# 5.4. Expressions¶

Expressions are combinations of operands and operators.
Operands are values in themselves, which may be expressions surrounded by `( )`

.
Operators are logical, arithmetic, or string and the valid operators depend on the types of the operands.

## 5.4.1. Literal Values¶

Literal values can be individual lexical elements such as identifiers, numbers, and strings.

Literal arrays are sequences of comma-separated values surrounded by brackets `[ ]`

.

Example:

```
LET a: Array<Number> := [1, 2, 3]
```

Literal dictionaries are sequences of comma-separated name/value pairs surrounded by braces `{ }`

.

Example:

```
LET d: Dictionary<Number> := {
"one": 1,
"two": 2,
"three": 3
}
```

For convenience, both literal arrays and dictionaries accept a trailing comma after the final element:

```
LET numbers: Array<String> := [
"zero",
"one",
"two",
]
```

## 5.4.2. Boolean Operators¶

The following operator takes one boolean value.

Operator | Description |
---|---|

`NOT` |
logical negation |

The following operators take two boolean values.

Operator | Description |
---|---|

`=` |
equality |

`<>` |
inequality |

`AND` |
logical conjunction |

`OR` |
logical disjunction |

## 5.4.3. Numeric Operators¶

The following operators take one number value.

Operator | Description |
---|---|

`+` |
identity (provided for symmetry with `-` ) |

`-` |
arithmetic negation |

The following operators take two number values.

Operator | Description |
---|---|

`+` |
addition |

`-` |
subtraction |

`*` |
multiplication |

`/` |
division |

`INTDIV` |
integer division |

`MOD` |
modulo (remainder) |

`^` |
exponentiation |

`=` |
equality |

`<>` |
inequality |

`<` |
less than |

`>` |
greater than |

`<=` |
less than or equal |

`>=` |
greater than or equal |

## 5.4.4. String Operators¶

The following operators take two string values.

Operator | Description |
---|---|

`&` |
concatenation |

`=` |
equality |

`<>` |
inequality |

`<` |
lexicographical less than |

`>` |
lexicographical greater than |

`<=` |
lexicographical less than or equal |

`>=` |
lexicographical greater than or equal |

## 5.4.5. Array Operators¶

Operator | Description |
---|---|

`IN` |
membership test (O(n) complexity) |

`NOT IN` |
membership test (O(n) complexity) |

## 5.4.6. Dictionary Operators¶

Operator | Description |
---|---|

`IN` |
membership test (O(log n) complexity) |

`NOT IN` |
membership test (O(log n) complexity) |

## 5.4.7. Object Operator¶

Operator | Description |
---|---|

`ISA` |
type test (`object ISA Type` ) |

## 5.4.8. Pointer Operator¶

Operator | Description |
---|---|

`->` |
pointer dereference |

## 5.4.9. Operator Precedence¶

The operator precedence is as follows, highest to lowest:

Operator | Description |
---|---|

`( )` |
subexpression |

`^` |
exponentiation |

`*` `/` `MOD` `INTDIV` |
multiplication, division, modulo |

`+` `-` `&` |
addition, subtraction, concatenation |

`=` `<>` `<` `>` `<=` `>=` |
comparison |

`IN` `NOT IN` |
membership |

`AND` |
conjunction |

`OR` |
disjunction |

`IF` |
conditional |

## 5.4.10. Array Subscripts¶

Array subscripts are normally integers greater than or equal to zero:

```
LET a: Array<String> := ["foo", "bar", "baz"]
print(a[0])
print(a[2])
```

Two special values may be used, `FIRST`

and `LAST`

:

```
LET a: Array<String> := ["foo", "bar", "baz"]
print(a[FIRST])
print(a[LAST])
```

`FIRST`

always means the same as 0 and is provided for completeness.
`LAST`

refers to the index of the last element of the array (if the array is not empty).

Array slices are also possible using the `TO`

keyword.
Both indexes are inclusive:

```
LET a: Array<String> := ["foo", "bar", "baz"]
LET b: Array<String> := a[0 TO 1]
LET c: Array<String> := a[LAST-1 TO LAST]
```

In the above example, `b`

contains `["foo", "bar"]`

and `c`

contains `["bar", "baz"]`

.

## 5.4.11. Dictionary Subscripts¶

Dictionary subscripts are strings:

```
LET d: Dictionary<String> := {
"apple": "red",
"orange": "orange",
"banana": "yellow"
}
print(d["apple"])
print(d["banana"])
```

## 5.4.12. Conditional Expression¶

A conditional expression is like an inline `IF`

statement:

```
LET n: Number := 5
LET s: String := (IF n >= 0 THEN "positive" ELSE "negative")
```

The condition following `IF`

is evaluated.
If it is true, then the `THEN`

expression is evaluated and is the result of the expression.
Otherwise, the `ELSE`

expression is evaluated and is the result of the expression.

The parentheses around the entire conditional expression are required.

Note

The branch not taken is *not* evaluated.
This means that if a branch not taken is a function call, the function will not be called.

## 5.4.13. Try Expression¶

A try expression is like an inline `TRY`

statement:

```
LET a: Number := 5
LET b: Number := 0
LET n: Number := (TRY a / b TRAP DivideByZeroException GIVES -1)
```

The expression following `TRY`

is evaluated.
If an exception is raised, then it is matched against the `TRAP`

clauses.
A matching `TRAP`

clause with a `GIVES`

keyword evalues the `GIVES`

expression and returns that as the value of the try expression.

The keyword `DO`

can be used instead of `GIVES`

.
The `DO`

keywords introduces a new statement block which must end with a block-exiting statement (`EXIT`

, `NEXT`

, `RAISE`

, or `RETURN`

).

## 5.4.14. Expression Substitution¶

Literal strings may contain embedded expressions surrounded by the special escape `\( )`

.
These expressions are evaluated at run time.
The type of the embedded expression must have a `.toString()`

method which will be called automatically to convert the result to a string.

Example:

```
LET a: Array<String> := ["one", "two", "three"]
FOR i := 0 TO 2 DO
print("i is \(i) and the array element is \(a[i])")
END FOR
```

TODO

formatting specifiers