Block Comments

This comparison of programming languages compares the features of language syntax (format) for over 50 computer programming languages.

Expressions

Programming language expressions can be broadly classified into four syntax structures:

;prefix notation
* Lisp (* (+ 2 3) (expt 4 5))
;infix notation
* Fortran (2 + 3) * (4 ** 5)
;suffix, postfix, or Reverse Polish notation
* Forth 2 3 + 4 5 ** *
;math-like notation
* TUTOR (2 + 3)(45) $$ note implicit multiply operator

Statements

When a programming languages has statements, they typically have conventions for:

* statement separators;
* statement terminators; and
* line continuation

A statement separator demarcates the boundary between two separate statements. A statement terminator defines the end of an individual statement. Languages that interpret the end of line to be the end of a statement are called "line-oriented" languages.

"Line continuation" is a convention in line-oriented languages where the newline character could potentially be misinterpreted as a statement terminator. In such languages, it allows a single statement to span more than just one line.

Line continuation

Line continuation is generally done as part of lexical analysis: a newline normally results in a token being added to the token stream, unless line continuation is detected.

;Whitespace – Languages that do not need continuations
* Ada – Lines terminate with semicolon
* C# – Lines terminate with semicolon
* JavaScript – Lines terminate with semicolon (which may be inferred)
* Lua
* OCaml

;Ampersand as last character of line
* Fortran 90, Fortran 95, Fortran 2003, Fortran 2008

;Backslash as last character of line
* bash and other Unix shells
* C and C++ preprocessor
* Mathematica and Wolfram Language
* Python
* Ruby
* JavaScript – only within single- or double-quoted strings

;Backtick as last character of line
* PowerShell

;Hyphen as last character of line
* SQL*Plus

;Underscore as last character of line
* AutoIt
* Cobra
* Visual Basic
* Xojo

;Ellipsis (as three periods–not one special character)
* MATLAB: The ellipsis token need not be the last characters on the line, but any following it will be ignored. (In essence, it begins a comment that extends ''through'' (i.e. including) the first subsequent newline character. Contrast this with an inline comment, which extends ''until'' the first subsequent newline.)

; Comma delimiter as last character of line
* Ruby (comment may follow delimiter)

; Left bracket delimiter as last character of line
* Batch file: starting a parenthetical block can allow line continuation
* Ruby: left parenthesis, left square bracket, or left curly bracket

; Operator as last object of line
* Ruby (comment may follow operator)

; Operator as first character of continued line
* AutoHotkey: Any expression operators except ++ and --, as well as a comma or a period

;Backslash as first character of continued line
* Vimscript

;Some form of inline comment serves as line continuation
* Turbo Assembler: \
* m4: dnl
* TeX: %

;Character position
* Fortran 77: A non-comment line is a continuation of the previous non-comment line if any non-space character appears in column 6. Comment lines cannot be continued.
* COBOL: String constants may be continued by not ending the original string in a PICTURE clause with ', then inserting a - in column 7 (same position as the * for comment is used.)
* TUTOR: Lines starting with a tab (after any indentation required by the context) continue the previous command.

; nd and Beginusing normal quotes
* C and C++ preprocessor: The string is ended normally and continues by starting with a quote on the next line.

Libraries

To import a library is a way to read external, possibly compiled, routines, programs or packages. Imports can be classified by level (module, package, class, procedure,...) and by syntax (directive name, attributes,...)

;File import
* addpath(''directory'')MATLAB
* COPY filename.COBOL
* :-include("''filename''"). Prolog
* #include file="''filename''" ASP
* #include "''filename''", AutoHotkey, AutoIt, C, C++
* #include <''filename''>AutoHotkey, AutoIt, C, C++
* #import "''filename''", Objective-C
* #import <''filename''> Objective-C
* Import filename"/code>Mathematica and Wolfram Language
* include 'filename' Fortran
* include "''filename''";PHP
* include ilenameprogram, Pick Basic
* #include ilenameprogram Pick Basic
* include!("''filename''");Rust
* load "''filename''"Ruby
* load %filenameRed
* require("''filename''")Lua
* require "''filename''"; Perl, PHP
* require "filename"Ruby
* source(""filename"") R

;Package import
* #include ''filename'' C, C++
* # ath = "''filename''"mod ''altname'';, Rust
* @import ''module''; Objective-C
* <<''name'' Mathematica and Wolfram Language
* :-use_module(module). Prolog:
* from ''module'' import * Python
* extern crate ''libname'';, Rust
* extern crate ''libname'' as ''altname''; Rust
* mod ''modname'';, Rust
* library("''package''") R:
* IMPORT ''module'' Oberon
* import ''altname'' "''package/name''" Go:
* import ''package.module'';, D
* import ''altname'' = ''package.module''; D
* import ''Module'', Haskell
* import qualified ''Module'' as ''M'' Haskell
* import ''package''.* Java, MATLAB, kotlin
* import "''modname''"; JavaScript:
* import ''altname'' from "''modname''";, JavaScript:
* import ''package'' Scala
* import ''package''._, Scala
* import ''module'' Swift
* import ''module'', Python
* require("''modname''") Lua:
* require "gem", Ruby
* use ''module'', Fortran 90+
* use ''module'', only : ''identifier'' Fortran 90+
* use ''Module'';, Perl
* use Module qw(''import options'');Perl
* use ''Package''.''Name'' Cobra
* uses ''unit'' Pascal
* with ''package'' Ada

;Class import
* from ''module'' import ''Class'' Python
* import ''package''.''class'' Java, MATLAB, kotlin
* import ''class'' from "''modname''";, JavaScript
* import from "''modname''";, JavaScript
* import from "''modname''";JavaScript
* import ''package''.''class'', Scala
* import ''package''., Scala
* import ''package''._ Scala
* use ''Namespace\ClassName'';, PHP
* use ''Namespace\ClassName as AliasName''; PHP

;Procedure/function import
* from ''module'' import ''function'' Python:
* import ''package.module'' : ''symbol'';, D:
* import ''package.module'' : ''altsymbolname'' = ''symbol''; D:
* import ''Module'' (''function'') Haskell:
* import ''function'' from "modname";, JavaScript:
* import from "''modname''";, JavaScript:
* import from "''modname''";JavaScript:
* import ''package''.''function'' MATLAB:
* import ''package''.''class''.''function'', Scala:
* import ''package''.''class''. Scala:
* use Module ('symbol');Perl:
* use function ''Namespace\function_name'';, PHP:
* use ''Namespace\function_name as function_alias_name''; PHP:
* use ''module''::''submodule''::''symbol'';, Rust:
* use ''module''::''submodule''::;, Rust:
* use ''module''::''submodule''::''symbol'' as ''altname''; Rust:

;Constant import
* use const ''Namespace\CONST_NAME''; PHP

The above statements can also be classified by whether they are a syntactic convenience (allowing things to be referred to by a shorter name, but they can still be referred to by some fully qualified name without import), or whether they are actually required to access the code (without which it is impossible to access the code, even with fully qualified names).

;Syntactic convenience
* import ''package''.* Java
* import ''package''.''class'' Java
* open ''module'' OCaml

;Required to access code
* import altname "''package/name''" Go
* import ''altname'' from "''modname''";JavaScript
* import ''module''Python


Blocks

A block is a notation for a group of two or more statements, expressions or other units of code that are related in such a way as to comprise a whole.

;Braces (a.k.a. curly brackets) :
* Curly bracket programming languages: C, C++, Objective-C, Go, Java, JavaScript/ECMAScript, C#, D, Perl, PHP (for & loop loops, or pass a block as argument), R, Rust, Scala, S-Lang, Swift, PowerShell, Haskell (in do-notation), AutoHotkey

;Parentheses ( ... )
* Batchfile, F# (lightweight syntax), OCaml, Prolog, Standard ML
;Square brackets /code> ... /code>
* Rebol, Red, Self, Smalltalk (blocks are first class objects. a.k.a. closures)
;begin ... end:
* Ada, ALGOL, F# (verbose syntax), Pascal, Ruby (for, do/while & do/until loops), OCaml, SCL, Simula, Erlang.
;do ... end:
* PL/I, REXX
;do ... done:
* Bash (for & while loops), F# (verbose syntax) Visual Basic, Fortran, TUTOR (with mandatory indenting of block body), Visual Prolog
;do ... end
* Lua, Ruby (pass blocks as arguments, for loop), Seed7 (encloses loop bodies between do and end)
;X ... end (e.g. if ... end):
* Ruby (if, while, until, def, class, module statements), OCaml (for & while loops), MATLAB (if & switch conditionals, for & while loops, try clause, package, classdef, properties, methods, events, & function blocks), Lua (then / else & function)
;(begin ...):
* Scheme
;(progn ...):
* Lisp
;(do ...):
* Clojure

;Indentation
* Off-side rule languages: Boo, Cobra, CoffeeScript, F#, Haskell (in do-notation when braces are omitted), LiveScript, occam, Python, Nemerle (Optional; the user may use white-space sensitive syntax instead of the curly-brace syntax if they so desire), Nim, Scala (Optional, as in Nemerle)
* Free-form languages: most descendants from ALGOL (including C, Pascal, and Perl); Lisp languages

;Others
* Ada, Visual Basic, Seed7: if ... end if
* APL: :If ... :EndIf or :If ... :End
* Bash, sh, and ksh: if ... fi, do ... done, case ... esac;
* ALGOL 68: begin ... end, ( ... ), if ... fi, do ... od
* Lua, Pascal, Modula-2, Seed7: repeat ... until
* COBOL: IF ... END-IF, PERFORM ... END-PERFORM, etc. for statements; ... . for sentences.
* Visual Basic .Net: If ... End If, For ... Next, Do ... Loop
* Small Basic: If ... EndIf, For ... EndFor, While ... EndWhile


Comments

Comments can be classified by:
* style (inline/block)
* parse rules (ignored/interpolated/stored in memory)
* recursivity (nestable/non-nestable)
* uses (docstrings/throwaway comments/other)

Inline comments

Inline comments are generally those that use a newline character to indicate the end of a comment, and an arbitrary delimiter or sequence of tokens to indicate the beginning of a comment.

Examples:

Block comments

Block comments are generally those that use a delimiter to indicate the beginning of a comment, and another delimiter to indicate the end of a comment. In this context, whitespace and newline characters are not counted as delimiters. In the examples, the symbol ~ represents the comment; and, the symbols surrounding it are understood by the interpreters/compilers as the delimiters.

Examples:

Unique variants

;Fortran
* Indenting lines in Fortran 66/77 is significant. The actual statement is in columns 7 through 72 of a line. Any non-space character in column 6 indicates that this line is a continuation of the previous line. A 'C' in column 1 indicates that this entire line is a comment. Columns 1 though 5 may contain a number which serves as a label. Columns 73 though 80 are ignored and may be used for comments; in the days of punched cards, these columns often contained a sequence number so that the deck of cards could be sorted into the correct order if someone accidentally dropped the cards. Fortran 90 removed the need for the indentation rule and added inline comments, using the ! character as the comment delimiter.

;COBOL
* In fixed format code, line indentation is significant. Columns 1–6 and columns from 73 onwards are ignored. If a * or / is in column 7, then that line is a comment. Until COBOL 2002, if a D or d was in column 7, it would define a "debugging line" which would be ignored unless the compiler was instructed to compile it.

;Cobra
* Cobra supports block comments with "/# ... #/" which is like the "/* ... */" often found in C-based languages, but with two differences. The # character is reused from the single-line comment form "# ...", and the block comments can be nested which is convenient for commenting out large blocks of code.

;Curl
* Curl supports block comments with user-defined tags as in , foo# ... #foo, .

;Lua
* Like raw strings, there can be any number of equals signs between the square brackets, provided both the opening and closing tags have a matching number of equals signs; this allows nesting as long as nested block comments/raw strings use a different number of equals signs than their enclosing comment: -- comment --[=[ nested comment ">[_nested_comment_.html" ;"title="comment --[=[ nested comment ">comment --[=[ nested comment . Lua discards the first newline (if present) that directly follows the opening tag.

;Perl
* Block comments in Perl are considered part of the documentation, and are given the name Plain Old Documentation (POD). Technically, Perl does not have a convention for including block comments in source code, but POD is routinely used as a workaround.

;PHP

* PHP supports standard C/C++ style comments, but supports Perl style as well.

;Python
* The use of the triple-quotes to comment-out lines of source, does not actually form a comment. The enclosed text becomes a string literal, which Python usually ignores (except when it is the first statement in the body of a module, class or function; see docstring).

;Raku
* Raku (programming language), Raku uses #`(...) to denote block comments. Raku actually allows the use of any "right" and "left" paired brackets after #` (i.e. #`(...), #` ../code>, #`, #`<...>, and even the more complicated #`{} are all valid block comments). Brackets are also allowed to be nested inside comments (i.e. #` goes to the last closing brace).

;Ruby
* Block comment in Ruby opens at =begin line and closes at =end line.

;S-Lang
* The region of lines enclosed by the #<tag> and #</tag> delimiters are ignored by the interpreter. The tag name can be any sequence of alphanumeric characters that may be used to indicate how the enclosed block is to be deciphered. For example, #<latex> could indicate the start of a block of LaTeX formatted documentation.

;Scheme and Racket
* The next complete syntactic component (s-expression) can be commented out with #; .

;ABAP
ABAP supports two different kinds of comments. If the first character of a line, including indentation, is an asterisk (*) the whole line is considered as a comment, while a single double quote (") begins an in-line comment which acts until the end of the line. ABAP comments are not possible between the statements EXEC SQL and ENDEXEC because Native SQL has other usages for these characters. In the most SQL dialects the double dash (--) can be used instead.

;Esoteric languages
* Many esoteric programming languages follow the convention that any text not executed by the instruction pointer (e.g., Befunge) or otherwise assigned a meaning (e.g., Brainfuck), is considered a "comment".


Comment comparison

There is a wide variety of syntax styles for declaring comments in source code.
''BlockComment'' in italics is used here to indicate block comment style.
''InlineComment'' in italics is used here to indicate inline comment style.

See also

* C syntax
* C++ syntax
* Curly bracket programming languages, a broad family of programming language syntaxes
* Java syntax
* JavaScript syntax
* PHP syntax and semantics
* Python syntax and semantics

References

Notes

{{Notelist

*Syntax