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Fix struct_expr grammar: make base update part optional. #38957

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Fix struct_expr grammar: make base update part optional. #38957

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2 changes: 1 addition & 1 deletion src/doc/grammar.md
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Original file line number Diff line number Diff line change
Expand Up @@ -512,7 +512,7 @@ unit_expr : "()" ;
```antlr
struct_expr : expr_path '{' ident ':' expr
[ ',' ident ':' expr ] *
[ ".." expr ] '}' |
[ ".." expr ] ? '}' |
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@nagisa nagisa Jan 10, 2017 •
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IIRC [ <rules> ] already means <rules> are optional.

Side note: this rule says A { a: 0 .. A { ..Default::default() } (note the missing comma after the 0) would do base update twice, but the comma between fields and base update cannot be omitted here. It also disallows A { ..Default::default() } which is valid, because it requires at least one field. Want to fix this rule?

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@theduke theduke Jan 10, 2017 •
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Often in BNF () is used for grouping and [] for optional but in grammar.md it's used just for grouping. ( quote: Square brackets are used to group rules. )

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@theduke theduke Jan 10, 2017 •
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But yeah I'll fix the rule to be valid, but this will be affected by my other PR.

I think I'll just close this down and do some more thought out work on the grammar.

expr_path '(' expr
[ ',' expr ] * ')' |
expr_path ;
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