Ada Reference Manual (Ada 2022) — Legal Information

# 5.1 Simple and Compound Statements - Sequences of Statements

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A statement is either simple or compound. A simple_statement encloses no other statement. A compound_statement can enclose simple_statements and other compound_statements. A parallel construct is a construct that introduces additional logical threads of control (see Clause 9) without creating a new task. Parallel loops (see 5.5) and parallel_block_statements (see 5.6.1) are parallel constructs.

#### Syntax

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sequence_of_statements ::= statement {statement} {label}
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statement ::=
{label} simple_statement | {label} compound_statement
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simple_statement ::= null_statement
| assignment_statement | exit_statement
| goto_statement | procedure_call_statement
| simple_return_statement | entry_call_statement
| requeue_statement | delay_statement
| abort_statement | raise_statement
| code_statement
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compound_statement ::=
if_statement | case_statement
| loop_statement | block_statement
| extended_return_statement
| parallel_block_statement
| accept_statement | select_statement
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null_statement ::= null;
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label ::= <<label_statement_identifier>>
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statement_identifier ::= direct_name
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The direct_name of a statement_identifier shall be an identifier (not an operator_symbol).

#### Name Resolution Rules

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The direct_name of a statement_identifier shall resolve to denote its corresponding implicit declaration (see below).

#### Legality Rules

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Distinct identifiers shall be used for all statement_identifiers that appear in the same body, including inner block_statements but excluding inner program units.

#### Static Semantics

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For each statement_identifier, there is an implicit declaration (with the specified identifier) at the end of the declarative_part of the innermost block_statement or body that encloses the statement_identifier. The implicit declarations occur in the same order as the statement_identifiers occur in the source text. If a usage name denotes such an implicit declaration, the entity it denotes is the label, loop_statement, or block_statement with the given statement_identifier.
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If one or more labels end a sequence_of_statements, an implicit null_statement follows the labels before any following constructs.

#### Dynamic Semantics

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The execution of a null_statement has no effect.
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A transfer of control is the run-time action of an exit_statement, return statement, goto_statement, or requeue_statement, selection of a terminate_alternative, raising of an exception, or an abort, which causes the next action performed to be one other than what would normally be expected from the other rules of the language. As explained in 7.6.1, a transfer of control can cause the execution of constructs to be completed and then left, which may trigger finalization.
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The execution of a sequence_of_statements consists of the execution of the individual statements in succession until the sequence_ is completed.
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Within a parallel construct, if a transfer of control out of the construct is initiated by one of the logical threads of control, an attempt is made to cancel all other logical threads of control initiated by the parallel construct. Once all other logical threads of control of the construct either complete or are canceled, the transfer of control occurs. If two or more logical threads of control of the same construct initiate such a transfer of control concurrently, one of them is chosen arbitrarily and the others are canceled.
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When a logical thread of control is canceled, the cancellation causes it to complete as though it had performed a transfer of control to the point where it would have finished its execution. Such a cancellation is deferred while the logical thread of control is executing within an abort-deferred operation (see 9.8), and may be deferred further, but not past a point where the logical thread initiates a new nested parallel construct or reaches an exception handler that is outside such an abort-deferred operation.

#### Bounded (Run-Time) Errors

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During the execution of a parallel construct, it is a bounded error to invoke an operation that is potentially blocking (see 9.5). Program_Error is raised if the error is detected by the implementation; otherwise, the execution of the potentially blocking operation can either proceed normally, or it can result in the indefinite blocking of some or all of the logical threads of control making up the current task.
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NOTE A statement_identifier that appears immediately within the declarative region of a named loop_statement or an accept_statement is nevertheless implicitly declared immediately within the declarative region of the innermost enclosing body or block_statement; in other words, the expanded name for a named statement is not affected by whether the statement occurs inside or outside a named loop or an accept_statement — only nesting within block_statements is relevant to the form of its expanded name.

#### Examples

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Examples of labeled statements:
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<<Here>> <<Ici>> <<Aqui>> <<Hier>> null;
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<<After>> X := 1;

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