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added flux conditional expressions to the language specification

pull/199/head
Scott Anderson 2019-04-24 15:39:15 -06:00
parent c11a026db9
commit 6b7a609f69
1 changed files with 34 additions and 16 deletions
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content/v2.0/reference/flux/language/expressions.md
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@ -152,30 +152,48 @@ DotExpression = "." identifer
MemberBracketExpression = "[" string_lit "]" .
```
### Operators
## Conditional expressions
Conditional expressions evaluate a boolean-valued condition.
If the result is _true_, the expression that follows the `then` keyword is evaluated and returned.
If the result is _false_, the expression that follows the `else` keyword is evaluated and returned.
In either case, only the branch taken is evaluated and only side effects associated this branch will occur.
```js
ConditionalExpression = "if" Expression "then" Expression "else" Expression .
```
Example:
```js
color = if code == 0 then "green" else if code == 1 then "yellow" else "red"
```
## Operators
Operators combine operands into expressions.
The precedence of the operators is given in the table below.
Operators with a lower number have higher precedence.
|Precedence| Operator | Description |
|----------|----------|---------------------------|
| 1 | `a()` | Function call |
| | `a[]` | Member or index access |
| | `.` | Member access |
| 2 | `*` `/` |Multiplication and division|
| 3 | `+` `-` | Addition and subtraction |
| 4 |`==` `!=` | Comparison operators |
| | `<` `<=` | |
| | `>` `>=` | |
| |`=~` `!~` | |
| 5 | `not` | Unary logical expression |
| 6 | `and` | Logical AND |
| 7 | `or` | Logical OR |
| Precedence | Operator | Description |
|:----------:|:--------: |:--------------------------|
| 1 | `a()` | Function call |
| | `a[]` | Member or index access |
| | `.` | Member access |
| 2 | `*` `/` |Multiplication and division|
| 3 | `+` `-` | Addition and subtraction |
| 4 |`==` `!=` | Comparison operators |
| | `<` `<=` | |
| | `>` `>=` | |
| |`=~` `!~` | |
| 5 | `not` | Unary logical expression |
| 6 | `and` | Logical AND |
| 7 | `or` | Logical OR |
| 8 | `if` `then` `else` | Conditional |
The operator precedence is encoded directly into the grammar as the following.
```js
Expression = LogicalExpression .
Expression = ConditionalExpression .
ConditionalExpression = LogicalExpression
| "if" Expression "then" Expression "else" Expression .
LogicalExpression = UnaryLogicalExpression
| LogicalExpression LogicalOperator UnaryLogicalExpression .
LogicalOperator = "and" | "or" .