Nonreturning Subprograms

{AI95-00329-01} {AI95-00414-01} {AI05-0229-1} {AI12-0269-1} {AI12-0418-1} Specifying aspect No_Return to have the value True indicates that a subprogram procedure cannot return normally[; it may, for example, propagate an exception or loop forever].

Discussion: Aspect No_Deposit will have to wait for Ada 2028 2020. :-)

Static Semantics

{AI05-0229-1} {AI12-0269-1} For a subprogram procedure or generic subprogram procedure, the following language-defined representation aspect may be specified:

{AI12-0423-1} If directly specified, the aspect_definition shall be a static expression. When not directly specified, if the subprogram is a primitive subprogram inherited by a derived type, then the aspect is True if any corresponding subprogram of the parent or progenitor types is nonreturning. Otherwise [This aspect is never inherited;] if not directly specified, the aspect is False.

Aspect Description for No_Return: A subprogram procedure will not return normally.

{AI05-0229-1} {AI12-0269-1} If a generic subprogram procedure is nonreturning, then so are its instances. If a subprogram procedure declared within a generic unit is nonreturning, then so are the corresponding copies of that subprogram procedure in instances.

Legality Rules

Reason: A null procedure cannot have the appropriate nonreturning semantics, as it does not raise an exception or loop forever.

Ramification: {AI05-0229-1} {AI12-0269-1} The procedure can be abstract. If a nonreturning subprogram procedure is renamed (anywhere) calls through the new name still have the nonreturning semantics.

{AI12-0269-1} Any return statement that applies to a nonreturning function or generic function shall be a simple_return_statement with an expression that is a raise_expression, a call on a nonreturning function, or a parenthesized expression of one of these.

Ramification: We still require at least one return statement in a function; we just require that all such return statements don't actually return a value.

{AI95-00414-01} {AI12-0269-1} A subprogram procedure shall be nonreturning if it overrides a dispatching nonreturning subprogram procedure. In addition to the places where Legality Rules normally apply (see 12.3), this rule applies also in the private part of an instance of a generic unit.

Reason: This ensures that dispatching calls to nonreturning subprograms procedures will, in fact, not return.

{AI95-00414-01} {AI12-0269-1} If a renaming-as-body completes a nonreturning subprogram procedure declaration, then the renamed subprogram procedure shall be nonreturning.

Reason: This ensures that no extra code is needed to implement the renames (that is, no wrapper is needed) as the body has the same property.

Dynamic Semantics

Discussion: {AI12-0269-1} Note that there is no name for suppressing this check, since the check represents a bug, imposes no time overhead, and minimal space overhead (since it can usually be statically eliminated as dead code). We don't need a similar rule for nonreturning functions as this is standard semantics for all functions that normally complete without a transfer of control (such as a return statement).

Implementation Note: If a nonreturning procedure tries to return, we raise Program_Error. This is stated as happening at the call site, because we do not wish to allow the procedure to handle the exception (and then, perhaps, try to return again!). However, the expected run-time model is that the compiler will generate raise Program_Error at the end of the procedure body (but not handleable by the procedure itself), as opposed to doing it at the call site. (This is just like the typical run-time model for functions that fall off the end without returning a value). The reason is indirect calls: in P.all(...);, the compiler cannot know whether P designates a nonreturning procedure or a normal one. Putting the raise Program_Error in the procedure's generated code solves this problem neatly.

Similarly, if one passes a nonreturning procedure to a generic formal parameter, the compiler cannot know this at call sites (in shared code implementations); the raise-in-body solution deals with this neatly.

Examples

{AI95-00433-01} {AI05-0229-1} procedure Fail(Msg : String)  -- raises Fatal_Error exception
   with No_Return;
   -- Inform compiler and reader that procedure never returns normally

Extensions to Ada 95

{AI95-00329-01} {AI95-00414-01} Pragma No_Return is new.

Extensions to Ada 2005

{AI05-0229-1} Aspect No_Return is new; pragma No_Return is now obsolescent.

Incompatibilities With Ada 2012

{AI12-0423-1} Correction: Clarified that the nonreturning property is inherited by derivation. A literal implementation of the Ada 2012 would mean that no overriding would ever be rejected, as the inherited routine would never be nonreturning. Ada 2022 requires overridings of dispatching nonreturning subprograms to be rejected. This is formally incompatible, but practically such overridings have been rejected in practice.

6.5.1 Nonreturning SubprogramsPragma No_Return