Editing Common Lisp (section)

====The function namespace====
<!-- This section name is linked from several places; if you change it, update the links. -->

The namespace for function names is separate from the namespace for data variables. This is a key difference between Common Lisp and [[Scheme (programming language)|Scheme]]. For Common Lisp, operators that define names in the function namespace include <code>defun</code>, <code>flet</code>, <code>labels</code>, <code>defmethod</code> and <code>defgeneric</code>.

To pass a function by name as an argument to another function, one must use the <code>function</code> special operator, commonly abbreviated as <code>#'</code>. The first <code>sort</code> example above refers to the function named by the symbol <code>&gt;</code> in the function namespace, with the code <code>#'&gt;</code>. Conversely, to call a function passed in such a way, one would use the <code>funcall</code> operator on the argument.

[[Scheme (programming language)|Scheme's]] evaluation model is simpler: there is only one namespace, and all positions in the form are evaluated (in any order) – not just the arguments. Code written in one dialect is therefore sometimes confusing to programmers more experienced in the other. For instance, many Common Lisp programmers like to use descriptive variable names such as ''list'' or ''string'' which could cause problems in Scheme, as they would locally shadow function names.

Whether a separate namespace for functions is an advantage is a source of contention in the Lisp community. It is usually referred to as the ''Lisp-1 vs. Lisp-2 debate''. Lisp-1 refers to Scheme's model and Lisp-2 refers to Common Lisp's model. These names were coined in a 1988 paper by [[Richard P. Gabriel]] and [[Kent Pitman]], which extensively compares the two approaches.<ref>{{cite journal |title=Technical Issues of Separation in Function Cells and Value Cells |url=http://www.nhplace.com/kent/Papers/Technical-Issues.html |author=Richard P. Gabriel |author2=Kent M. Pitman |journal=LISP and Symbolic Computation |date=June 1988|volume=1 |issue=1 |pages=81–101 |doi=10.1007/bf01806178|s2cid=26716515}}</ref>