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If and if let grammar #137

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Dec 1, 2017
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If and if let grammar #137

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37 changes: 37 additions & 0 deletions src/expressions/if-expr.md
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,14 @@

## `if` expressions

> **<sup>Syntax</sup>**
> _IfExpression_ :
> &nbsp;&nbsp; `if` [_Expression_]<sub>_except struct expression_</sub> [_BlockExpression_]
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Can we link to the struct expression here?

> &nbsp;&nbsp; (`else` (
> [_BlockExpression_]
> | _IfExpression_
> | _IfLetExpression_ ) )<sup>\?</sup>

An `if` expression is a conditional branch in program control. The form of an
`if` expression is a condition expression, followed by a consequent block, any
number of `else if` conditions and blocks, and an optional trailing `else`
Expand Down Expand Up @@ -31,8 +39,18 @@ let y = if 12 * 15 > 150 {
};
assert_eq!(y, "Bigger");
```

## `if let` expressions

> **<sup>Syntax</sup>**
> _IfLetExpression_ :
> &nbsp;&nbsp; `if` `let` _Pattern_ `=` [_Expression_]<sub>_except struct expression_</sub>
> [_BlockExpression_]
> &nbsp;&nbsp; (`else` (
> [_BlockExpression_]
> | _IfExpression_
> | _IfLetExpression_ ) )<sup>\?</sup>

An `if let` expression is semantically similar to an `if` expression but in
place of a condition expression it expects the keyword `let` followed by a
refutable pattern, an `=` and an expression. If the value of the expression on
Expand All @@ -57,3 +75,22 @@ if let ("Ham", b) = dish {
println!("Ham is served with {}", b);
}
```

`if` and `if let` expressions can be intermixed:

```rust
let x = Some(3);
let a = if let Some(1) = x {
1
} else if x == Some(2) {
2
} else if let Some(y) = x {
y
} else {
-1
};
assert_eq!(a, 3);
```

[_Expression_]: expressions.html
[_BlockExpression_]: expressions/block-expr.html