Curly bracket programming language

Curly brace or bracket programming languages are those which use balanced brackets ({ and }, also known as "brace brackets" or simply "braces") to make blocks in their syntax or formal grammar, mainly due to being C-influenced. The main alternate style is the use of paired keywords, although some languages (notably Python and Occam) instead use an off-side style.

History

Curly-bracket syntax pre-dates C. BCPL was the first language to use curly brackets to outline multi-statement function bodies. Ken Thompson used the feature in B, his cut-down version of BCPL. Because C was initially designed after B, it has retained the bracket syntax of B, as have many subsequent languages (C++, Java, JavaScript and its generalized standard ECMAScript, C#, D, etc.). Pico is a non-C descendant that also uses this style.

One common part of curly bracket style is the common style of terminating a statement with a semicolon (;), which is one way for languages to ignore whitespace. BCPL and Pico do not have this rule; a newline is used as a statement terminator in such languages. The Pico indent style is then used, as below (BCPL):

LET FUNC foo(a) = VALOF
{ b := a + 1
  RESULTIS b }

Statement blocks

The name derives from the common syntax of the languages, where blocks of statements are enclosed in curly brackets. For example (using BSD/Allman indent style, one of many stylistic ways to format a program):

for (int i = 0; i < 10; i++)
{
    printf("%d", i);
    doTask(i);
}

Languages in this family are sometimes referred to as C-style, because they tend to have syntax that is strongly influenced by C syntax. Beside the curly brackets, they often inherit other syntactic features, such as using the semicolon as a statement terminator (not as a separator), and the three-part "for" statement syntax as shown above.

Generally, these languages are also considered "free-form languages", meaning that the compiler considers all whitespace to be the same as one blank space, much like HTML. Considering that, the above code could be written:

for(int i=0;i<10;i++){printf("%d",i);doTask(i);}

but this is not recommended, as it becomes difficult to read after the program grows beyond a few statements.

A popular way to work with curly braces is with the K&R style:

int i;
for(i = 0; i < 10; i++) {
   printf("%d", i);
   doTask(i);
}

There are many other ways to identify statement blocks, such as ending keywords that may match beginning keywords (in Ada, Pascal, REXX, and Visual Basic), the Off-side rule of indentation (in Python), or other symbols such as parentheses (in Lisp). These ways are not necessarily exclusive: whereas indentation is the default in Haskell, curly brackets can be used when desired.

Loops

In C, C++, C#, D, Java, and Perl:

while (boolean expression)
{
    statement(s)
}

do
{
    statement(s)
} while (boolean expression);

for (initialization; continuation condition; incrementing expr)
{
    statement(s)
}

Conditional statements

In C, C++, C#, D, and Java:

if (boolean expression)
{
    statement(s)
}

if (boolean expression)
{
    statement(s)
} 
else
{
    statement(s)
}

if (boolean expression)
{
    statement(s)
} 
else if (boolean expression)
{
    statement (s)
}
...
else
{
    statement(s)
}

switch (integer expression)
{
    case constant integer expr:
        statement(s)
        break;
    ...
    default:
        statement(s)
        break;
}

Exception handling

In C# and Java:

try
{
    statement(s)
}
catch (exception type)
{
    statement(s)
}
catch (exception type)
{
    statement(s)
}
finally
{
    statement(s)
}

Objective-C has the same syntax starting with gcc 3.3 and Apple Mac OS X 10.3 , but with an at sign in front of the keywords (@try, @catch, @finally).

C++ does not have finally, but otherwise looks similar. C has nothing like this, though some vendors have added the keywords __try and __finally to their compilers.

Typographical concerns

Some 7 bit national ISO/IEC 646 sets redefine curly brackets to characters that make programs hardly readable on such designed terminals. To address this problem, BCPL had digraphs (* and *) for { and } (same as Pascal), and ANSI C introduced trigraphs that can be used instead of such problematic characters. All trigraphs consist of two question marks (“??”) followed by a character that is not redefined in the national 7 bit ASCII character sets. The trigraphs for ‘{’ and ‘}’, respectively, are “??<” and “??>”.

Languages

See Category:Curly bracket programming languages.