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Add information about || and && to grammar describing while let. #772

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Apr 8, 2020
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Add information about || and && to grammar describing while let. #772

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71b0551
Add information about || and && to Grammar describing `while let`.
steffahn Mar 4, 2020
532060a
Remove duplicated explanation about lazy boolean operators in while let,
steffahn Mar 4, 2020
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27 changes: 24 additions & 3 deletions src/expressions/loop-expr.md
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Expand Up @@ -45,7 +45,7 @@ have type compatible with the value of the `break` expression(s).

> **<sup>Syntax</sup>**\
> _PredicateLoopExpression_ :\
> &nbsp;&nbsp; `while` [_Expression_]<sub>except struct expression</sub> [_BlockExpression_]
> &nbsp;&nbsp; `while` [_Expression_]<sub>_except struct expression_</sub> [_BlockExpression_]

A `while` loop begins by evaluating the boolean loop conditional expression. If
the loop conditional expression evaluates to `true`, the loop body block
Expand All @@ -67,7 +67,7 @@ while i < 10 {

> **<sup>Syntax</sup>**\
> [_PredicatePatternLoopExpression_] :\
> &nbsp;&nbsp; `while` `let` [_MatchArmPatterns_] `=` [_Expression_]<sub>except struct expression</sub>
> &nbsp;&nbsp; `while` `let` [_MatchArmPatterns_] `=` [_Expression_]<sub>_except struct or lazy boolean operator expression_</sub>
> [_BlockExpression_]

A `while let` loop is semantically similar to a `while` loop but in place of a
Expand Down Expand Up @@ -123,11 +123,32 @@ while let Some(v @ 1) | Some(v @ 2) = vals.pop() {
}
```

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:

<!-- ignore: psuedo code -->
```rust,ignore
// Before...
while let PAT = EXPR && EXPR { .. }

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

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

// After...
while let PAT = ( EXPR || EXPR ) { .. }
```
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This is user advice and discussion of unstable features, which is off-topic for the reference.

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Then perhaps it should also be removed from the page about if expressions?

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Probably yes, cc @ehuss

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I think it's fine to keep this information, though place it in a "note" block. I don't see a reason to never talk about such things, since it is relevant here (explaining a strange peculiarity of the language). The C++ reference does the same with side notes mentioning future changes or how to disambiguate things that aren't obvious.

The information only needs to live in one place. if let would be fine, with while let linking to if let for more information. I might even trim while let even more to remove redundant information, but doesn't necessarily need to be done in this PR.

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Putting it in a note and deduplicating seems like a reasonable resolution to me.


## Iterator loops

> **<sup>Syntax</sup>**\
> _IteratorLoopExpression_ :\
> &nbsp;&nbsp; `for` [_Pattern_] `in` [_Expression_]<sub>except struct expression</sub>
> &nbsp;&nbsp; `for` [_Pattern_] `in` [_Expression_]<sub>_except struct expression_</sub>
> [_BlockExpression_]

A `for` expression is a syntactic construct for looping over elements provided
Expand Down