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RFC: let-expression #3159

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RFC: let-expression #3159

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3405606
init let expression rfc
HKalbasi Aug 8, 2021
c2bdecd
change first paragraph of #why now?
HKalbasi Aug 9, 2021
c86f4c2
Update 0000-let-expression.md
HKalbasi Aug 10, 2021
30e09b4
Prepare to remove not operator
HKalbasi Aug 11, 2021
9e77c59
Add future of matches section
HKalbasi Aug 11, 2021
d6c3ac1
update hard to read let expression section
HKalbasi Aug 11, 2021
8561362
fix a typo
HKalbasi Aug 11, 2021
8c2609f
Add Let expression type section in drawbacks
HKalbasi Aug 11, 2021
39ab42d
Add a practical example
HKalbasi Aug 12, 2021
1903255
Add another practical example
HKalbasi Aug 12, 2021
1b0a4bf
Add another practical example
HKalbasi Aug 13, 2021
6310916
Try to make examples more readable
HKalbasi Aug 13, 2021
9b38567
Move compare with let-else to let-else section
HKalbasi Aug 13, 2021
8d72621
Remove parenthesis from let expressions
HKalbasi Aug 17, 2021
f52c822
Move operator not to future possibilities
HKalbasi Aug 17, 2021
0a8b92c
Update practical examples
HKalbasi Aug 19, 2021
e2e24b9
Fix English grammar error
HKalbasi Aug 19, 2021
7746b6e
Update section Why now
HKalbasi Aug 19, 2021
6e91ca7
Fix a problem in examples
HKalbasi Aug 24, 2021
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40 changes: 40 additions & 0 deletions text/0000-let-expression.md
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Original file line number Diff line number Diff line change
Expand Up @@ -102,6 +102,46 @@ assert!(matches!(a, Some(x) if let Some(y) = b(x) && x == y));
assert!(if let Some(x) = a && let Some(y) = b(x) && x == y { true } else { false });
```

### Practical usage of this features

People find let expressions theoretical at first look, and only traditional cases (if-let-chain and let-else)
Can become useful. In this section there are some usages for new features of this RFC in real codes.

This is an example from rust-clippy repository:
```rust
for w in block.stmts.windows(2) {
if_chain! {
if let StmtKind::Semi(first) = w[0].kind;
if let StmtKind::Semi(second) = w[1].kind;
if !differing_macro_contexts(first.span, second.span);
if let ExprKind::Assign(lhs0, rhs0, _) = first.kind;
if let ExprKind::Assign(lhs1, rhs1, _) = second.kind;
if eq_expr_value(cx, lhs0, rhs1);
if eq_expr_value(cx, lhs1, rhs0);
then {
// 30 lines of code with massive rightward drift
}
}
}
```

Which by a generalized let-else can become:
```rust
for w in block.stmts.windows(2) {
(let StmtKind::Semi(first) = w[0].kind)
&& (let StmtKind::Semi(second) = w[1].kind)
&& !differing_macro_contexts(first.span, second.span)
&& (let ExprKind::Assign(lhs0, rhs0, _) = first.kind)
&& (let ExprKind::Assign(lhs1, rhs1, _) = second.kind)
&& eq_expr_value(cx, lhs0, rhs1)
&& eq_expr_value(cx, lhs1, rhs0)
|| continue;
// 30 lines of code with two less tab
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@kennytm kennytm Aug 12, 2021 •
edited
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this is more like a practical example of the existing if_let_chains feature (if you must include a continue for the number of indentations)

    if let StmtKind::Semi(first) = w[0].kind 
        && let StmtKind::Semi(second) = w[1].kind
        && !differing_macro_contexts(first.span, second.span)
        ...
        && eq_expr_value(cx, lhs1, rhs0)
    {
    } else {
        continue;
    }

this is also more consistent with the way people would write

if next_condition {
    continue;
}

rather than

!next_condition || continue;

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if-let-chain still need one indentation, because variables are bound inside of if body (like if-let) but in the example with || variables are bound after statement (like let-else). If you believe if-let can't meet the needs that let-else provide, The same statement is true for if-let-chain and this RFC.

this is also more consistent with the way people would write

I am agree and prefer explicit if over || in normal cases. Without bindings, both behavior is the same (both in current rust and in this RFC) but because binding rules for them are different (for good reasons; for example accessing if-let variables after body is surprising) you can save one indentation by || one.

And it is worth noting that !next_condition || continue; is completely valid and working in today rust. But !next_condition else { continue }; isn't and won't, so let-else syntax has a negative point here and this RFC is more consistent with rust.

}
```
Every `if let` or `if_chain!` that fill body of a loop or function can refactored in this way. You can easily find
dozens of them just in rust-clippy.

## Why now?
This RFC exists thanks to people who choose `if let` for syntax we know today.
That syntax wasn't the only choice and there was other options like `iflet`, `if match`, `let if`, `if is` or another `keyword`.
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