From 754bbf7a25a8dda49b5d08ef0d0443bbf5af0e36 Mon Sep 17 00:00:00 2001 From: Craig Jennings Date: Sun, 7 Apr 2024 13:41:34 -0500 Subject: new repository --- devdocs/elisp/operator-precedence-grammars.html | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+) create mode 100644 devdocs/elisp/operator-precedence-grammars.html (limited to 'devdocs/elisp/operator-precedence-grammars.html') diff --git a/devdocs/elisp/operator-precedence-grammars.html b/devdocs/elisp/operator-precedence-grammars.html new file mode 100644 index 00000000..7b36c16d --- /dev/null +++ b/devdocs/elisp/operator-precedence-grammars.html @@ -0,0 +1,23 @@ +

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.

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Function: smie-merge-prec2s &rest tables +

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

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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.

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

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  • 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).
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  • 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.
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Additionally, conflicts can occur:

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  • 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.
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  • 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").
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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.

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+ Copyright © 1990-1996, 1998-2022 Free Software Foundation, Inc.
Licensed under the GNU GPL license.
+ https://www.gnu.org/software/emacs/manual/html_node/elisp/Operator-Precedence-Grammars.html +

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