expression evaluated. It is a common style to leave off the semicolon after an expression in return position.
Modules vs. namespace
In many object-oriented languages, classes are the main means of encapsulation and modularity; each class defines a namespace and controls which definitions are externally visible. Further, classes in many languages define an indivisible unit that must be used as a whole. For example, using a String concatenation function requires importing and compiling against all of String.
Some languages, including Dylan, also include a separate, explicit namespace or module system that performs encapsulation in a more general way.
In Dylan, the concepts of compile-unit and import-unit are separated, and classes have nothing specifically to do with either. A ''library'' defines items that should be compiled and handled together, while a ''module'' defines a namespace. Classes can be placed together in modules, or cut across them, as the programmer wishes. Often the complete definition for a class does not exist in a single module, but is spread across several that are optionally collected together. Different programs can have different definitions of the same class, including only what they need.
For example, consider an add-on library for regex support on String. In some languages, for the functionality to be included in strings, the functionality must be added to the String namespace. As soon as this occurs, the String class becomes larger, and functions that don't need to use regex still must "pay" for it in increased library size. For this reason, these sorts of add-ons are typically placed in their own namespaces and objects. The downside to this approach is that the new functions are no longer a ''part of'' String; instead, it is isolated in its own set of functions that must be called separately. Instead of myString.parseWith(myPattern), which would be the natural organization from an OO viewpoint, something like myPattern.parseString(myString) is used, which effectively reverses the ordering.
Under Dylan, many interfaces can be defined for the same code, for instance the String concatenation method could be placed in both the String interface, and the "concat" interface which collects together all of the different concatenation functions from various classes. This is more commonly used in math libraries, where functions tend to be applicable to widely differing object types.
A more practical use of the interface construct is to build public and private versions of a module, something that other languages include as a ''bolt on'' feature that invariably causes problems and adds syntax. Under Dylan, every function call can be simply placed in the "Private" or "Development" interface, and collect up publicly accessible functions in Public. Under
Java
Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...
or C++ the visibility of an object is defined in the code, meaning that to support a similar change, a programmer would be forced to rewrite the definitions fully, and could not have two versions at the same time.
Classes
Classes in Dylan describe slots (data members, fields, ivars, etc.) of objects in a fashion similar to most OO languages. All access to slots is via methods, as in
Smalltalk
Smalltalk is a purely object oriented programming language (OOP) that was originally created in the 1970s for educational use, specifically for constructionist learning, but later found use in business. It was created at Xerox PARC by Learni ...
. Default getter and setter methods are automatically generated based on the slot names. In contrast with most other OO languages, other methods applicable to the class are often defined outside of the class, and thus class definitions in Dylan typically include the definition of the storage only. For instance:
define class ()
slot title :: = "untitled", init-keyword: title:;
slot position :: , required-init-keyword: position:;
end class;
In this example, the class "<window>" is defined. The <class name> syntax is convention only, to make the class names stand out—the angle brackets are merely part of the class name. In contrast, in some languages the convention is to capitalize the first letter of the class name or to prefix the name with a ''C'' or ''T'' (for example). <window> inherits from a single class, <view>, and contains two slots, title holding a string for the window title, and position holding an X-Y point for a corner of the window. In this example, the title has been given a default value, while the position has not. The optional ''init-keyword'' syntax allows the programmer to specify the initial value of the slot when instantiating an object of the class.
In languages such as C++ or Java, the class would also define its interface. In this case the definition above has no explicit instructions, so in both languages access to the slots and methods is considered protected, meaning they can be used only by subclasses. To allow unrelated code to use the window instances, they must be declared public.
In Dylan, these sorts of visibility rules are not considered part of the code, but of the module/interface system. This adds considerable flexibility. For instance, one interface used during early development could declare everything public, whereas one used in testing and deployment could limit this. With C++ or Java these changes would require changes to the source code, so people won't do it, whereas in Dylan this is a fully unrelated concept.
Although this example does not use it, Dylan also supports
multiple inheritance
Multiple inheritance is a feature of some object-oriented computer programming languages in which an object or class can inherit features from more than one parent object or parent class. It is distinct from single inheritance, where an object ...
.
Methods and generic functions
In Dylan, methods are not intrinsically associated with any specific class; methods can be thought of as existing outside of classes. Like CLOS, Dylan is based on
multiple dispatch
Multiple dispatch or multimethods is a feature of some programming languages in which a Subroutine, function or Method (computer programming), method can be dynamic dispatch, dynamically dispatched based on the run time (program lifecycle phase), ...
(multimethods), where the specific method to be called is chosen based on the types of all its arguments. The method need not be known at compile time, the understanding being that the required function may be available, or not, based on a user's preferences.
Under Java the same methods would be isolated in a specific class. To use that functionality the programmer is forced to ''import'' that class and refer to it explicitly to call the method. If that class is unavailable, or unknown at compile time, the application simply won't compile.
In Dylan, code is isolated from storage in ''functions''. Many classes have methods that call their own functions, thereby looking and feeling like most other OO languages. However code may also be located in ''generic functions'', meaning they are not attached to a specific class, and can be called natively by anyone. Linking a specific generic function to a method in a class is accomplished thusly:
define method turn-blue (w :: )
w.color := $blue;
end method;
This definition is similar to those in other languages, and would likely be encapsulated within the <window> class. Note the := setter call, which is
syntactic sugar
In computer science, syntactic sugar is syntax within a programming language that is designed to make things easier to read or to express. It makes the language "sweeter" for human use: things can be expressed more clearly, more concisely, or in an ...
for color-setter($blue, w).
The utility of generic methods comes into its own when you consider more "generic" examples. For instance, one common function in most languages is the to-string, which returns some human-readable form for the object. For instance, a window might return its title and its position in parens, while a string would return itself. In Dylan these methods could all be collected into a single module called "to-string", thereby removing this code from the definition of the class itself. If a specific object did not support a to-string, it could be easily added in the to-string module.
Extensibility
This whole concept might strike some readers as very odd. The code to handle to-string for a window isn't defined in <window>? This might not make any sense until you consider how Dylan handles the call of the to-string. In most languages when the program is compiled the to-string for <window> is looked up and replaced with a pointer (more or less) to the method. In Dylan this occurs when the program is first run; the runtime builds a table of method-name/parameters details and looks up methods dynamically via this table. That means that a function for a specific method can be located anywhere, not just in the compile-time unit. In the end the programmer is given considerable flexibility in terms of where to place their code, collecting it along class lines where appropriate, and functional lines where it's not.
The implication here is that a programmer can add functionality to existing classes by defining functions in a separate file. For instance, you might wish to add spell checking to all <string>s, which in C++ or Java would require access to the source code of the string class—and such basic classes are rarely given out in source form. In Dylan (and other "extensible languages") the spell checking method could be added in the spell-check module, defining all of the classes on which it can be applied via the define method construct. In this case the actual functionality might be defined in a single generic function, which takes a string and returns the errors. When the spell-check module is compiled into your program, all strings (and other objects) will get the added functionality.
Apple Dylan
Apple Dylan is the implementation of Dylan produced by
Apple Computer
Apple Inc. is an American multinational corporation and technology company headquartered in Cupertino, California, in Silicon Valley. It is best known for its consumer electronics, software, and services. Founded in 1976 as Apple Computer Co ...
. It was originally developed for the
Apple Newton
The Newton is a specified standard and series of personal digital assistants (PDAs) developed and marketed by Apple Inc., Apple Computer, Inc. from 1993 to 1998. An early device in the PDA categorythe term itself originating with the Newtonit w ...