Types and Operators

Types

ASTRA is a typed language whose type system is based on the Java type system. This makes it easy to understand how values in ASTRA are passed to methods in Java. How variables are declared is explained in Variables, Choice and Repetition.

Type	Description	Example Values	Java Mapping
int	An integer number (4 bytes)	5, -11, 127	int / java.lang.Integer
long	An integer number (8 bytes)	55l, -225l, 4000l	long / java.lang.Long
float	A real number (4 bytes)	3.14f, -4.567f	float / java.lang.Float
double	A real number (8 bytes)	4.01, -1234.5678	double / java.lang.Double
char	A character	'a', '3', '@'	char / java.lang.Character
boolean	true or false	true, false	boolean / java.lang.Boolean
string	A string of characters	"hello", "12\t34"	java.lang.String
list	A list of values (replaces Java arrays)	[1, 2, 3], ["the", 4, 'a', true]	astra.term.ListTerm (implements java.util.List)
funct	A logical function	fatherOf("rem")	astra.term.Funct
Java class	A reference to a Java object, where the type is the class name	no literal form	Any Java class

Operators

Values can be combined with the following operators:

Operator	Meaning	Applies to
*, /, %	multiplication, division, remainder	numbers
+, -	addition, subtraction	numbers
+	concatenation	strings (and lists)

*, / and % are applied before + and -, and operators of the same level are applied from left to right, as in Java:

int a = 10 - 3 - 2;     // 5, i.e. (10 - 3) - 2
int b = 24 / 4 / 2;     // 3
int c = 2 + 3 * 4;      // 14
int d = (2 + 3) * 4;    // 20
int e = 2 * -3;         // -6

When one side of a + is a string, the other side is converted to a string, so + can be used to build messages from any kind of value, including funct, list and speechact values. Because evaluation is from left to right, "total: " + 1 + 2 is "total: 12", while 1 + 2 + " in total" is "3 in total"; use brackets to add the numbers first: "total: " + (1 + 2). Using + on two lists joins them: [1, 2] + [3] is [1, 2, 3].

An expression can only be evaluated once all of its variables have values. If a variable has not been bound (for example, because a sub-goal that was meant to set it did not), the statement that uses the expression fails with a message such as Cannot evaluate: "total: "+N - unbound variable(s): N. Like other failures, this can be handled with try/recover or a failure plan (see Failure Handling).

Comparisons

Two values are compared with the following operators, which give a formula that is true or false. They can be used wherever a formula can: in the context of a rule, in an if or while condition, and in a query.

Operator	Meaning	Example
==	equal	X == "kitchen"
~=	not equal	D ~= R
<, <=	less than, less than or equal	N < 10
>, >=	greater than, greater than or equal	N >= 0

“Not equal” is written ~=, as ~ is ASTRA’s “not”. ASTRA has no !=: from ASTRA 2.8.3, the compiler reports D != R as ASTRA has no != operator: write "not equal" as ~=.

rule +!go(string Room) : at(string Here) & Here ~= Room {
    // move towards Room
}