REBOL
Encyclopedia
REBOL is a cross-platform
data exchange language and a multi-paradigm
dynamic programming language
originally designed by Carl Sassenrath
for network communications and distributed computing
. The language and its official implementation, which is a proprietary freely redistributable software
are developed by REBOL Technologies. REBOL Technologies also claims the name REBOL as a trademark.
Douglas Crockford
of JavaScript
fame has described REBOL as "a more modern language, but with some very similar ideas to Lisp, in that it's all built upon a representation of data which is then executable as programs.", and as one of the JSON
influences.
REBOL introduces the concept of dialecting
: small, optimized
, domain-specific languages for code and data, which is also the most notable property of the language according to its designer:
REBOL has been used to program Internet
application
s (both client-
and server-side
), database
applications, utilities
, and multimedia
applications.
, based on his study of denotational semantics
and using concepts from the programming languages Lisp, Forth, Logo
, and Self.
REBOL Technologies was founded in 1998.
REBOL 2, the interpreter, which became the core of extended interpreter editions, was first released in 1999.
REBOL/Command, which added strong encryption and ODBC
access, was released in September 2000.
REBOL/View was released in April 2001, adding graphical abilities on the core language.
REBOL/IOS, an extensible collaboration environment built with REBOL was released in August 2001.
REBOL/SDK, providing a choice of kernels to bind against, as well as a preprocessor, was released in December 2002.
REBOL 3, the newest version of the REBOL interpreter
, is currently in development. Alpha versions have been released publicly since January 2008.
by functions processing REBOL blocks (or parsing strings) in a specific way.
An example of REBOL's dialecting abilities can be seen with the word return. In the data exchange dialect return is just a word not having any specific meaning. In the do dialect, return is a global variable
referring to a native function passing back a function result value. In the visual interface dialect (VID), return is a keyword
causing the layout engine to simulate a carriage return
, moving the "rendering pen" down to the beginning of the next line.
A REBOL interpreter with graphical abilities must understand and interpret many dialects. The table below lists the most important ones in order of significance.
, not requiring specific positioning. However, indentation
is recommended to better convey the structure of the text to human readers.
Syntactic properties of different dialects may differ. The common platform for all REBOL dialects is the data exchange dialect; other dialects are usually derived from it. In addition to being the common platform for all dialects, the data exchange dialect is directly used to represent data and metadata, populate data structures, send data over Internet, and save them in data storage.
In contrast to programming languages like C
, the data exchange dialect does not consist of declarations
, statement
s, expressions
or keywords. A valid data exchange dialect text stream is a tree data structure consisting of blocks (the root block is implicit, subblocks are delimited
by square brackets
), parens (delimited by round brackets
), strings
(delimited by double quotes
or curly brackets
suitable for multi-line strings; caret notation
is used for unprintable characters), URLs
, e-mail addresses, files, paths or other composite values
. Unlike ALGOL
blocks, REBOL blocks are composite values similar to quoted s-expression
s in Lisp.
Blocks as well as parens may contain other composite values (a block may contain subblocks, parens, strings, ...) or scalar values
like words, set-words (words suffixed by the colon
), get-words (words prefixed by the colon), lit-words (words prefixed by the apostrophe), numbers, money, characters
, etc., separated by whitespace
. Note that special characters are allowed in words, so
Comment
s may appear following the semicolon
until the end of the line. Multi-line comments or comments not ignored by the lexical parser can be written using "ordinary" datatypes like multi-line strings.
The do dialect interpreted by the do function, is an expression-oriented
sublanguage of the data exchange dialect. The main semantic unit of the language is the expression
. In contrast to imperative programming languages descending from ALGOL
, the do dialect has neither keywords, nor statements.
Words are used as case-insensitive
variables
. Like in all dynamically typed languages, variables don't have an associated type, type is associated with values. The result, i.e. the evaluation of a word is returned, when a word is encountered by the do function. The set-word form of a word can be used for assignment
. While not having statements, assignment, together with functions with side-effects can be used for imperative programming
.
Subblocks of the root block evaluate to themselves. This property is used to handle data blocks, for structured programming
by submitting blocks as arguments to control
functions like if, either, loop, etc., and for dialecting, when a block is passed to a specific interpreter function.
A specific problem worth noting is, that composite values, assigned to variables, are not copied. To make a copy, the value must be passed to the copy function.
The do function normally follows a prefix style of evaluation
, where a function processes the arguments that follow it. However, infix evaluation
using infix operator
s exists too. Infix evaluation takes precedence over the prefix evaluation. For example,
returns 1, since the infix addition takes precedence over the computation of the absolute value. When evaluating infix expressions, the order of evaluation is left to right, no operator takes precedence
over another. For example,
returns 20, while an evaluation giving precedence to multiplication would yield 14. All operators have prefix versions. Do usually evaluates arguments before passing them to a function. So, the below expression:
first reads
, the usage of parentheses in expressions can be avoided.
The simple precedence rules are both an advantage:
as well as a disadvantage:
Parse expressions are written in the parse dialect, which, like the do dialect, is an expression-oriented sublanguage of the data exchange dialect. Unlike the do dialect, the parse dialect uses keywords representing operators and the most important nonterminals, infix parsing operators don't have prefix equivalents and use precedence rules (sequence has higher precedence than choice).
Actions can be included to be taken during the parsing process as well and the parse function can be used to process blocks or strings. At the string parsing level parse must handle the "low level" parsing, taking into account characters
and delimiters. Block parsing is higher level, handling the scanning at the level of REBOL values.
The parse dialect belongs to the family of grammars represented by the top-down parsing language
or the parsing expression grammar
. The main similarity is the presence of the sequence and choice operators all the family members have. Parse dialect syntax and the similarities between the parse dialect and the PEG are illustrated by this transliteration of a PEG example that parses an arithmetic expression:
with a graphical user interface:
view layout [text "Hello world!" button "Quit" [quit]]
is a proprietary, freely redistributable
interpreter available in several editions (/Core, /View, /Command, /SDK and /IOS). Reverse engineering of the interpreter is forbidden by the license. Parts of the interpreter are open source
. Example: the REBOL desktop
is an open source part of the REBOL/View interpreter linking the REBOL community on the Internet. The runtime environment is currently stored in a single executable file. REBOL/Core, the console edition, is about 300kB and REBOL/View, the graphical user interface
edition, is about 650kB in size. REBOL/View provides platform-independent
graphics and sound access, and comes with its own windowing toolkit and extensible set of styles (GUI widgets). Extended editions, such as REBOL/Command or REBOL/SDK require a paid license; they add features like ODBC data access, and the option to create standalone executable files.
There are also free software
/ open source alternatives to the REBOL interpreter (or parts of it), which are written as implementations of REBOL dialects.
Cross-platform
In computing, cross-platform, or multi-platform, is an attribute conferred to computer software or computing methods and concepts that are implemented and inter-operate on multiple computer platforms...
data exchange language and a multi-paradigm
Multi-paradigm programming language
Programming languages can be grouped by the number and types of paradigms supported.-Paradigm summaries:A concise reference for the programming paradigms listed in this article....
dynamic programming language
Dynamic programming language
Dynamic programming language is a term used broadly in computer science to describe a class of high-level programming languages that execute at runtime many common behaviors that other languages might perform during compilation, if at all...
originally designed by Carl Sassenrath
Carl Sassenrath
Carl Sassenrath is an architect of operating systems and computer languages. He brought multitasking to personal computers in 1985 with the creation of the Amiga Computer operating system kernel, and he is currently the designer of the REBOL computer language as well as the CTO of REBOL...
for network communications and distributed computing
Distributed computing
Distributed computing is a field of computer science that studies distributed systems. A distributed system consists of multiple autonomous computers that communicate through a computer network. The computers interact with each other in order to achieve a common goal...
. The language and its official implementation, which is a proprietary freely redistributable software
Freely redistributable software
Freely redistributable software is software that anyone is free to redistribute. The term has been used to mean two types of free to redistribute software, distinguished by the legal modifiability and limitations on purpose of use of the software. FRS which can be legally modified and used for any...
are developed by REBOL Technologies. REBOL Technologies also claims the name REBOL as a trademark.
Douglas Crockford
Douglas Crockford
Douglas Crockford is an American computer programmer and entrepreneur, best known for his ongoing involvement in the development of the JavaScript language, and for having popularized the data format JSON , and for developing JSLint...
of JavaScript
JavaScript
JavaScript is a prototype-based scripting language that is dynamic, weakly typed and has first-class functions. It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles....
fame has described REBOL as "a more modern language, but with some very similar ideas to Lisp, in that it's all built upon a representation of data which is then executable as programs.", and as one of the JSON
JSON
JSON , or JavaScript Object Notation, is a lightweight text-based open standard designed for human-readable data interchange. It is derived from the JavaScript scripting language for representing simple data structures and associative arrays, called objects...
influences.
REBOL introduces the concept of dialecting
Dialecting
Dialects are domain specific sub-languages of a programming language or a data exchange language. A language supporting this paradigm encourages users to create new dialects for specific problematic domains.- Languages :* FORTH* LISP* REBOL* Ruby* XL- See also :* Domain-specific...
: small, optimized
Optimization (computer science)
In computer science, program optimization or software optimization is the process of modifying a software system to make some aspect of it work more efficiently or use fewer resources...
, domain-specific languages for code and data, which is also the most notable property of the language according to its designer:
REBOL has been used to program Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
application
Application software
Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...
s (both client-
Client-side
Client-side refers to operations that are performed by the client in a client–server relationship in a computer network.Typically, a client is a computer application, such as a web browser, that runs on a user's local computer or workstation and connects to a server as necessary...
and server-side
Server-side
Server-side refers to operations that are performed by the server in a client–server relationship in computer networking.Typically, a server is a software program, such as a web server, that runs on a remote server, reachable from a user's local computer or workstation...
), database
Database
A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...
applications, utilities
Utility software
Utility software is system software designed to help analyze, configure, optimize or maintain a computer. A single piece of utility software is usually called a utility or tool....
, and multimedia
Multimedia
Multimedia is media and content that uses a combination of different content forms. The term can be used as a noun or as an adjective describing a medium as having multiple content forms. The term is used in contrast to media which use only rudimentary computer display such as text-only, or...
applications.
History
First released in 1997, REBOL was designed over a 20 year period by Carl Sassenrath, the architect and primary developer of AmigaOSAmigaOS
AmigaOS is the default native operating system of the Amiga personal computer. It was developed first by Commodore International, and initially introduced in 1985 with the Amiga 1000...
, based on his study of denotational semantics
Denotational semantics
In computer science, denotational semantics is an approach to formalizing the meanings of programming languages by constructing mathematical objects which describe the meanings of expressions from the languages...
and using concepts from the programming languages Lisp, Forth, Logo
Logo (programming language)
Logo is a multi-paradigm computer programming language used in education. It is an adaptation and dialect of the Lisp language; some have called it Lisp without the parentheses. It was originally conceived and written as functional programming language, and drove a mechanical turtle as an output...
, and Self.
REBOL Technologies was founded in 1998.
REBOL 2, the interpreter, which became the core of extended interpreter editions, was first released in 1999.
REBOL/Command, which added strong encryption and ODBC
Open Database Connectivity
In computing, ODBC is a standard C interface for accessing database management systems . The designers of ODBC aimed to make it independent of database systems and operating systems...
access, was released in September 2000.
REBOL/View was released in April 2001, adding graphical abilities on the core language.
REBOL/IOS, an extensible collaboration environment built with REBOL was released in August 2001.
REBOL/SDK, providing a choice of kernels to bind against, as well as a preprocessor, was released in December 2002.
REBOL 3, the newest version of the REBOL interpreter
Interpreter (computing)
In computer science, an interpreter normally means a computer program that executes, i.e. performs, instructions written in a programming language...
, is currently in development. Alpha versions have been released publicly since January 2008.
Dialects
REBOL dialects, now popular as domain-specific languages (DSLs), are micro-languages optimized for a specific purpose. Dialects can be used to define business rules, graphical user interfaces or sequences of screens during the installation of a program. Users can define their own dialects, reusing any existing REBOL word and giving it a specific meaning in that dialect. Dialects are interpretedInterpreted language
Interpreted language is a programming language in which programs are 'indirectly' executed by an interpreter program. This can be contrasted with a compiled language which is converted into machine code and then 'directly' executed by the host CPU...
by functions processing REBOL blocks (or parsing strings) in a specific way.
An example of REBOL's dialecting abilities can be seen with the word return. In the data exchange dialect return is just a word not having any specific meaning. In the do dialect, return is a global variable
Global variable
In computer programming, a global variable is a variable that is accessible in every scope . Interaction mechanisms with global variables are called global environment mechanisms...
referring to a native function passing back a function result value. In the visual interface dialect (VID), return is a keyword
Keyword (computer programming)
In computer programming, a keyword is a word or identifier that has a particular meaning to the programming language. The meaning of keywords — and, indeed, the meaning of the notion of keyword — differs widely from language to language....
causing the layout engine to simulate a carriage return
Carriage return
Carriage return, often shortened to return, refers to a control character or mechanism used to start a new line of text.Originally, the term "carriage return" referred to a mechanism or lever on a typewriter...
, moving the "rendering pen" down to the beginning of the next line.
A REBOL interpreter with graphical abilities must understand and interpret many dialects. The table below lists the most important ones in order of significance.
| Dialect name | Interpreted by | Purpose |
|---|---|---|
| Data exchange dialect | load function | represents data and metadata; common platform for REBOL dialects |
| Do dialect | do function | programming |
| Parse dialect | parse function | pattern matching Pattern matching In computer science, pattern matching is the act of checking some sequence of tokens for the presence of the constituents of some pattern. In contrast to pattern recognition, the match usually has to be exact. The patterns generally have the form of either sequences or tree structures... |
| Function specification dialect | make function | function definition; functional programming Functional programming In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state... |
| Object specification dialect | make function | object Object (computer science) In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure... definition/inheritance; prototype-based programming Prototype-based programming Prototype-based programming is a style of object-oriented programming in which classes are not present, and behavior reuse is performed via a process of cloning existing objects that serve as prototypes. This model can also be known as classless, prototype-oriented or instance-based programming... |
| Visual interface dialect (VID) or RebGUI |
layout function or display function |
specifies graphical user interface Graphical user interface In computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and... |
| Draw dialect | view function | defines graphical elements (lines, polygons, etc.) |
| Script specification dialect | do function | script definition |
| Security policy dialect | secure function | specifies security policy |
Syntax
REBOL syntax is free-formFree-form language
In computer programming, a free-form language is a programming language in which the positioning of characters on the page in program text is not significant. Program text does not need to be placed in specific columns as on old punched card systems, and frequently ends of lines are not significant...
, not requiring specific positioning. However, indentation
Indent style
In computer programming, an indent style is a convention governing the indentation of blocks of code to convey the program's structure. This article largely addresses the C programming language and its descendants, but can be applied to most other programming languages...
is recommended to better convey the structure of the text to human readers.
Syntactic properties of different dialects may differ. The common platform for all REBOL dialects is the data exchange dialect; other dialects are usually derived from it. In addition to being the common platform for all dialects, the data exchange dialect is directly used to represent data and metadata, populate data structures, send data over Internet, and save them in data storage.
In contrast to programming languages like C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
, the data exchange dialect does not consist of declarations
Declaration (computer science)
In programming languages, a declaration specifies the identifier, type, and other aspects of language elements such as variables and functions. It is used to announce the existence of the element to the compiler; this is important in many strongly-typed languages that require variables and their...
, statement
Statement (programming)
In computer programming a statement can be thought of as the smallest standalone element of an imperative programming language. A program written in such a language is formed by a sequence of one or more statements. A statement will have internal components .Many languages In computer programming...
s, expressions
Expression (programming)
An expression in a programming language is a combination of explicit values, constants, variables, operators, and functions that are interpreted according to the particular rules of precedence and of association for a particular programming language, which computes and then produces another value...
or keywords. A valid data exchange dialect text stream is a tree data structure consisting of blocks (the root block is implicit, subblocks are delimited
Delimiter
A delimiter is a sequence of one or more characters used to specify the boundary between separate, independent regions in plain text or other data streams. An example of a delimiter is the comma character, which acts as a field delimiter in a sequence of comma-separated values.Delimiters represent...
by square brackets
Bracket
Brackets are tall punctuation marks used in matched pairs within text, to set apart or interject other text. In the United States, "bracket" usually refers specifically to the "square" or "box" type.-List of types:...
), parens (delimited by round brackets
Bracket
Brackets are tall punctuation marks used in matched pairs within text, to set apart or interject other text. In the United States, "bracket" usually refers specifically to the "square" or "box" type.-List of types:...
), strings
String (computer science)
In formal languages, which are used in mathematical logic and theoretical computer science, a string is a finite sequence of symbols that are chosen from a set or alphabet....
(delimited by double quotes
Quotation mark glyphs
Different typefaces, character encodings and computer languages use various encodings and glyphs for quotation marks. This article lists some of these glyphs along with their Unicode code points and HTML entities...
or curly brackets
Bracket
Brackets are tall punctuation marks used in matched pairs within text, to set apart or interject other text. In the United States, "bracket" usually refers specifically to the "square" or "box" type.-List of types:...
suitable for multi-line strings; caret notation
Caret notation
Caret notation is a notation for unprintable control characters in ASCII encoding. The notation consists of a caret followed by a capital letter; this digraph stands for the ASCII code that has the numerical value equivalent to the letter's numerical value. For example the EOT character with a...
is used for unprintable characters), URLs
Uniform Resource Locator
In computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
, e-mail addresses, files, paths or other composite values
Scalar (computing)
In computing, a scalar variable or field is one that can hold only one value at a time; as opposed to composite variables like array, list, hash, record, etc. In some contexts, a scalar value may be understood to be numeric. A scalar data type is the type of a scalar variable...
. Unlike ALGOL
ALGOL
ALGOL is a family of imperative computer programming languages originally developed in the mid 1950s which greatly influenced many other languages and became the de facto way algorithms were described in textbooks and academic works for almost the next 30 years...
blocks, REBOL blocks are composite values similar to quoted s-expression
S-expression
S-expressions or sexps are list-based data structures that represent semi-structured data. An S-expression may be a nested list of smaller S-expressions. S-expressions are probably best known for their use in the Lisp family of programming languages...
s in Lisp.
Blocks as well as parens may contain other composite values (a block may contain subblocks, parens, strings, ...) or scalar values
Scalar (computing)
In computing, a scalar variable or field is one that can hold only one value at a time; as opposed to composite variables like array, list, hash, record, etc. In some contexts, a scalar value may be understood to be numeric. A scalar data type is the type of a scalar variable...
like words, set-words (words suffixed by the colon
Colon (punctuation)
The colon is a punctuation mark consisting of two equally sized dots centered on the same vertical line.-Usage:A colon informs the reader that what follows the mark proves, explains, or lists elements of what preceded the mark....
), get-words (words prefixed by the colon), lit-words (words prefixed by the apostrophe), numbers, money, characters
Character encoding
A character encoding system consists of a code that pairs each character from a given repertoire with something else, such as a sequence of natural numbers, octets or electrical pulses, in order to facilitate the transmission of data through telecommunication networks or storage of text in...
, etc., separated by whitespace
Whitespace (computer science)
In computer science, whitespace is any single character or series of characters that represents horizontal or vertical space in typography. When rendered, a whitespace character does not correspond to a visual mark, but typically does occupy an area on a page...
. Note that special characters are allowed in words, so
a+b is a word unlike a + b, which is a sequence of three words separated by spaces.Comment
Comment (computer programming)
In computer programming, a comment is a programming language construct used to embed programmer-readable annotations in the source code of a computer program. Those annotations are potentially significant to programmers but typically ignorable to compilers and interpreters. Comments are usually...
s may appear following the semicolon
Semicolon
The semicolon is a punctuation mark with several uses. The Italian printer Aldus Manutius the Elder established the practice of using the semicolon to separate words of opposed meaning and to indicate interdependent statements. "The first printed semicolon was the work of ... Aldus Manutius"...
until the end of the line. Multi-line comments or comments not ignored by the lexical parser can be written using "ordinary" datatypes like multi-line strings.
Semantics
Blocks containing domain-specific language can be submitted as arguments to specific evaluator functions.do
The most frequently used evaluator is the do function. It is used by default to interpret the text input to the interpreter console.The do dialect interpreted by the do function, is an expression-oriented
Expression-oriented programming languages
An expression-oriented programming language is a programming language where every construction is an expression and thus yields a value. The typical exceptions are macro definitions, preprocessor commands, and declarations, which expression-oriented languages often treat as statements rather than...
sublanguage of the data exchange dialect. The main semantic unit of the language is the expression
Expression (programming)
An expression in a programming language is a combination of explicit values, constants, variables, operators, and functions that are interpreted according to the particular rules of precedence and of association for a particular programming language, which computes and then produces another value...
. In contrast to imperative programming languages descending from ALGOL
ALGOL
ALGOL is a family of imperative computer programming languages originally developed in the mid 1950s which greatly influenced many other languages and became the de facto way algorithms were described in textbooks and academic works for almost the next 30 years...
, the do dialect has neither keywords, nor statements.
Words are used as case-insensitive
Case sensitivity
Text sometimes exhibits case sensitivity; that is, words can differ in meaning based on differing use of uppercase and lowercase letters. Words with capital letters do not always have the same meaning when written with lowercase letters....
variables
Variable (programming)
In computer programming, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents...
. Like in all dynamically typed languages, variables don't have an associated type, type is associated with values. The result, i.e. the evaluation of a word is returned, when a word is encountered by the do function. The set-word form of a word can be used for assignment
Assignment (computer science)
In computer programming, an assignment statement sets or re-sets the value stored in the storage location denoted by a variable name. In most imperative computer programming languages, assignment statements are one of the basic statements...
. While not having statements, assignment, together with functions with side-effects can be used for imperative programming
Imperative programming
In computer science, imperative programming is a programming paradigm that describes computation in terms of statements that change a program state...
.
Subblocks of the root block evaluate to themselves. This property is used to handle data blocks, for structured programming
Structured programming
Structured programming is a programming paradigm aimed on improving the clarity, quality, and development time of a computer program by making extensive use of subroutines, block structures and for and while loops - in contrast to using simple tests and jumps such as the goto statement which could...
by submitting blocks as arguments to control
Control flow
In computer science, control flow refers to the order in which the individual statements, instructions, or function calls of an imperative or a declarative program are executed or evaluated....
functions like if, either, loop, etc., and for dialecting, when a block is passed to a specific interpreter function.
A specific problem worth noting is, that composite values, assigned to variables, are not copied. To make a copy, the value must be passed to the copy function.
The do function normally follows a prefix style of evaluation
Polish notation
Polish notation, also known as prefix notation, is a form of notation for logic, arithmetic, and algebra. Its distinguishing feature is that it places operators to the left of their operands. If the arity of the operators is fixed, the result is a syntax lacking parentheses or other brackets that...
, where a function processes the arguments that follow it. However, infix evaluation
Infix notation
Infix notation is the common arithmetic and logical formula notation, in which operators are written infix-style between the operands they act on . It is not as simple to parse by computers as prefix notation or postfix notation Infix notation is the common arithmetic and logical formula notation,...
using infix operator
Operator (programming)
Programming languages typically support a set of operators: operations which differ from the language's functions in calling syntax and/or argument passing mode. Common examples that differ by syntax are mathematical arithmetic operations, e.g...
s exists too. Infix evaluation takes precedence over the prefix evaluation. For example,
abs -2 + 3
returns 1, since the infix addition takes precedence over the computation of the absolute value. When evaluating infix expressions, the order of evaluation is left to right, no operator takes precedence
Order of operations
In mathematics and computer programming, the order of operations is a rule used to clarify unambiguously which procedures should be performed first in a given mathematical expression....
over another. For example,
2 + 3 * 4
returns 20, while an evaluation giving precedence to multiplication would yield 14. All operators have prefix versions. Do usually evaluates arguments before passing them to a function. So, the below expression:
print read http://en.wikipedia.org/wiki/REBOL
first reads
http://en.wikipedia.org/wiki/REBOL Polish notation
Polish notation, also known as prefix notation, is a form of notation for logic, arithmetic, and algebra. Its distinguishing feature is that it places operators to the left of their operands. If the arity of the operators is fixed, the result is a syntax lacking parentheses or other brackets that...
, the usage of parentheses in expressions can be avoided.
The simple precedence rules are both an advantage:
- No need to "consult" precedence tables when writing expressions
- No need to rewrite precedence tables when a new operator is defined
- Expressions can be easily transliteratedTransliterationTransliteration is a subset of the science of hermeneutics. It is a form of translation, and is the practice of converting a text from one script into another...
from infix to prefix notation and vice versa
as well as a disadvantage:
- Users accustomed to more conventional precedence rules may easily make a mistake
parse
The parse function is preferably used to specify, validate, transform and interpret dialects. It does so by matching parse expressions at run time.Parse expressions are written in the parse dialect, which, like the do dialect, is an expression-oriented sublanguage of the data exchange dialect. Unlike the do dialect, the parse dialect uses keywords representing operators and the most important nonterminals, infix parsing operators don't have prefix equivalents and use precedence rules (sequence has higher precedence than choice).
Actions can be included to be taken during the parsing process as well and the parse function can be used to process blocks or strings. At the string parsing level parse must handle the "low level" parsing, taking into account characters
Character (computing)
In computer and machine-based telecommunications terminology, a character is a unit of information that roughly corresponds to a grapheme, grapheme-like unit, or symbol, such as in an alphabet or syllabary in the written form of a natural language....
and delimiters. Block parsing is higher level, handling the scanning at the level of REBOL values.
The parse dialect belongs to the family of grammars represented by the top-down parsing language
Top-down parsing language
Top-Down Parsing Language is a type of analytic formal grammar developed by Alexander Birman in the early 1970s in order to study formally the behavior of a common class of practical top-down parsers that support a limited form of backtracking...
or the parsing expression grammar
Parsing expression grammar
A parsing expression grammar, or PEG, is a type of analytic formal grammar, i.e. it describes a formal language in terms of a set of rules for recognizing strings in the language...
. The main similarity is the presence of the sequence and choice operators all the family members have. Parse dialect syntax and the similarities between the parse dialect and the PEG are illustrated by this transliteration of a PEG example that parses an arithmetic expression:
Digit: charset [#"0" - #"9"]
Value: [some Digit | "(" Expr ")"]
Product: [Value any "/"] Value
Sum: [Product any "-"] Product
Expr: Sum
parse/all "12+13" Expr
Ease of use
One of the REBOL design principles is "to do simple things in simple ways". In the following example the Visual interface dialect is used to describe a simple Hello world programHello world program
A "Hello world" program is a computer program that outputs "Hello world" on a display device. Because it is typically one of the simplest programs possible in most programming languages, it is by tradition often used to illustrate to beginners the most basic syntax of a programming language, or to...
with a graphical user interface:
view layout [text "Hello world!" button "Quit" [quit]]
Implementations
The official REBOL implementationImplementation
Implementation is the realization of an application, or execution of a plan, idea, model, design, specification, standard, algorithm, or policy.-Computer Science:...
is a proprietary, freely redistributable
Freely redistributable software
Freely redistributable software is software that anyone is free to redistribute. The term has been used to mean two types of free to redistribute software, distinguished by the legal modifiability and limitations on purpose of use of the software. FRS which can be legally modified and used for any...
interpreter available in several editions (/Core, /View, /Command, /SDK and /IOS). Reverse engineering of the interpreter is forbidden by the license. Parts of the interpreter are open source
Open source
The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...
. Example: the REBOL desktop
Desktop environment
In graphical computing, a desktop environment commonly refers to a style of graphical user interface derived from the desktop metaphor that is seen on most modern personal computers. These GUIs help the user in easily accessing, configuring, and modifying many important and frequently accessed...
is an open source part of the REBOL/View interpreter linking the REBOL community on the Internet. The runtime environment is currently stored in a single executable file. REBOL/Core, the console edition, is about 300kB and REBOL/View, the graphical user interface
Graphical user interface
In computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and...
edition, is about 650kB in size. REBOL/View provides platform-independent
Cross-platform
In computing, cross-platform, or multi-platform, is an attribute conferred to computer software or computing methods and concepts that are implemented and inter-operate on multiple computer platforms...
graphics and sound access, and comes with its own windowing toolkit and extensible set of styles (GUI widgets). Extended editions, such as REBOL/Command or REBOL/SDK require a paid license; they add features like ODBC data access, and the option to create standalone executable files.
There are also free software
Free software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...
/ open source alternatives to the REBOL interpreter (or parts of it), which are written as implementations of REBOL dialects.
- Orca is "an interpreter for a REBOL-like language". Orca is written in CC (programming language)C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
, but cannot be compiled without a REBOL interpreter being installed (Orca can be used when available). - Boron is a C library which may be copied under the terms of the LGPLv3. It is an interpreter of a REBOL dialect. The Boron dialect is an interpreted, homoiconic language, which is meant for embedding domain specific languages.
- The Red/System compiler implements a REBOL dialect. It is currently bootstrapped using a REBOL interpreter and therefore needs a REBOL interpreter to run.
- The RebGUI dialect is an open alternative to REBOL/View's VID dialect for graphical user interfaces.
See also
- DialectingDialectingDialects are domain specific sub-languages of a programming language or a data exchange language. A language supporting this paradigm encourages users to create new dialects for specific problematic domains.- Languages :* FORTH* LISP* REBOL* Ruby* XL- See also :* Domain-specific...
- Domain-specific language
- Language-oriented programmingLanguage-oriented programmingLanguage oriented programming is a style of computer programming in which, rather than solving problems in general-purpose programming languages, the programmer creates one or more domain-specific languages for the problem first, and solves the problem in those languages...
- Parsing expression grammarParsing expression grammarA parsing expression grammar, or PEG, is a type of analytic formal grammar, i.e. it describes a formal language in terms of a set of rules for recognizing strings in the language...
- Pattern matchingPattern matchingIn computer science, pattern matching is the act of checking some sequence of tokens for the presence of the constituents of some pattern. In contrast to pattern recognition, the match usually has to be exact. The patterns generally have the form of either sequences or tree structures...
- Polish notationPolish notationPolish notation, also known as prefix notation, is a form of notation for logic, arithmetic, and algebra. Its distinguishing feature is that it places operators to the left of their operands. If the arity of the operators is fixed, the result is a syntax lacking parentheses or other brackets that...