Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

Mention syntax ambiguity for eRFC 2947. #437

Merged
merged 2 commits into from
Oct 3, 2018
Merged

Mention syntax ambiguity for eRFC 2947. #437

Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
165c11a
Mention syntax ambiguity for eRFC 2947.
davidtwco Sep 30, 2018
9f758ac
Mention a solution for eRFC 2947 ambiguity.
davidtwco Sep 30, 2018
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
24 changes: 23 additions & 1 deletion src/expressions/if-expr.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -44,7 +44,7 @@ assert_eq!(y, "Bigger");

> **<sup>Syntax</sup>**\
> _IfLetExpression_ :\
> &nbsp;&nbsp; `if` `let` [_Pattern_] `=` [_Expression_]<sub>_except struct expression_</sub>
> &nbsp;&nbsp; `if` `let` [_Pattern_] `=` [_Expression_]<sub>_except struct or lazy boolean operator expression_</sub>
> [_BlockExpression_]\
> &nbsp;&nbsp; (`else` (
> [_BlockExpression_]
Expand Down Expand Up @@ -111,6 +111,28 @@ match EXPR {
}
```

The expression cannot be a [lazy boolean operator expression][_LazyBooleanOperatorExpression_].
Use of a lazy boolean operator is ambiguous with a planned feature change
of the language (the implementation of if-let chains - see [eRFC 2947][_eRFCIfLetChain_]).
When lazy boolean operator expression is desired, this can be achieved
by using parenthesis as below:

```rust,ignore
// Before...
if let PAT = EXPR && EXPR { .. }

// After...
if let PAT = ( EXPR && EXPR ) { .. }

// Before...
if let PAT = EXPR || EXPR { .. }

// After...
if let PAT = ( EXPR || EXPR ) { .. }
```

[_Expression_]: expressions.html
[_BlockExpression_]: expressions/block-expr.html
[_Pattern_]: patterns.html
[_LazyBooleanOperatorExpression_]: expressions/operator-expr.html#lazy-boolean-operators
[_eRFCIfLetChain_]: https://github.com/rust-lang/rfcs/blob/master/text/2497-if-let-chains.md#rollout-plan-and-transitioning-to-rust-2018