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A computer programming language is said to adhere to the off-side rule if in it the scope of declarations (a statement block) is expressed by their indentation, i.e., blocks are formed (indicated) via indentation. The term and the idea are attributed to Peter J. Landin, and the term can be seen as a pun on the offside law of soccer.

Contents 1 Definition 2 Code examples 3 Alternatives 4 Off-side rule languages 5 References

“	Any non-whitespace token to the left of the first such token on the previous line is taken to be the start of a new declaration.	„
— Peter J. Landin ,  "The Next 700 Programming Languages"[1]

The following is an example of indentation blocks in Python (note also the obligatory colons — they are merely syntactical markers to make it easier to read) [1]:

def is_even(a):
    if a / 2.0 == a / 2:
        return True
    else:
        return False

Main article: Curly bracket programming language

The way popularized by C is to ignore whitespace and mark blocks between curly braces { and }. An advantage of this is that program code can be automatically reformatted and neatly indented without fear of the block structure changing. A disadvantage is that a human reader of the code pays attention to indentation and might miss the formal meaning that is communicated with braces.

Lisp doesn't differentiate statements from expressions, and parentheses are enough to control the scoping of all statements within the language. Like curly bracket languages, whitespace is ignored.

Another alternative is for each block to begin and end with explicit keywords. Often, this means that newlines are important (unlike in curly bracket languages), but the indentation is not.

Examples of this rule are the Pascal convention of starting blocks with keyword "begin" and ending them with "end". In BASIC, blocks begin with the block name (such as "IF") and end with the block name prepended with "END" (eg. "END IF"). The Bourne shell (sh, and bash) is similar, but the ending of the block is usually given by the name of the block written backwards (eg. "case" starts a conditional statement and it spans until the matching "esac").

• ISWIM, the abstract language that introduced the rule
• ABC
  • Python
• Miranda
  • Haskell
    • Curry
• Occam
• Pliant

1. ^ Landin, Peter J. (March 1966). "The next 700 programming languages". Communications of the ACM 9 (3): 157–166. DOI:10.1145/365230.365257.

• Off-side rule from FOLDOC
• The Haskell Report - Layout