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From: rockbrentwood@gmail.com
Newsgroups: comp.compilers
Subject: Segmentation in programming language grammars .. why?
Date: Sun, 29 Dec 2019 21:05:27 -0800 (PST)
Organization: Compilers Central
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Keywords: parse, design
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Xref: csiph.com comp.compilers:2415

I see this with many specifications -- redundant non-terminals and rules. I'll
illustrate it with a part of the grammar spec for C17.

Abbreviate opt, statement, labeled-statement, compound-statement,
expression-statement, selection-statement, iteration-statement,
jump-statement, block-item, block-item-list, declaration, expression,
constant-expression, identifier respectively as ?, S, Sx, Sc, Se, Sb, Sl, Sj,
SD, SDs, D, E, Ec, X for

The statement grammar is
(6.8)   S -> Sx | Sc | Se | Sb | Sl | Sj
(6.8.1) Sx -> X ':' S
        Sx -> 'case' Ec ':' S
        Sx -> 'default' ':' S
(6.8.2) Sc -> '{' SDs? '}'
        SDs -> SD | SDs SD
        SD -> D | S
(6.8.3) Se -> E? ';'
(6.8.4) Sb -> 'if' '(' E ')' S
        Sb -> 'if' '(' E ')' S 'else' S
        Sb -> 'switch' '(' E ')' S
(6.8.5) Sl -> 'while' '(' E ')' S
        Sl -> 'do' S 'while' '(' E ')' ';'
        Sl -> 'for' '(' E? ';' E? ';' E? ')' S
        Sl -> 'for' '(' D  E? ';' E? ')' S
(6.8.6) Sj -> 'goto' X ';'
        Sj -> 'continue' ';'
        Sj -> 'break' ';'
        Sj -> 'return' E? ';'

and is segmented into subgroups Sx, Sc, Se, Sb, Sl, Sj of S. Why? Why not just
write it as one segment like this? It's not creating new conflicts in so
doing.

(6.8)
(6.8.1) S -> X ':' S
        S -> 'case' Ec ':' S
        S -> 'default' ':' S
(6.8.2) S -> '{' SDs? '}'
(6.8.3) S -> E? ';'
        SDs -> SD | SDs SD
        SD -> D | S
(6.8.4) S -> 'if' '(' E ')' S
        S -> 'if' '(' E ')' S 'else' S
        S -> 'switch' '(' E ')' S
(6.8.5) S -> 'while' '(' E ')' S
        S -> 'do' S 'while' '(' E ')' ';'
        S -> 'for' '(' E? ';' E? ';' E? ')' S
        S -> 'for' '(' D  E? ';' E? ')' S
(6.8.6) S -> 'goto' X ';'
        S -> 'continue' ';'
        S -> 'break' ';'
        S -> 'return' E? ';'