* doc/autoconf.texi: Document @$.

[bison.git] / doc / bison.info-4

diff --git a/doc/bison.info-4 b/doc/bison.info-4
index 05b064961d244f3830bdccb1803298b5b912da7b..644b95a3a79405410954d5211593d8458cec7ab2 100644 (file)
--- a/doc/bison.info-4
+++ b/doc/bison.info-4
@@ -1,5 +1,5 @@
-Ceci est le fichier Info bison.info, produit par Makeinfo version 4.0 à
-partir bison.texinfo.
+Ceci est le fichier Info bison.info, produit par Makeinfo version 4.0b
+à partir bison.texinfo.
 
 START-INFO-DIR-ENTRY
 * bison: (bison).      GNU Project parser generator (yacc replacement).
@@ -28,6 +28,65 @@ License", "Conditions for Using Bison" and this permission notice may be
 included in translations approved by the Free Software Foundation
 instead of in the original English.
 
+\1f
+File: bison.info,  Node: Precedence,  Next: Contextual Precedence,  Prev: Shift/Reduce,  Up: Algorithm
+
+Operator Precedence
+===================
+
+   Another situation where shift/reduce conflicts appear is in
+arithmetic expressions.  Here shifting is not always the preferred
+resolution; the Bison declarations for operator precedence allow you to
+specify when to shift and when to reduce.
+
+* Menu:
+
+* Why Precedence::    An example showing why precedence is needed.
+* 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.
+
+\1f
+File: bison.info,  Node: Why Precedence,  Next: Using Precedence,  Up: Precedence
+
+When Precedence is Needed
+-------------------------
+
+   Consider the following ambiguous grammar fragment (ambiguous because
+the input `1 - 2 * 3' can be parsed in two different ways):
+
+     expr:     expr '-' expr
+             | expr '*' expr
+             | expr '<' expr
+             | '(' expr ')'
+             ...
+             ;
+
+Suppose the parser has seen the tokens `1', `-' and `2'; should it
+reduce them via the rule for the subtraction operator?  It depends on
+the next token.  Of course, if the next token is `)', we must reduce;
+shifting is invalid because no single rule can reduce the token
+sequence `- 2 )' or anything starting with that.  But if the next token
+is `*' or `<', we have a choice: either shifting or reduction would
+allow the parse to complete, but with different results.
+
+   To decide which one Bison should do, we must consider the results.
+If the next operator token OP is shifted, then it must be reduced first
+in order to permit another opportunity to reduce the difference.  The
+result is (in effect) `1 - (2 OP 3)'.  On the other hand, if the
+subtraction is reduced before shifting OP, the result is
+`(1 - 2) OP 3'.  Clearly, then, the choice of shift or reduce should
+depend on the relative precedence of the operators `-' and OP: `*'
+should be shifted first, but not `<'.
+
+   What about input such as `1 - 2 - 5'; should this be `(1 - 2) - 5'
+or should it be `1 - (2 - 5)'?  For most operators we prefer the
+former, which is called "left association".  The latter alternative,
+"right association", is desirable for assignment operators.  The choice
+of left or right association is a matter of whether the parser chooses
+to shift or reduce when the stack contains `1 - 2' and the look-ahead
+token is `-': shifting makes right-associativity.
+
 \1f
 File: bison.info,  Node: Using Precedence,  Next: Precedence Examples,  Prev: Why Precedence,  Up: Precedence
 
@@ -1038,7 +1097,7 @@ Bison Symbols
 
 `YYLTYPE'
      Macro for the data type of `yylloc'; a structure with four
-     members.  *Note Textual Positions of Tokens: Token Positions.
+     members.  *Note Data Types of Locations: Location Type.
 
 `yyltype'
      Default value for YYLTYPE.