with-at-most-one-form

Function

Package: excl

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Allegro CL version 10.1
New since the initial 10.1 release.

with-at-most-one-form

Arguments: (&key if-null-body) &body body

The purpose of this macro is to ensure that one and only one of a set of conditionalized forms results from the evaluation of body. So in an example below, we conditionalize for Linux and Windows. If later we start running on a Mac, the with-at-most-one-form will error because we forgot to add a mac-conditionalized form.

When a with-at-most-one-form is macroexpanded, the result is either:

The single form that body is read and returned by the Lisp reader.
The value of the if-null-body argument if that argument is specified and no forms are returned for the body by the Lisp reader.
An error being signaled (meaning with body consisted of more than one form or body was empty and if-null-body is not given a value).

Examples

;; This would signal an error at macroexpand time if run on a
;; non-Linux or non-Windows version of Allegro CL:
(with-at-most-one-form ()
  #+linux (do-linux-stuff)
  #+mswindows (do-windows-stuff))

;; The result of this form would be 10 on Windows and 11 elsewhere:
(with-at-most-one-form (:if-null-body 11)
  #+mswindows 10)

;; This call will always error (at macroexpand time) as body has
;; two forms:
(with-at-most-one-form ()
  (setq author "Bill")
  (setq editor "Helen"))
-> ERROR