Issue with reduce/reduce conflicts

[TorbenMogensen]

Consider this grammar file:

%token ONE MINUS MIN

%left MINUS
%right UMINUS
%right MIN

%start Exp
%type Exp

%%

Exp:
ONE { 1 }
| Exp MINUS Exp { $1 - $3) }
| MINUS Exp %prec UMINUS { - $2 }
| MIN Exp Exp { min $2 $3 }
;

%%

With mosmlyac, this generates no conflict, but with fsyacc, it generates three reduce/reduce conflicts:

state 4: reduce/reduce error on ONE
state 4: reduce/reduce error on MINUS
state 4: reduce/reduce error on MIN

State 4 has the following items:

Exp -> Exp . 'MINUS' Exp
Exp -> Exp 'MINUS' Exp . 
Exp -> 'MINUS' Exp .

Given that the unary minus has higher precedence than the infix minus, there should be no conflict: There should be a reduce on the last production in the item set. So it seems that precedence declarations are ignored when resolving reduce/reduce conflicts.

Peter Sestoft tells me that he resolved this issue in mosmlyac (which is also based on ocamlyacc), as well as an isse with %nonassoc, around 10 years ago.