[m-dev.] for review: improve documentation of if-then-else

[Fergus Henderson]

Estimated hours taken: 0.5

doc/reference_manual.texi:
	Improve the documentation for if-then-else.
	This change is in response to a suggestion from
	Don Smith <dsmith at now.cirl.uoregon.edu> that we
	say in the reference manual that Mercury's if-then-else
	(in contrast to Prolog) does not prune away all but the
	first solution of condition.

Workspace: /d-drive/home/hg/fjh/mercury
Index: doc/reference_manual.texi
===================================================================
RCS file: /home/mercury1/repository/mercury/doc/reference_manual.texi,v
retrieving revision 1.153
diff -u -r1.153 reference_manual.texi
--- doc/reference_manual.texi	1999/10/03 04:17:17	1.153
+++ doc/reference_manual.texi	1999/10/07 12:40:13
@@ -500,6 +500,16 @@
 @var{CondGoal}, @var{ThenGoal}, and @var{ElseGoal} must be valid goals.
 Note that the ``else'' part is @emph{not} optional.
 
+The declarative semantics of an if-then-else is given by
+ at code{( @var{CondGoal}, @var{ThenGoal} ; not(@var{CondGoal}), @var{ElseGoal})},
+but the operational semantics are different, and it is treated
+differently for the purposes of determinism inference (@pxref{Determinism}).
+Operationally, it executes the @var{CondGoal}, and if that succeeds, then
+execution continues with the @var{ThenGoal}; otherwise, i.e. if @var{CondGoal}
+fails, it executes the @var{ElseGoal}.  Note that @var{CondGoal} can be
+nondeterministic -- unlike Prolog, Mercury's if-then-else does not commit
+to the first solution of the condition if the condition succeeds.
+
 @item @code{@var{Term1} = @var{Term2}}
 A unification.
 @var{Term1} and @var{Term2} must be valid data-terms.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
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