23.7.1.2 Operator Precedence Grammars

SMIE’s precedence grammars simply give to each token a pair of precedences: the left-precedence and the right-precedence. We say T1 < T2 if the right-precedence of token T1 is less than the left-precedence of token T2. A good way to read this < is as a kind of parenthesis: if we find ... T1 something T2 ... then that should be parsed as ... T1 (something T2 ... rather than as ... T1 something) T2 .... The latter interpretation would be the case if we had T1 > T2. If we have T1 = T2, it means that token T2 follows token T1 in the same syntactic construction, so typically we have "begin" = "end". Such pairs of precedences are sufficient to express left-associativity or right-associativity of infix operators, nesting of tokens like parentheses and many other cases.

Function: smie-prec2->grammar table

This function takes a prec2 grammar table and returns an alist suitable for use in smie-setup. The prec2 table is itself meant to be built by one of the functions below.

Function: smie-merge-prec2s &rest tables

This function takes several prec2 tables and merges them into a new prec2 table.

Function: smie-precs->prec2 precs

This function builds a prec2 table from a table of precedences precs. precs should be a list, sorted by precedence (for example "+" will come before "*"), of elements of the form (assoc op ...), where each op is a token that acts as an operator; assoc is their associativity, which can be either left, right, assoc, or nonassoc. All operators in a given element share the same precedence level and associativity.

Function: smie-bnf->prec2 bnf &rest resolvers

This function lets you specify the grammar using a BNF notation. It accepts a bnf description of the grammar along with a set of conflict resolution rules resolvers, and returns a prec2 table.

bnf is a list of nonterminal definitions of the form (nonterm rhs1 rhs2 ...) where each rhs is a (non-empty) list of terminals (aka tokens) or non-terminals.

Not all grammars are accepted:

• An rhs cannot be an empty list (an empty list is never needed, since SMIE allows all non-terminals to match the empty string anyway).
• An rhs cannot have 2 consecutive non-terminals: each pair of non-terminals needs to be separated by a terminal (aka token). This is a fundamental limitation of operator precedence grammars.

Additionally, conflicts can occur:

• The returned prec2 table holds constraints between pairs of tokens, and for any given pair only one constraint can be present: T1 < T2, T1 = T2, or T1 > T2.
• A token can be an opener (something similar to an open-paren), a closer (like a close-paren), or neither of the two (e.g., an infix operator, or an inner token like "else").

Precedence conflicts can be resolved via resolvers, which is a list of precs tables (see smie-precs->prec2): for each precedence conflict, if those precs tables specify a particular constraint, then the conflict is resolved by using this constraint instead, else a conflict is reported and one of the conflicting constraints is picked arbitrarily and the others are simply ignored.