fkifejezések

fkifejezések, or F-expressions, denote a historical notation used to express Lisp programs in the early days of the language. In F-expression form, function application is written as a sequence of the function name followed by its arguments, without the explicit parentheses that later became standard in Lisp. For example, the arithmetic expression plus 2 3 would be an F-expression, corresponding to the S-expression (+ 2 3).

F-expressions were designed to be closer to conventional mathematical notation and thus easier for humans to read. They emphasize the functional structure of expressions, with a clear separation between function and arguments. However, the lack of explicit punctuation makes nested expressions harder to parse unambiguously by machines, and the notation introduced challenges for compiler design.

Because of these practical concerns, Lisp adopted S-expressions, a parenthesized prefix notation that unambiguously encodes tree structure and simplifies parsing. In S-expressions, the same example is written as (+ 2 3).

Today, F-expressions are primarily of historical interest. They illustrate an early design choice in Lisp's development and are discussed in historical overviews of Lisp notation and in discussions of the evolution of programming language syntax.

See also: Lisp, S-expression, John McCarthy, Lisp 1.5 Manual.