Before a variable or a named constant value can be referenced in a program, it must be declared.

The declaration gives the variable or named constant a unique name, a data type and, optionally except for constants, an initial value. There are three forms of variable declarations, the PrimitiveVariableDeclaration, the ReferenceVariableDeclaration and the Constant. You must supply a literal value for a constant. Aside from the types, the main difference is that the declarations for the primitive types are simpler than for reference types.

Variable declarations in actions and blocks are statements that are executed when the program’s control flow reaches them.