diff options
| -rw-r--r-- | lispref/control.texi | 37 |
1 files changed, 35 insertions, 2 deletions
diff --git a/lispref/control.texi b/lispref/control.texi index 4973599d877..4d01661a9a1 100644 --- a/lispref/control.texi +++ b/lispref/control.texi @@ -143,8 +143,9 @@ following @var{forms}, in textual order, returning the result of @cindex conditional evaluation Conditional control structures choose among alternatives. Emacs Lisp -has two conditional forms: @code{if}, which is much the same as in other -languages, and @code{cond}, which is a generalized case statement. +has four conditional forms: @code{if}, which is much the same as in +other languages; @code{when} and @code{unless}, which are variants of +@code{if}; and @code{cond}, which is a generalized case statement. @defspec if condition then-form else-forms@dots{} @code{if} chooses between the @var{then-form} and the @var{else-forms} @@ -171,6 +172,38 @@ never evaluated---it is ignored. Thus, in the example below, @end example @end defspec +@defspec when condition then-forms@dots{} +This is a variant of @code{if} where there are no @var{else-forms}, +and possibly several @var{then-forms}. In particular, + +@example +(when @var{condition} @var{a} @var{b} @var{c}) +@end example + +@noindent +is entirely equivalent to + +@example +(if @var{condition} (progn @var{a} @var{b} @var{c}) nil) +@end example +@end defspec + +@defspec unless condition forms@dots{} +This is a variant of @code{if} where there is no @var{then-form}: + +@example +(unless @var{condition} @var{a} @var{b} @var{c}) +@end example + +@noindent +is entirely equivalent to + +@example +(if @var{condition} nil + @var{a} @var{b} @var{c}) +@end example +@end defspec + @defspec cond clause@dots{} @code{cond} chooses among an arbitrary number of alternatives. Each @var{clause} in the @code{cond} must be a list. The @sc{car} of this |