doc: fix the dangling else with precedence directives

[bison.git] / doc / bison.texi

diff --git a/doc/bison.texi b/doc/bison.texi
index 7e1d9617fb0c0d4682014da22680fcdeef12ec1e..bc3237ae8421c961fb10af2741f781444d47c5df 100644 (file)
--- a/doc/bison.texi
+++ b/doc/bison.texi
@@ -276,6 +276,7 @@ Operator Precedence
 * Using Precedence::  How to specify precedence in Bison grammars.
 * Precedence Examples::  How these features are used in the previous example.
 * How Precedence::    How they work.
+* Non Operators::     Using precedence for general conflicts.
 
 Tuning LR
 
@@ -6665,11 +6666,16 @@ This particular ambiguity was first encountered in the specifications of
 Algol 60 and is called the ``dangling @code{else}'' ambiguity.
 
 To avoid warnings from Bison about predictable, legitimate shift/reduce
-conflicts, use the @code{%expect @var{n}} declaration.
+conflicts, you can use the @code{%expect @var{n}} declaration.
 There will be no warning as long as the number of shift/reduce conflicts
 is exactly @var{n}, and Bison will report an error if there is a
 different number.
-@xref{Expect Decl, ,Suppressing Conflict Warnings}.
+@xref{Expect Decl, ,Suppressing Conflict Warnings}.  However, we don't
+recommend the use of @code{%expect} (except @samp{%expect 0}!), as an equal
+number of conflicts does not mean that they are the @emph{same}.  When
+possible, you should rather use precedence directives to @emph{fix} the
+conflicts explicitly (@pxref{Non Operators,, Using Precedence For Non
+Operators}).
 
 The definition of @code{if_stmt} above is solely to blame for the
 conflict, but the conflict does not actually appear without additional
@@ -6714,6 +6720,7 @@ shift and when to reduce.
 * Using Precedence::  How to specify precedence in Bison grammars.
 * Precedence Examples::  How these features are used in the previous example.
 * How Precedence::    How they work.
+* Non Operators::     Using precedence for general conflicts.
 @end menu
 
 @node Why Precedence
@@ -6828,6 +6835,44 @@ resolved.
 Not all rules and not all tokens have precedence.  If either the rule or
 the lookahead token has no precedence, then the default is to shift.
 
+@node Non Operators
+@subsection Using Precedence For Non Operators
+
+Using properly precedence and associativity directives can help fixing
+shift/reduce conflicts that do not involve arithmetics-like operators.  For
+instance, the ``dangling @code{else}'' problem (@pxref{Shift/Reduce, ,
+Shift/Reduce Conflicts}) can be solved elegantly in two different ways.
+
+In the present case, the conflict is between the token @code{"else"} willing
+to be shifted, and the rule @samp{if_stmt: "if" expr "then" stmt}, asking
+for reduction.  By default, the precedence of a rule is that of its last
+token, here @code{"then"}, so the conflict will be solved appropriately
+by giving @code{"else"} a precedence higher than that of @code{"then"}, for
+instance as follows:
+
+@example
+@group
+%nonassoc "then"
+%nonassoc "else"
+@end group
+@end example
+
+Alternatively, you may give both tokens the same precedence, in which case
+associativity is used to solve the conflict.  To preserve the shift action,
+use right associativity:
+
+@example
+%right "then" "else"
+@end example
+
+Neither solution is perfect however.  Since Bison does not provide, so far,
+support for ``scoped'' precedence, both force you to declare the precedence
+of these keywords with respect to the other operators your grammar.
+Therefore, instead of being warned about new conflicts you would be unaware
+of (e.g., a shift/reduce conflict due to @samp{if test then 1 else 2 + 3}
+being ambiguous: @samp{if test then 1 else (2 + 3)} or @samp{(if test then 1
+else 2) + 3}?), the conflict will be already ``fixed''.
+
 @node Contextual Precedence
 @section Context-Dependent Precedence
 @cindex context-dependent precedence