add when, unless

* module/ice-9/boot-9.scm (when, unless): New forms.

* doc/ref/api-control.texi (Conditionals): Add docs.  Rename this
  node from "if cond case".

* doc/ref/r6rs.texi:
* doc/ref/scheme-ideas.texi:
* doc/ref/srfi-modules.texi: Update referrers.

doc/ref/api-control.texi

@@ -12,7 +12,7 @@ flow of Scheme affects C code.
 
 @menu
 * begin::                       Sequencing and splicing.
-* if cond case::                Simple conditional evaluation.
+* Conditionals::                If, when, unless, case, and cond.
 * and or::                      Conditional evaluation of a sequence.
 * while do::                    Iteration mechanisms.
 * Prompts::                     Composable, delimited continuations.
@@ -103,11 +103,13 @@ good idea.  But it is useful to be able to write macros that expand out
 to multiple definitions, as in @code{define-sealant} above, so Scheme
 abuses the @code{begin} form for these two tasks.
 
-@node if cond case
+@node Conditionals
 @subsection Simple Conditional Evaluation
 
 @cindex conditional evaluation
 @cindex if
+@cindex when
+@cindex unless
 @cindex case
 @cindex cond
 
@@ -121,14 +123,44 @@ values.
 All arguments may be arbitrary expressions.  First, @var{test} is
 evaluated.  If it returns a true value, the expression @var{consequent}
 is evaluated and @var{alternate} is ignored.  If @var{test} evaluates to
-@code{#f}, @var{alternate} is evaluated instead.  The value of the
-evaluated branch (@var{consequent} or @var{alternate}) is returned as
-the value of the @code{if} expression.
+@code{#f}, @var{alternate} is evaluated instead.  The values of the
+evaluated branch (@var{consequent} or @var{alternate}) are returned as
+the values of the @code{if} expression.
 
 When @var{alternate} is omitted and the @var{test} evaluates to
 @code{#f}, the value of the expression is not specified.
 @end deffn
 
+When you go to write an @code{if} without an alternate (a @dfn{one-armed
+@code{if}}), part of what you are expressing is that you don't care
+about the return value (or values) of the expression.  As such, you are
+more interested in the @emph{effect} of evaluating the consequent
+expression.  (By convention, we use the word @dfn{statement} to refer to
+an expression that is evaluated for effect, not for value).
+
+In such a case, it is considered more clear to express these intentions
+with these special forms, @code{when} and @code{unless}.  As an added
+bonus, these forms accept multiple statements to evaluate, which are
+implicitly wrapped in a @code{begin}.
+
+@deffn {Scheme Syntax} when test statement1 statement2 ...
+@deffnx {Scheme Syntax} unless test statement1 statement2 ...
+The actual definitions of these forms are in many ways their most clear
+documentation:
+
+@example
+(define-syntax-rule (when test stmt stmt* ...)
+  (if test (begin stmt stmt* ...)))
+
+(define-syntax-rule (unless condition stmt stmt* ...)
+  (if (not test) (begin stmt stmt* ...)))
+@end example
+
+That is to say, @code{when} evaluates its consequent statements in order
+if @var{test} is true.  @code{unless} is the opposite: it evaluates the
+statements if @var{test} is false.
+@end deffn
+
 @deffn syntax cond clause1 clause2 @dots{}
 Each @code{cond}-clause must look like this:
 

doc/ref/api-data.texi

@@ -1,6 +1,6 @@
 @c -*-texinfo-*-
 @c This is part of the GNU Guile Reference Manual.
-@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2006, 2007, 2008, 2009, 2010, 2011
+@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2006, 2007, 2008, 2009, 2010, 2011, 2012
 @c   Free Software Foundation, Inc.
 @c See the file guile.texi for copying conditions.
 
@@ -78,10 +78,10 @@ equality predicates @code{eq?}, @code{eqv?} and @code{equal?}
 #t
 @end lisp
 
-In test condition contexts like @code{if} and @code{cond} (@pxref{if
-cond case}), where a group of subexpressions will be evaluated only if a
-@var{condition} expression evaluates to ``true'', ``true'' means any
-value at all except @code{#f}.
+In test condition contexts like @code{if} and @code{cond}
+(@pxref{Conditionals}), where a group of subexpressions will be
+evaluated only if a @var{condition} expression evaluates to ``true'',
+``true'' means any value at all except @code{#f}.
 
 @lisp
 (if #t "yes" "no")

doc/ref/r6rs.texi

@@ -1,6 +1,6 @@
 @c -*-texinfo-*-
 @c This is part of the GNU Guile Reference Manual.
-@c Copyright (C)  2010, 2011
+@c Copyright (C)  2010, 2011, 2012
 @c   Free Software Foundation, Inc.
 @c See the file guile.texi for copying conditions.
 
@@ -316,7 +316,7 @@ grouped below by the existing manual sections to which they correspond.
 @deffn {Scheme Syntax} if test consequence [alternate]
 @deffnx {Scheme Syntax} cond clause1 clause2 ...
 @deffnx {Scheme Syntax} case key clause1 clause2 ...
-@xref{if cond case}, for documentation.
+@xref{Conditionals}, for documentation.
 @end deffn
 
 @deffn {Scheme Syntax} and expr ...
@@ -1146,7 +1146,7 @@ exception handler that binds a raised exception to @var{variable} and
 then evaluates the specified @var{clause}s as if they were part of a 
 @code{cond} expression, with the value of the first matching clause 
 becoming the value of the @code{guard} expression 
-(@pxref{if cond case}).  If none of the clause's test expressions 
+(@pxref{Conditionals}).  If none of the clause's test expressions 
 evaluates to @code{#t}, the exception is re-raised, with the exception
 handler that was current before the evaluation of the @code{guard} form.
 

doc/ref/scheme-ideas.texi

@@ -1,6 +1,6 @@
 @c -*-texinfo-*-
 @c This is part of the GNU Guile Reference Manual.
-@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005
+@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005, 2012
 @c   Free Software Foundation, Inc.
 @c See the file guile.texi for copying conditions.
 
@@ -964,11 +964,11 @@ same as a procedure which returns its last argument, because the
 evaluation of a procedure invocation expression does not guarantee to
 evaluate the arguments in order.
 
-@code{if} and @code{cond} (@pxref{if cond case}) provide conditional
+@code{if} and @code{cond} (@pxref{Conditionals}) provide conditional
 evaluation of argument expressions depending on whether one or more
 conditions evaluate to ``true'' or ``false''.
 
-@code{case} (@pxref{if cond case}) provides conditional evaluation of
+@code{case} (@pxref{Conditionals}) provides conditional evaluation of
 argument expressions depending on whether a variable has one of a
 specified group of values.
 

doc/ref/srfi-modules.texi

@@ -4193,7 +4193,7 @@ This SRFI extends RnRS @code{cond} to support test expressions that
 return multiple values, as well as arbitrary definitions of test
 success.  SRFI 61 is implemented in the Guile core; there's no module
 needed to get SRFI-61 itself.  Extended @code{cond} is documented in
-@ref{if cond case,, Simple Conditional Evaluation}.
+@ref{Conditionals,, Simple Conditional Evaluation}.
 
 @node SRFI-67
 @subsection SRFI-67 - Compare procedures

module/ice-9/boot-9.scm

@@ -412,6 +412,12 @@ If there is no handler at all, Guile prints an error and then exits."
     ((_ x) x)
     ((_ x y ...) (let ((t x)) (if t t (or y ...))))))
 
+(define-syntax-rule (when test stmt stmt* ...)
+  (if test (begin stmt stmt* ...)))
+
+(define-syntax-rule (unless test stmt stmt* ...)
+  (if (not test) (begin stmt stmt* ...)))
+
 ;; The "maybe-more" bits are something of a hack, so that we can support
 ;; SRFI-61. Rewrites into a standalone syntax-case macro would be
 ;; appreciated.