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56-534: BCPL (" Basic Combined Programming Language ") is a procedural , imperative , and structured programming language . Originally intended for writing compilers for other languages, BCPL is no longer in common use. However, its influence is still felt because a stripped down and syntactically changed version of BCPL, called B , was the language on which the C programming language was based. BCPL introduced several features of many modern programming languages, including using curly braces to delimit code blocks. BCPL

112-439: A Windows Script File running under Windows Script Host . Although a section marking the code appears as a comment, the diagram itself actually appears in an XML CDATA section, which is technically considered distinct from comments, but can serve similar purposes. Although this identical diagram could easily have been included as a comment, the example illustrates one instance where a programmer may opt not to use comments as

168-466: A byte or on machines with larger word sizes such as 32-bit or 64-bit. The interpretation of any value was determined by the operators used to process the values. (For example, + added two values together, treating them as integers ; ! indirected through a value, effectively treating it as a pointer.) In order for this to work, the implementation provided no type checking . The mismatch between BCPL's word orientation and byte-oriented hardware

224-449: A combination of both styles. It is common for input/output code in functional languages to be written in a procedural style. There do exist a few esoteric functional languages (like Unlambda ) that eschew structured programming precepts for the sake of being difficult to program in (and therefore challenging). These languages are the exception to the common ground between procedural and functional languages. In logic programming ,

280-406: A comment. Further, copyright notices can be embedded within source code as comments. Binary data may also be encoded in comments through a process known as binary-to-text encoding , although such practice is uncommon and typically relegated to external resource files. The following code fragment is a simple ASCII diagram depicting the process flow for a system administration script contained in

336-400: A novel or noteworthy solution to a specific problem. In such cases, comments may contain an explanation of the methodology. Such explanations may include diagrams and formal mathematical proofs. This may constitute explanation of the code, rather than a clarification of its intent; but others tasked with maintaining the code base may find such explanation crucial. This might especially be true in

392-521: A number that associates the name with the corresponding numerically addressed word in the global vector. As well as variables, the global vector contains bindings for external procedures. This makes dynamic loading of compilation units very simple to achieve. Instead of relying on the link loader of the underlying implementation, effectively, BCPL gives the programmer control of the linking process. The global vector also made it very simple to replace or augment standard library routines. A program could save

448-470: A particular community. Experts have varying viewpoints on whether, and when, comments are appropriate in source code. Some assert that source code should be written with few comments, on the basis that the source code should be self-explanatory or self-documenting . Others suggest code should be extensively commented (it is not uncommon for over 50% of the non- whitespace characters in source code to be contained within comments). In between these views

504-419: A program implementation into objects that expose behavior (methods) and data (members) via a well-defined interface. In contrast, procedural programming is about dividing the program implementation into variables , data structures , and subroutines . An important distinction is that while procedural involves procedures to operate on data structures, OOP bundles the two together. An object is a data structure and

560-442: A program is a set of premises, and computation is performed by attempting to prove candidate theorems. From this point of view, logic programs are declarative , focusing on what the problem is, rather than on how to solve it. However, the backward reasoning technique, implemented by SLD resolution , used to solve problems in logic programming languages such as Prolog , treats programs as goal-reduction procedures. Thus clauses of

616-548: A program, such as " conditional comments ". Such "hot comments" may be the only practical solution that maintains backward-compatibility, but are widely regarded as a kludge . One specific example are docblocks , which are specially-formatted comments used to document a specific segment of code. This makes the DocBlock format independent of the target language (as long as it supports comments); however, it may also lead to multiple or inconsistent standards. There are cases where

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672-468: A project starts, or evolve as a matter of convention or need as a project grows. Usually programmers prefer styles that are consistent, non-obstructive, easy to modify, and difficult to break. The following code fragments in C demonstrate just a tiny example of how comments can vary stylistically, while still conveying the same basic information: Factors such as personal preference, flexibility of programming tools, and other considerations tend to influence

728-418: A way of including resources in source code. Comments in a computer program often store metadata about a program file. In particular, many software maintainers put submission guidelines in comments to help people who read the source code of that program to send any improvements they make back to the maintainer. Other metadata includes: the name of the creator of the original version of the program file and

784-406: A wide degree of variability, but formal conventions for their use are commonly part of programming style guides. Comments are generally formatted as either block comments (also called prologue comments or stream comments ) or line comments (also called inline comments ). Block comments delimit a region of source code which may span multiple lines or a part of a single line. This region

840-576: Is C++ (with ++ being C's increment operator ), although meanwhile, a D programming language also exists. In 1979, implementations of BCPL existed for at least 25 architectures; the language gradually fell out of favour as C became popular on non-Unix systems. Martin Richards maintains a modern version of BCPL on his website, last updated in 2023. This can be set up to run on various systems including Linux, FreeBSD, and Mac OS X. The latest distribution includes graphics and sound libraries, and there

896-519: Is a comprehensive manual. He continues to program in it, including for his research on musical automated score following . A common informal MIME type for BCPL is text/x-bcpl . Richards and Whitby-Strevens provide an example of the "Hello, World!" program for BCPL using a standard system header, 'LIBHDR': If these programs are run using Richards' current version of Cintsys (December 2018), LIBHDR, START and WRITEF must be changed to lower case to avoid errors. Print factorials: Count solutions to

952-514: Is a global vector, similar to "blank common" in Fortran . All data shared between different compilation units comprises scalars and pointers to vectors stored in a pre-arranged place in the global vector. Thus, the header files (files included during compilation using the "GET" directive) become the primary means of synchronizing global data between compilation units, containing "GLOBAL" directives that present lists of symbolic names, each paired with

1008-418: Is possible, like Lisp machines or Java processors , but no attempt was commercially successful. Certain software development practices are often employed with procedural programming in order to enhance quality and lower development and maintenance costs. Modularity is about organizing the procedures of a program into separate modules—each of which has a specific and understandable purpose. Minimizing

1064-414: Is specified with a start delimiter and an end delimiter. Some programming languages (such as MATLAB ) allow block comments to be recursively nested inside one another, but others (such as Java ) do not. Line comments either start with a comment delimiter and continue until the end of the line, or in some cases, start at a specific column (character line offset) in the source code, and continue until

1120-402: Is the assertion that comments are neither beneficial nor harmful by themselves, and what matters is that they are correct and kept in sync with the source code, and omitted if they are superfluous, excessive, difficult to maintain or otherwise unhelpful. Comments are sometimes used to document contracts in the design by contract approach to programming. Depending on the intended audience of

1176-409: Is unusual in having only one data type : a word , a fixed number of bits, usually chosen to align with the architecture's machine word and of adequate capacity to represent any valid storage address. For many machines of the time, this data type was a 16-bit word. This choice later proved to be a significant problem when BCPL was used on machines in which the smallest addressable item was not a word but

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1232-478: The N queens problem : Procedural programming The first major procedural programming languages appeared c.  1957 –1964, including Fortran , ALGOL , COBOL , PL/I and BASIC . Pascal and C were published c.  1970 –1972. Computer processors provide hardware support for procedural programming through a stack register and instructions for calling procedures and returning from them. Hardware support for other types of programming

1288-729: The PDP-11s used in BBN's implementation of the second generation IMPs used in the ARPANET . There was also a version produced for the BBC Micro in the mid-1980s, by Richards Computer Products, a company started by John Richards, the brother of Martin Richards. The BBC Domesday Project made use of the language. Versions of BCPL for the Amstrad CPC and Amstrad PCW computers were also released in 1986 by UK software house Arnor Ltd. MacBCPL

1344-457: The scope of variables and procedures can enhance software quality by reducing the cognitive load of procedures and modules. A program lacking modularity or wide scoping tends to have procedures that consume many variables that other procedures also consume. The resulting code is relatively hard to understand and to maintain. Since a procedure can specify a well-defined interface and be self-contained it supports code reuse —in particular via

1400-678: The software library . Procedural programming is classified as an imperative programming , because it involves direct command of execution. Procedural is a sub-class of imperative since procedural includes block and scope concepts, whereas imperative describes a more general concept that does not require such features. Procedural languages generally use reserved words that define blocks, such as if , while , and for , to implement control flow , whereas non-structured imperative languages (i.e. assembly language ) use goto and branch tables for this purpose. Also classified as imperative, object-oriented programming (OOP) involves dividing

1456-585: The source code of a computer program . They are added with the purpose of making the source code easier for humans to understand, and are generally ignored by compilers and interpreters . The syntax of comments in various programming languages varies considerably. Comments are sometimes also processed in various ways to generate documentation external to the source code itself by documentation generators , or used for integration with source code management systems and other kinds of external programming tools . The flexibility provided by comments allows for

1512-547: The 1969 Spring Joint Computer Conference . BCPL has been rumored to have originally stood for "Bootstrap Cambridge Programming Language", but CPL was never created since development stopped at BCPL, and the acronym was later reinterpreted for the BCPL book. BCPL is the language in which the original "Hello, World!" program was written. The first MUD was also written in BCPL ( MUD1 ). Several operating systems were written partially or wholly in BCPL (for example, TRIPOS and

1568-455: The behavior associated with that data structure. Some OOP languages support the class concept which allows for creating an object based on a definition. Nomenclature varies between the two, although they have similar semantics: The principles of modularity and code reuse in functional languages are fundamentally the same as in procedural languages, since they both stem from structured programming . For example: The main difference between

1624-437: The case of highly specialized problem domains; or rarely used optimizations, constructs or function-calls. For example, a programmer may add a comment to explain why an insertion sort was chosen instead of a quicksort , as the former is, in theory, slower than the latter. This could be written as follows: Logos , diagrams, and flowcharts consisting of ASCII art constructions can be inserted into source code formatted as

1680-417: The code and other considerations, the level of detail and description may vary considerably. For example, the following Java comment would be suitable in an introductory text designed to teach beginning programming: This level of detail, however, would not be appropriate in the context of production code, or other situations involving experienced developers. Such rudimentary descriptions are inconsistent with

1736-477: The code in plain English is considered superfluous; the need to re-explain code may be a sign that it is too complex and should be rewritten, or that the naming is bad. Comments may also be used to explain why a block of code does not seem to fit conventions or best practices. This is especially true of projects involving very little development time, or in bug fixing. For example: Sometimes source code contains

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1792-404: The code rather than the code itself. If this type of comment is left in, it simplifies the review process by allowing a direct comparison of the code with the intended results. A common logical fallacy is that code that is easy to understand does what it's supposed to do. Comments can be used to summarize code or to explain the programmer's intent. According to this school of thought, restating

1848-465: The date when the first version was created, the name of the current maintainer of the program, the names of other people who have edited the program file so far, the URL of documentation about how to use the program, the name of the software license for this program file, etc. When an algorithm in some section of the program is based on a description in a book or other reference, comments can be used to give

1904-518: The day, source programs often used the sequences $ ( and $ ) or [ and ] in place of the symbols { and } . The single-line // comments of BCPL, which were not adopted by C , reappeared in C++ and later in C99 . The book BCPL: The language and its compiler describes the philosophy of BCPL as follows: The philosophy of BCPL is not one of the tyrant who thinks he knows best and lays down

1960-707: The earliest versions of AmigaDOS ). BCPL was also the initial language used in the Xerox PARC Alto project. Among other projects, the Bravo document preparation system was written in BCPL. An early compiler, bootstrapped in 1969, by starting with a paper tape of the O-code of Richards's Atlas 2 compiler, targeted the ICT 1900 series . The two machines had different word-lengths (48 vs 24 bits), different character encodings, and different packed string representations—and

2016-406: The end of the line. Some programming languages employ both block and line comments with different comment delimiters. For example, C++ has block comments delimited by /* and */ that can span multiple lines and line comments delimited by // . Other languages support only one type of comment. For example, Ada comments are line comments: they start with -- and continue to the end of

2072-422: The file's syntax highlighting mode, or the file's revision number . These functional control comments are also commonly referred to as annotations . Keeping documentation within source code comments is considered as one way to simplify the documentation process, as well as increase the chances that the documentation will be kept up to date with changes in the code. Examples of documentation generators include

2128-423: The form: have a dual interpretation, both as procedures and as logical implications: A skilled logic programmer uses the procedural interpretation to write programs that are effective and efficient, and uses the declarative interpretation to help ensure that programs are correct. Comment (computer programming) In computer programming , a comment is a human-readable explanation or annotation in

2184-428: The guideline: "Good comments ... clarify intent." Further, for professional coding environments, the level of detail is ordinarily well defined to meet a specific performance requirement defined by business operations. There are many stylistic alternatives available when considering how comments should appear in source code. For larger projects involving a team of developers, comment styles are either agreed upon before

2240-594: The law on what is and what is not allowed; rather, BCPL acts more as a servant offering his services to the best of his ability without complaint, even when confronted with apparent nonsense. The programmer is always assumed to know what he is doing and is not hemmed in by petty restrictions. BCPL was first implemented by Martin Richards of the University of Cambridge in 1967. BCPL was a response to difficulties with its predecessor, Cambridge Programming Language, later renamed Combined Programming Language (CPL), which

2296-399: The line. How best to make use of comments is subject to dispute; different commentators have offered varied and sometimes opposing viewpoints. There are many different ways of writing comments and many commentators offer conflicting advice. Comments can be used as a form of pseudocode to outline intention prior to writing the actual code. In this case it should explain the logic behind

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2352-445: The normal comment characters are co-opted to create a special directive for an editor or interpreter. Two examples of this directing an interpreter are: The script below for a Unix-like system shows both of these uses: Somewhat similar is the use of comments in C to communicate to a compiler that a default "fallthrough" in a case statement has been done deliberately: Inserting such a /* Fall thru */ comment for human readers

2408-401: The page number and title of the book or Request for Comments or other reference. A common developer practice is to comment out a code snippet, meaning to add comment syntax causing that block of code to become a comment, so that it will not be executed in the final program. This may be done to exclude certain pieces of code from the final program, or (more commonly) it can be used to find

2464-446: The pointer from the global vector to the original routine and replace it with a pointer to an alternative version. The alternative might call the original as part of its processing. This could be used as a quick ad hoc debugging aid. BCPL was the first brace programming language and the braces survived the syntactical changes and have become a common means of denoting program source code statements. In practice, on limited keyboards of

2520-550: The programs Javadoc for use with Java , Ddoc for D , Doxygen for C , C++ , Java, IDL , Visual Expert for PL/SQL , Transact-SQL , PowerBuilder , and PHPDoc for PHP . Forms of docstring are supported by Python , Lisp , Elixir , and Clojure . C# , F# and Visual Basic .NET implement a similar feature called "XML Comments" which are read by IntelliSense from the compiled .NET assembly. Occasionally syntax elements that were originally intended to be comments are re-purposed to convey additional information to

2576-542: The source of an error. By systematically commenting out and running parts of the program, the source of an error can be determined, allowing it to be corrected. Many IDEs allow quick adding or removing such comments with single menu options or key combinations. The programmer has only to mark the part of text they want to (un)comment and choose the appropriate option. Programming tools sometimes store documentation and metadata in comments. These may include insert positions for automatic header file inclusion, commands to set

2632-458: The styles is that functional programming languages remove or at least deemphasize the imperative elements of procedural programming. The feature set of functional languages is therefore designed to support writing programs as much as possible in terms of pure functions : Many functional languages, however, are in fact impurely functional and offer imperative/procedural constructs that allow the programmer to write programs in procedural style, or in

2688-402: The stylistic variants used in source code. For example, Variation Two might be disfavored among programmers who do not have source code editors that can automate the alignment and visual appearance of text in comments. Software consultant and technology commentator Allen Holub is one expert who advocates aligning the left edges of comments: The use of /* and */ as block comment delimiters

2744-506: The successful bootstrapping increased confidence in the practicality of the method. By late 1970, implementations existed for the Honeywell 635 and Honeywell 645 , IBM 360 , PDP-10 , TX-2 , CDC 6400 , UNIVAC 1108 , PDP-9 , KDF 9 and Atlas 2. In 1974 a dialect of BCPL was implemented at BBN without using the intermediate O-code. The initial implementation was a cross-compiler hosted on BBN's TENEX PDP-10s , and directly targeted

2800-569: The top of the file: Sometimes programmers will add comments as a way to relieve stress by commenting about development tools, competitors, employers, working conditions, or the quality of the code itself. The occurrence of this phenomenon can be easily seen from online resources that track profanity in source code. There are various normative views and long-standing opinions regarding the proper use of comments in source code. Some of these are informal and based on personal preference, while others are published or promulgated as formal guidelines for

2856-403: Was addressed in several ways. One was by providing standard library routines for packing and unpacking words into byte strings. Later, two language features were added: the bit-field selection operator and the infix byte indirection operator (denoted by % ). BCPL handles bindings spanning separate compilation units in a unique way. There are no user-declarable global variables; instead, there

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2912-413: Was already a common convention, but in 2017 the gcc compiler began looking for these (or other indications of deliberate intent), and, if not found, emitting: "warning: this statement may fall through". Many editors and IDEs will read specially formatted comments. For example, the "modeline" feature of Vim ; which would change its handling of tabs while editing a source with this comment included near

2968-540: Was designed during the early 1960s. Richards created BCPL by "removing those features of the full language which make compilation difficult". The first compiler implementation, for the IBM 7094 under Compatible Time-Sharing System , was written while Richards was visiting Project MAC at the Massachusetts Institute of Technology in the spring of 1967. The language was first described in a paper presented to

3024-435: Was first implemented by Martin Richards of the University of Cambridge in 1967. BCPL was designed so that small and simple compilers could be written for it; reputedly some compilers could be run in 16 kilobytes . Furthermore, the original compiler, itself written in BCPL, was easily portable. BCPL was thus a popular choice for bootstrapping a system. A major reason for the compiler's portability lay in its structure. It

3080-474: Was released for the Apple Macintosh in 1985 by Topexpress Ltd, of Kensington, England. Both the design and philosophy of BCPL strongly influenced B , which in turn influenced C . Programmers at the time debated whether an eventual successor to C would be called "D", the next letter in the alphabet, or "P", the next letter in the parent language name. The language most accepted as being C's successor

3136-434: Was split into two parts: the front end parsed the source and generated O-code, an intermediate language . The back end took the O-code and translated it into the machine code for the target machine. Only 1 ⁄ 5 of the compiler's code needed to be rewritten to support a new machine, a task that usually took between 2 and 5 person-months . This approach became common practice later (e.g. Pascal , Java ). The language

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