Expressions
An expression computes a single strongly typed value and consists of operands, operators, and executable functions.
switchPosition == POS.OFF || output.current > 0
The above example is a boolean expression, i.e., an expression producing a boolean value. It produces the logical disjunction, i.e., logical OR (||) between it’s left and right hand side. On the left hand side, the expressions references the switchPosition toggle and checks whether the switch is in the OFF position. On the right hand side, the output current is checked to be greater then 0.
Operands
The operands in expressions can be either of the following:
- basic literals such as
1,"message in a bottle"or enumeration literals such asPOS.OFF - references to contextually available handles such as value properties, ports and their channels, and local variables
- expressions themselves, allowing complex formulations
Operators
Operators perform operations on their operands. Operators define the number and type of their arguments, as well as their return type. Any non type- correct use of an operator or non-resolvable reference will lead to validation failured.
switchPos == POS.OFF will raise a validation error because the reference to switchPosition is spelled incorrectly.
switchPosition == DIRECTION.UP will raise a validation error because the types of left and right hand side are not compatible.
Operators evaluate in a specific order that is determined by their precedence and associativity. The operator with the higher precedence evaluates first. For example, multiplication has higher precedence than addition.
1 + 2 * 3 evaluates as 1 + (2 * 3) which is 7.
If two operators have the same precedence, then the operators are applied in order of associativity. For example, multiplication and division have the same precedence. The associativity of these operators is left to right; therefore, the operator on the left evaluates first.
6 / 2 * 3 evaluates as (6 / 2) * 3 which is 9.
Operators are classified into arithmetic, assignment, comparison, and logical operators.
Arithmetic Operators
Arithmetic operators perform mathematical operations. They apply to numbers and return a number.
| Operator | Name | Example | Description |
|---|---|---|---|
+ | Addition | a + b | Addition a and b |
- | Subtraction | a - b | Substract b from a |
* | Multiplication | a * b | Multiply a and b |
/ | Division | a / b | Devide a by b |
% | Moduls | a % b | Reminder of deviding a by b |
++ | Increment | ++a | Increment a by 1 |
-- | Decrement | --a | Decrement a by 1 |
Assignment Operators
An assignment operator assigns a value to a variable or port. The variable or port the value is assigned to is defined on the left-hand side. The value is defined on the right-hand side. The variable or port and assigned value must have compatible types.
a = 2 assigns the value 2 to a variable or port called a.
Variations of the assignment operator abbreviate combinations of assignment and other operators.
| Operator | Example | Abbreviation for |
|---|---|---|
= | a = 5 | a = 5 |
+= | a = 5 | a = a + 5 |
-= | a = 5 | a = a - 5 |
*= | a = 5 | a = a * 5 |
/= | a = 5 | a = a / 5 |
%= | a = 5 | a = a % 5 |
Comparison Operators
Comparison operators compare two values. They apply to primitives and return boolean values.
| Operator | Name | Example | Description |
|---|---|---|---|
== | Equal to | a == b | true if a is equal to b |
!= | Not equal to | a != b | true if a is not equal to b |
> | Greather than | a > b | true if a is greather than b |
< | Less than | a < b | true if a is less than b |
>= | Greather than or equal to | a >= b | true if a is greather than or equal to b |
<= | Less than or equal to | a <= b | true if a is less than or equal to b |
Logical Operators
Logical operators compare two variables or values. They apply and return boolean values.
| Operator | Name | Example | Description |
|---|---|---|---|
&& | Logical and | a && b | true if both a and b are true |
|| | Logical or | a || b | true if a or b is true |
! | Logical not | !a | true if a is false |
Executable Functions
Executable functions serve as means to store functionality. This functionality can then be accessed in expressions by calling the functions.
The expression ArithmeticModule.abs(-4) == 4 calls a function abs stored in the module ArithmeticModule with input value -4. The computed absolute value is then compared to the desired outcome, 4.
Automatic Checks (Well-formedness Rules)
This list is limited, as it only applies to the most general expressions. Find detailed explanations pertaining to specific use cases in the respective chapters:
| Error Description | Solution |
|---|---|
| Type of expr. could not be determined | Make sure that referenced context elements exists |
| Make sure referenced elements are of compatible types | |
| Make sure to formulate type-correct expressions |
Further Thoughts
- Bit expressions
- Short circuit