Logic Operators
===============

{{#include ../links.md}}


Comparison Operators
--------------------

| Operator | Description<br/>(`x` _operator_ `y`) | `x`, `y` same type or are numeric | `x`, `y` different types |
| :------: | ------------------------------------ | :-------------------------------: | :----------------------: |
|   `==`   | `x` is equals to `y`                 |       error if not defined        |  `false` if not defined  |
|   `!=`   | `x` is not equals to `y`             |       error if not defined        |  `true` if not defined   |
|   `>`    | `x` is greater than `y`              |       error if not defined        |  `false` if not defined  |
|   `>=`   | `x` is greater than or equals to `y` |       error if not defined        |  `false` if not defined  |
|   `<`    | `x` is less than `y`                 |       error if not defined        |  `false` if not defined  |
|   `<=`   | `x` is less than or equals to `y`    |       error if not defined        |  `false` if not defined  |

Comparison operators between most values of the same type are built in for all [standard types].

Others are defined in the [`LogicPackage`][built-in packages] but excluded when using a [raw `Engine`].


### Floating-point numbers interoperate with integers

Comparing a floating-point number (`FLOAT`) with an integer is also supported.

```rust
42 == 42.0;         // true

42.0 == 42;         // true

42.0 > 42;          // false

42 >= 42.0;         // true

42.0 < 42;          // false
```

### Decimal numbers interoperate with integers

Comparing a [`Decimal`][rust_decimal] number with an integer is also supported.

```rust
let d = parse_decimal("42");

42 == d;            // true

d == 42;            // true

d > 42;             // false

42 >= d;            // true

d < 42;             // false
```

### Strings interoperate with characters

Comparing a [string] with a [character] is also supported, with the character first turned into a
[string] before performing the comparison.

```rust
'x' == "x";         // true

"" < 'a';           // true

'x' > "hello";      // false
```

### Comparing different types defaults to `false`

Comparing two values of _different_ data types defaults to `false` unless the appropriate operator
functions have been registered.

The exception is `!=` (not equals) which defaults to `true`. This is in line with intuition.

```rust
42 > "42";          // false: i64 cannot be compared with string

42 <= "42";         // false: i64 cannot be compared with string

let ts = new_ts();  // custom type

ts == 42;           // false: different types cannot be compared

ts != 42;           // true: different types cannot be compared

ts == ts;           // error: '==' not defined for the custom type
```

### Safety valve: Comparing different _numeric_ types has no default

Beware that the above default does _NOT_ apply to numeric values of different types
(e.g. comparison between `i64` and `u16`, `i32` and `f64`) &ndash; when multiple numeric types are
used it is too easy to mess up and for subtle errors to creep in.

```rust
// Assume variable 'x' = 42_u16, 'y' = 42_u16 (both types of u16)

x == y;             // true: '==' operator for u16 is built-in

x == "hello";       // false: different non-numeric operand types default to false

x == 42;            // error: ==(u16, i64) not defined, no default for numeric types

42 == y;            // error: ==(i64, u16) not defined, no default for numeric types
```

### Caution: Beware operators for custom types

```admonish tip.side.wide "Tip: Always the full set"

It is strongly recommended that, when defining operators for [custom types], always define the
**full set** of six operators together, or at least the `==` and `!=` pair.
```

Operators are completely separate from each other.  For example:

* `!=` does not equal `!(==)`

* `>` does not equal `!(<=)`

* `<=` does not equal `<` plus `==`

* `<=` does not imply `<`

Therefore, if a [custom type] misses an [operator] definition, it simply raises an error
or returns the default.

This behavior can be counter-intuitive.

```rust
let ts = new_ts();  // custom type with '<=' and '==' defined

ts <= ts;           // true: '<=' defined

ts < ts;            // error: '<' not defined, even though '<=' is

ts == ts;           // true: '==' defined

ts != ts;           // error: '!=' not defined, even though '==' is
```


Boolean Operators
-----------------

```admonish note.side

All boolean operators are [built in][built-in operators] for the `bool` data type.
```

|     Operator      | Description | Arity  | Short-circuits? |
| :---------------: | :---------: | :----: | :-------------: |
|  `!` _(prefix)_   |    _NOT_    | unary  |       no        |
|       `&&`        |    _AND_    | binary |     **yes**     |
|        `&`        |    _AND_    | binary |       no        |
| <code>\|\|</code> |    _OR_     | binary |     **yes**     |
|  <code>\|</code>  |    _OR_     | binary |       no        |

Double boolean operators `&&` and `||` _short-circuit_ &ndash; meaning that the second operand will not be evaluated
if the first one already proves the condition wrong.

Single boolean operators `&` and `|` always evaluate both operands.

```rust
a() || b();         // b() is not evaluated if a() is true

a() && b();         // b() is not evaluated if a() is false

a() | b();          // both a() and b() are evaluated

a() & b();          // both a() and b() are evaluated
```


Null-Coalescing Operator
------------------------

| Operator |  Description  | Arity  | Short-circuits? |
| :------: | :-----------: | :----: | :-------------: |
|   `??`   | Null-coalesce | binary |       yes       |

The null-coalescing operator (`??`) returns the first operand if it is not [`()`], or the second
operand if the first operand is [`()`].

It _short-circuits_  &ndash; meaning that the second operand will not be evaluated if the first
operand is not [`()`].

```rust
a ?? b              // returns 'a' if it is not (), otherwise 'b'

a() ?? b();         // b() is only evaluated if a() is ()
```

~~~admonish tip.small "Tip: Default value for object map property"

Use the null-coalescing operator to implement default values for non-existent [object map] properties.

```rust
let map = #{ foo: 42 };

// Regular property access
let x = map.foo;            // x == 42

// Non-existent property
let x = map.bar;            // x == ()

// Default value for property
let x = map.bar ?? 42;      // x == 42
```
~~~

### Short-circuit loops and early returns

The following statements are allowed to follow the null-coalescing operator:

* `break`
* `continue`
* [`return`]
* [`throw`]

This means that you can use the null-coalescing operator to short-circuit loops and/or
early-return from functions when the value tested is [`()`].

```rust
let total = 0;

for value in list {
    // Whenever 'calculate' returns '()', the loop stops
    total += calculate(value) ?? break;
}
```