Special Form Entry: FUNCTION

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Special Form Entry: FUNCTION

Name

function

Class

Special Form

Syntax

(function arg)

Arguments and Values

  • arg: a function name (symbol) or a lambda expression; not evaluated.

Description

Returns arg without evaluating it, as quote does. The form additionally signals intent to the Visual LISP byte-compiler, which uses the hint to link and optimise arg as if it were a built-in operator or defun-named function (Autodesk: "The function function is identical to the quote function, except it tells the Visual LISP compiler to link and optimize the argument as if it were a built-in function or defun.").

Evaluation Rules

No subform evaluation occurs. The returned value is the unevaluated arg; resolution to a callable object — symbol lookup, lambda capture — happens at the point the value is used (typically inside apply, mapcar, vl-some, vl-every, or any other higher-order operator), not at the point the function form was reached.

Return Values

  • For a symbol arg: the symbol itself, dynamically resolved by the consumer.
  • For a (lambda …) form: the lambda form, reified to a function object when first applied.

Side Effects

None.

Exceptional Situations

  • arg neither a symbol nor a lambda form: :invalid-function-designator.
  • An undefined function name only surfaces when the value is actually applied — (function missing) itself does not signal.

Examples

  • (setq v (function +)) (apply v '(1 2)) => 3
  • (setq lam (function (lambda (x) (* x x)))) (apply lam '(5)) => 25
  • (mapcar (function 1+) '(1 2 3)) => (2 3 4)

Portable Higher-Order-Function Idiom

AutoLISP is nominally a Lisp-1 with dynamic scope (chapter 7), so a parameter bound to a function value should be callable as (v arg). The two reference implementations diverge on this point:

  • AutoCAD 2022/2026 warns ("parameter used as a function") and may refuse to dispatch through a parameter binding.
  • BricsCAD V25/V26 silently ignores the parameter shadow at call position and resolves the name to the surrounding (global) binding — including when the caller passed a lambda anonymously, which then becomes unreachable.

Code that must run on both vendors therefore treats functions as in a Lisp-2:

Goal Portable form
Define a named function (defun foo (x) …)
Obtain the function value of a name (function foo)
Anonymous function value (function (lambda (x) …))
Call a function held in a variable (parameter…) (apply v (list arg1 …))
Pass a function as parameter pass (function f), apply via apply

Avoid: 

(defun hof (object op)
  (op object))            ; non-portable direct call through a parameter

Prefer: 

(defun hof (object op)
  (apply op (list object)))

Compiled-Code Caveats

Two subtle effects fall out of the compiler hint:

  1. Visual LISP may link the argument of (function foo) at compile time. After such compilation, a later (defun foo …) from elsewhere does not necessarily replace the call site's binding. Example: 

    ;; foobar.lsp
(defun foo () 'foo)
(defun bar () (foo))

;; --- separately ---
(load (vlisp-compile 'st "foobar.lsp"))
(defun foo () 'new-foo)
(bar) ; => foo or new-foo, depending on compile-time linking

    Programs that need to override foo at runtime should avoid early-bind hints in the compiled producer (use (quote foo) rather than (function foo) for symbols that are intended to be redefinable from a host), or use apply with a symbol passed at runtime.

  2. For lambda forms, Autodesk recommends function over quote so the byte-compiler can optimise the body once, rather than rebuilding it on every call. Source: Lee Mac, The Apostrophe and the Quote Function.

Compatibility

Autodesk classifies function as a special form. Both vendors document the "identical to quote with compiler hint" semantics. clautolisp implements the same late-resolution semantic: the form returns its unevaluated argument.

Notes

  • The portable HOF rule lives at the language level: it does not depend on whether the producer is interpreted, byte-compiled (FAS/VLX), or hosted by clautolisp.
  • function is widely used with mapcar, vl-some, vl-every, vl-remove-if, vl-member-if, and other Visual LISP higher-order utilities. Whether the argument is a symbol or a lambda, the consumer resolves it via the same dispatch path used by apply.

Availability

  • AutoCAD 2026: documented (per Autodesk reference page).
  • BricsCAD V26: presumed compatible (no contradicting page found).
  • clautolisp: implemented (eval-function-form returns arg unevaluated; apply/mapcar/vl-* honour late resolution against the dynamic scope chain).
  • Status: AutoCAD documented; BricsCAD presumed-both pending Phase 4 verification.