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Fixes for the match grammar #262

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Merged
merged 2 commits into from
Mar 11, 2018
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Fixes for the match grammar #262

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59ad814
Fixes for the match grammar:
brauliobz Mar 10, 2018
ba7414d
Added back the optional pipe before the patterns
brauliobz Mar 10, 2018
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29 changes: 22 additions & 7 deletions src/expressions/match-expr.md
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Original file line number Diff line number Diff line change
Expand Up @@ -2,13 +2,26 @@

> **<sup>Syntax</sup>**
> _MatchExpression_ :
> &nbsp;&nbsp; `match` [_Expression_]<sub>_except struct expression_</sub> _MatchBlock_
> &nbsp;&nbsp; `match` [_Expression_]<sub>_except struct expression_</sub> `{`
> &nbsp;&nbsp; &nbsp;&nbsp; [_InnerAttribute_]<sup>\*</sup>
> &nbsp;&nbsp; &nbsp;&nbsp; _MatchArms_<sup>?</sup>
> &nbsp;&nbsp; `}`
>
> _MatchBlock_ :
> &nbsp;&nbsp; &nbsp;&nbsp; `{` `}`
> &nbsp;&nbsp; | `{` (`|`<sup>?</sup> _Pattern_ (`|` _Pattern_)<sup>\*</sup> (`if` [_Expression_])<sup>?</sup> `=>` ([_BlockExpression_] `,`<sup>?</sup> | [_Expression_] `,`))<sup>\*</sup>
> &nbsp;&nbsp; &nbsp;&nbsp; &nbsp;&nbsp; &nbsp; (`|`<sup>?</sup> _Pattern_ (`|` _Pattern_)<sup>\*</sup> (`if` [_Expression_])<sup>?</sup> `=>` ([_BlockExpression_] `,`<sup>?</sup> | [_Expression_] `,`<sup>?</sup>))
> &nbsp;&nbsp; &nbsp;&nbsp; `}`
> _MatchArms_ :
> &nbsp;&nbsp; ( _MatchArm_ `=>`
> ( [_BlockExpression_] `,`<sup>?</sup>
> | [_Expression_] `,` )
> )<sup>\*</sup>
> &nbsp;&nbsp; _MatchArm_ `=>` ( [_BlockExpression_] | [_Expression_] ) `,`<sup>?</sup>
>
> _MatchArm_ :
> &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup> _MatchArmPatterns_ _MatchArmGuard_
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Can you give an example of the OuterAttribute here?

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I can't think of a useful example...

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@brauliobz brauliobz Mar 11, 2018 •
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I tried something like this, but the attribute doesn't apply to the use of deprecated USBType::A and rustc warns about both things instead (unused attribute and use of deprecated item):

enum USBType {
    #[deprecated] A,
    B,
    C,
}

fn describe(ty: USBType) -> &'static str {
    use USBType::*;
    match ty {
        #[allow(deprecated)] A => "",
        B => "",
        C => ""
    }
}

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So it's allowed by the grammar, but doesn't seem to have a use case? Weird, but as long as it's legal...

>
> _MatchArmPatterns_ :
> &nbsp;&nbsp; `|`<sup>?</sup> _Pattern_ ( `|` _Pattern_ )<sup>*</sup>
>
> _MatchArmGuard_ :
> &nbsp;&nbsp; `if` [_Expression_]

A `match` expression branches on a *pattern*. The exact form of matching that
occurs depends on the pattern. Patterns consist of some combination of
Expand Down Expand Up @@ -141,4 +154,6 @@ let message = match maybe_digit {
[place expression]: expressions.html#place-expressions-and-value-expressions
[value expression]: expressions.html#place-expressions-and-value-expressions
[`char`]: types.html#textual-types
[numeric types]: types.html#numeric-types
[numeric types]: types.html#numeric-types
[_InnerAttribute_]: attributes.html
[_OuterAttribute_]: attributes.html