11.3 Raise Statements and Raise Expressions

Reason: Unlike conditional expressions, this doesn't say "immediately surrounded"; the only requirement is that it is somehow within a pair of parentheses that is part of the expression. We need this restriction in order that raise_expressions cannot be syntactically confused with immediately following constructs (such as aspect_specifications).

Discussion: We only need to require that a right parenthesis appear somewhere between the raise_expression and the surrounding context; that's all we need to specify in order to eliminate the ambiguities. Moreover, we don't care at all where the left parenthesis is (so long as it is legal, of course).

For instance, the following is illegal by this rule: 

as the "with Atomic" could be part of the raise_expression or part of the object declaration. Both of the following are legal: 

and if the with belongs to the raise_expression, then both of the following are legal: 

This rule only requires parentheses for raise_expressions that are part of the "top-level" of an expression in one of the named contexts; the raise_expression is either the entire expression, or part of a chain of logical operations. In practice, the raise_expression will almost always be last in interesting top-level expressions; anything that follows it could never be executed, so that should be rare. Other contexts such as conditional expressions, qualified expressions, aggregates, and even function calls, provide the needed parentheses. All of the following are legal, no additional parens are needed: 

Parentheses that are part of the context of the expression don't count. For instance, the parentheses around the raise_expression are required in the following: 

as ancestor_part is one of the contexts that triggers the rule.

This English-language rule could have been implemented instead by adding nonterminals initial_expression and initial_relation, which are the same as choice_expression and choice_relation except for the inclusion of membership in initial_relation. Then, initial_expresion could be used in place of expression in all of the contexts noted. We did not do that because of the large amount of change required, both to the grammar and to language rules that refer to the grammar. A complete grammar is given in AI12-0152-1.

The use of a raise_expression is illegal in each of modular_type_definition, floating_point_definition, ordinary_fixed_point_definition, and decimal_fixed_point_definition as these uses are required to be static and a raise_expression is never static. We include these in this rule so that Ada text has an unambiguous syntax in these cases. 

Proof: {AI95-00361-01} The definition of Exceptions.Exception_Message includes a statement that the string is returned (see 11.4.1). We describe the use of the string here so that we don't have an unexplained parameter in this subclause. 

Implementation Note: For a re-raise statement, the implementation does not create a new Exception_Occurrence, but instead propagates the same Exception_Occurrence value. This allows the original cause of the exception to be determined. 

Term entry: raise an exception — abandon normal program execution so as to draw attention to the fact that the corresponding situation has arisen

The fact that the name in a raise_statement has to denote an exception is not clear from RM83. Clearly that was the intent, since the italicized part of the syntax rules so indicate, but there was no explicit rule. RM83-1.5(11) doesn't seem to give the italicized parts of the syntax any force. 

{AI95-00361-01} The syntax of a raise_statement is extended to include a string message. This is more convenient than calling Exceptions.Exception_Message (exception_name'Identity, string_expression), and should encourage the use of message strings when raising exceptions.

{AI12-0022-1} {AI12-0152-1} {AI12-0159-1} Corrigendum: The raise_expression is new. This construct is necessary to allow conversion of existing specifications to use preconditions and predicates without changing the exceptions raised. It is considered important enough to be added to Ada 2012 rather than waiting for Ada 2022.