Manual

Chapter 5: The Bison Parser Algorithm 73
The conflict exists because the grammar as written is ambiguous: either parsing of the
simple nested if-statement is legitimate. The established convention is that these ambiguities
are resolved by attaching the else-clause to the innermost if-statement; this is what Bison
accomplishes by choosing to shift rather than reduce. (It would ideally be cleaner to write an
unambiguous grammar, but that is very hard to do in this case.) This particular ambiguity
was first encountered in the specifications of Algol 60 and is called the “dangling else”
ambiguity.
To avoid warnings from Bison about predictable, legitimate shift/reduce conflicts, use
the %expect n declaration. There will be no warning as long as the number of shift/reduce
conflicts is exactly n. See Section 3.7.8 [Suppressing Conflict Warnings], page 57.
The definition of if_stmt above is solely to blame for the conflict, but the conflict
does not actually appear without additional rules. Here is a complete Bison input file that
actually manifests the conflict:
%token IF THEN ELSE variable
%%
stmt: expr
| if_stmt
;
if_stmt:
IF expr THEN stmt
| IF expr THEN stmt ELSE stmt
;
expr:
;
variable
5.3 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.
5.3.1 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 ’