Special Form Entry: DEFUN

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Special Form Entry: DEFUN

Name

defun

Class

Special Form

Syntax

(defun sym ([arguments] [/ locals ...]) expr ...)

Arguments and Values

  • sym: function name
  • arguments: parameter names
  • / locals: local variable declarations within the AutoLISP parameter syntax
  • expr: body forms

Description

Defines a named function. Names beginning with C: define CAD commands callable at the command line.

Evaluation Rules

The parameter/local syntax is AutoLISP-specific and must not be replaced with Common Lisp lambda-list semantics.

Return Values

Under-specified in the current vendor corpus sampled here; implementations commonly return the function name, but this should be version-tested before being treated as normative.

Side Effects

Writes the resulting function value into the symbol's single binding cell (see chapter 7, "Single-Cell Symbol Binding"). Any previous binding on sym — variable value, prior function, or built-in — is overwritten and is no longer reachable through the symbol.

Compatibility

  • local variable syntax is vendor-documented
  • duplicate-name behavior is uniform across vendors at the per-symbol level: the most recent setq or defun on a symbol wins. This was confirmed for BricsCAD V26 by direct probe (results/bricscad/macos/20260426T155521Z-namespace/) and is consistent with the BricsCAD release-note defect SR44723 fix.

Notes

The slash separating arguments and locals must be surrounded by spaces in Autodesk documentation. AutoLISP is Lisp-1: defun and setq share the same per-symbol binding cell.

Extension (clautolisp): variadic lambda lists with &rest

This subsection documents a clautolisp extension to the defun (and lambda) lambda list; it is not part of the Autodesk-documented syntax. It exists so that user code can cleanly override the variadic built-ins (princ, load, …): a fixed-arity redefinition cannot accept the same range of calls the original does, whereas a &rest parameter can.

Extended lambda-list grammar: 

lambda-list      ::= { parameter-name } [ '&rest' rest-parameter-name ] [ '/' { local-name } ] .
parameter-name   ::= symbol .
rest-parameter-name ::= parameter-name .
local-name       ::= symbol .

&rest and / stand for the symbols &REST and / themselves. The optional &rest clause, when present, must appear after the mandatory parameter names and before the / locals.

Semantics (mirrors Common Lisp &rest, which BricsCAD V26 also implements as an undocumented extension):

  • the call must supply at least as many arguments as there are mandatory parameter-name entries; they are bound in order;
  • any excess arguments are gathered into a freshly-consed list bound to rest-parameter-name (nil when there is no excess);
  • with a &rest parameter present there is no upper bound on the argument count — extras accumulate in the list rather than being passed positionally;
  • local-name entries after / behave exactly as in the base syntax.

Example — redefining the variadic princ over a module output port: 

(defun funcall (fun &rest arguments)
  (apply fun arguments))

(setq original-princ (function princ))
(defun princ (&rest arguments / value fileHandle)
  (if (null arguments)
      (funcall original-princ)
      (progn
        (setq value (car arguments))
        (setq fileHandle (mymodule-output))
        (if (cdr arguments)
            (setq fileHandle (cadr arguments)))
        (mymodule-write fileHandle value))))

Diagnostics surface at call time, matching AutoLISP's late-binding error model:

  • :wrong-number-of-arguments — fewer arguments than the mandatory formals require;
  • :invalid-rest-parameter — &rest followed by other than exactly one symbol, or &rest appearing after / (where it would name a local rather than introduce a rest parameter).

Dialect handling: the default clautolisp dialect accepts &rest silently. dialect bricscad accepts it (and is expected to warn that it is an undocumented BricsCAD extension); dialect strict / autocad reject it at defun time with :strict-lambda-list-extension. The dialect-warning wiring is tracked in issues/open/bricscad-undocumented-clisms.issue.

Availability

  • AutoCAD 2026: documented (per Autodesk reference page).
  • BricsCAD V26: presumed compatible (no contradicting page found).
  • Status: AutoCAD documented; BricsCAD presumed-both pending Phase 4 verification.
  • &rest extension: clautolisp (implemented 1.1.16); BricsCAD V26 (undocumented, confirmed by Bricsys support); AutoCAD: not available.

Source Notes