11. Exceptions

11.1. Exception Declarations

No additions or restrictions

11.2. Exception Handlers

Exception handlers are supported in SPARK, but the verification rules associated to language mandated checks and contracts make it so that only exceptions raised in actual raise statements can be handled.

Legality Rules

  1. Exception handlers shall not have a choice parameter.

11.3. Raise Statements and Raise Expressions

Raise statements and raise expressions are in SPARK. An exception is said to be expected if it is covered by a choice of an exception handler in an enclosing handled sequence of statements, or if its enclosing entity is a procedure body and the exception is covered by a choice in its Exceptional_Cases aspect whose associated consequence is not statically False.

As described below, all raise expressions must be provably never executed. The same holds true for raise statements if they raise unexpected exceptions.

Verification Rules

  1. A raise_expression introduces an obligation to prove that the expression will not be evaluated, much like the verification condition associated with

    pragma Assert (False);

    [In other words, the verification conditions introduced for a raise expression are the same as those introduced for a run-time check which fails unconditionally.]

  2. A raise_statement introduces an obligation to prove that the exception raised is expected. [For raise statements with an exception name which is unexpected, this amounts to proving that the statement will not be executed.]

11.4. Exception Handling

No additions or restrictions.

11.4.1. The Package Exceptions

11.4.2. Pragmas Assert and Assertion_Policy

Legality Rules

  1. The pragmas Assertion_Policy, Suppress, and Unsuppress are allowed in SPARK, but have no effect on the generation of verification conditions. [For example, an array index value must be shown to be in bounds regardless of whether Index_Check is suppressed at the point of the array indexing.]

  2. The following SPARK defined aspects and pragmas are assertions and their Boolean_expressions are assertion expressions:

    • Assert_And_Cut;

    • Assume;

    • Contract_Cases;

    • Default_Initial_Condition;

    • Initial_Condition;

    • Loop_Invariant;

    • Loop_Variant; and

    • Refined_Post.

    There is an assertion_aspect_mark for each of these aspects and pragmas with the same identifier as the corresponding aspect or pragma. In addition, Ghost is a SPARK defined assertion_aspect_mark.

    An implementation may introduce further implementation defined assertion_aspect_marks some of which may apply to groups of these assertions.