Declarations and Statements

1. The body of a program unit generally contains two parts: a declarative part, which defines the logical entities to be used in the program unit, and a sequence of statements, which defines the execution of the program unit.
2. The declarative part associates names with declared entities. For example, a name may denote a type, a constant, a variable, or an exception. A declarative part also introduces the names and parameters of other nested subprograms, packages, task units, protected units, and generic units to be used in the program unit.
3. The sequence of statements describes a sequence of actions that are to be performed. The statements are executed in succession (unless a transfer of control causes execution to continue from another place).
4. An assignment statement changes the value of a variable. A procedure call invokes execution of a procedure after associating any actual parameters provided at the call with the corresponding formal parameters.
5. Case statements and if statements allow the selection of an enclosed sequence of statements based on the value of an expression or on the value of a condition.
6. The loop statement provides the basic iterative mechanism in the language. A loop statement specifies that a sequence of statements is to be executed repeatedly as directed by an iteration scheme, or until an exit statement is encountered.
7. A block statement comprises a sequence of statements preceded by the declaration of local entities used by the statements.
8. Certain statements are associated with concurrent execution. A delay statement delays the execution of a task for a specified duration or until a specified time. An entry call statement is written as a procedure call statement; it requests an operation on a task or on a protected object, blocking the caller until the operation can be performed. A called task may accept an entry call by executing a corresponding accept statement, which specifies the actions then to be performed as part of the rendezvous with the calling task. An entry call on a protected object is processed when the corresponding entry barrier evaluates to true, whereupon the body of the entry is executed. The requeue statement permits the provision of a service as a number of related activities with preference control. One form of the select statement allows a selective wait for one of several alternative rendezvous. Other forms of the select statement allow conditional or timed entry calls and the asynchronous transfer of control in response to some triggering event.
9. Execution of a program unit may encounter error situations in which normal program execution cannot continue. For example, an arithmetic computation may exceed the maximum allowed value of a number, or an attempt may be made to access an array component by using an incorrect index value. To deal with such error situations, the statements of a program unit can be textually followed by exception handlers that specify the actions to be taken when the error situation arises. Exceptions can be raised explicitly by a raise statement.

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