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5.5.2 Generalized Loop Iteration

1/3
{AI05-0139-2} Generalized forms of loop iteration are provided by an iterator_specification.

Syntax

2/3
{AI05-0139-2} {AI05-0292-1} iterator_specification ::= 
    defining_identifier in [reverseiterator_name
  | defining_identifier [: subtype_indicationof [reverseiterable_name

Name Resolution Rules

3/3
{AI05-0139-2} {AI05-0292-1} For the first form of iterator_specification, called a generalized iterator, the expected type for the iterator_name is any iterator type. For the second form of iterator_specification, the expected type for the iterable_name is any array or iterable container type. If the iterable_name denotes an array object, the iterator_specification is called an array component iterator; otherwise it is called a container element iterator.
3.a.1/3
Glossary entry: An iterator is a construct that is used to loop over the elements of an array or container. Iterators may be user defined, and may perform arbitrary computations to access elements from a container.

Legality Rules

4/3
{AI05-0139-2} If the reserved word reverse appears, the iterator_specification is a reverse iterator; otherwise it is a forward iterator. In a reverse generalized iterator, the iterator_name shall be of a reversible iterator type. In a reverse container element iterator, the default iterator type for the type of the iterable_name shall be a reversible iterator type.
5/4
{AI05-0139-2} {AI12-0151-1} The subtype defined by the subtype_indication, if any, of an array component iterator shall statically match the component subtype of the type of the iterable_name. The subtype defined by the subtype_indication, if any, of a container element iterator shall statically match the default element subtype for the type of the iterable_name.
6/3
{AI05-0139-2} In a container element iterator whose iterable_name has type T, if the iterable_name denotes a constant or the Variable_Indexing aspect is not specified for T, then the Constant_Indexing aspect shall be specified for T.
6.1/4
 {AI12-0047-1} The iterator_name or iterable_name of an iterator_specification shall not denote a subcomponent that depends on discriminants of an object whose nominal subtype is unconstrained, unless the object is known to be constrained.
6.a/4
Reason: This is the same rule that applies to renames; it serves the same purpose of preventing the object from disappearing while the iterator is still using it. 
6.2/4
 {AI12-0120-1} A container element iterator is illegal if the call of the default iterator function that creates the loop iterator (see below) is illegal.
6.b/4
Ramification: This can happen if the parameter to the default iterator function is in out and the iterable_name is a constant. The wording applies to any reason that the call would be illegal, as it's possible that one of the default parameters would be illegal, or that some accessibility check would fail. 
6.3/4
 {AI12-0120-1} A generalized iterator is illegal if the iteration cursor subtype of the iterator_name is a limited type at the point of the generalized iterator. A container element iterator is illegal if the default cursor subtype of the type of the iterable_name is a limited type at the point of the container element iterator.
6.c/4
Reason: If the cursor type is limited, the assignment to the loop parameter for a generalized iterator would be illegal. The same is true for a container element iterator. We have to say "at the point of the iterator" as the limitedness of a type can change due to visibility. 

Static Semantics

7/3
{AI05-0139-2} {AI05-0269-1} {AI05-0292-1} An iterator_specification declares a loop parameter. In a generalized iterator, the nominal subtype of the loop parameter is the iteration cursor subtype. In an array component iterator or a container element iterator, if a subtype_indication is present, it determines the nominal subtype of the loop parameter. In an array component iterator, if a subtype_indication is not present, the nominal subtype of the loop parameter is the component subtype of the type of the iterable_name. In a container element iterator, if a subtype_indication is not present, the nominal subtype of the loop parameter is the default element subtype for the type of the iterable_name.
8/3
{AI05-0139-2} {AI05-0292-1} In a generalized iterator, the loop parameter is a constant. In an array component iterator, the loop parameter is a constant if the iterable_name denotes a constant; otherwise it denotes a variable. In a container element iterator, the loop parameter is a constant if the iterable_name denotes a constant, or if the Variable_Indexing aspect is not specified for the type of the iterable_name; otherwise it is a variable.
8.a/4
Ramification: {AI12-0093-1} The loop parameter of a generalized iterator has the same accessibility as the loop statement. This means that the loop parameter object is finalized when the loop statement is left. (It also may be finalized as part of assigning a new value to the loop parameter.) For array component iterators and container element iterators, the loop parameter directly denotes an element of the array or container and has the accessibility of the associated array or container. 

Dynamic Semantics

9/3
{AI05-0139-2} For the execution of a loop_statement with an iterator_specification, the iterator_specification is first elaborated. This elaboration elaborates the subtype_indication, if any.
10/3
{AI05-0139-2} For a generalized iterator, the loop parameter is created, the iterator_name is evaluated, and the denoted iterator object becomes the loop iterator. In a forward generalized iterator, the operation First of the iterator type is called on the loop iterator, to produce the initial value for the loop parameter. If the result of calling Has_Element on the initial value is False, then the execution of the loop_statement is complete. Otherwise, the sequence_of_statements is executed and then the Next operation of the iterator type is called with the loop iterator and the current value of the loop parameter to produce the next value to be assigned to the loop parameter. This repeats until the result of calling Has_Element on the loop parameter is False, or the loop is left as a consequence of a transfer of control. For a reverse generalized iterator, the operations Last and Previous are called rather than First and Next.
10.a/4
Ramification: {AI12-0093-1} The loop parameter of a generalized iterator is a variable of which the user only has a constant view. It follows the normal rules for a variable of its nominal subtype. In particular, if the nominal subtype is indefinite, the variable is constrained by its initial value. Similarly, if the nominal subtype is class-wide, the variable (like all variables) has the tag of the initial value. Constraint_Error may be raised by a subsequent iteration if Next or Previous return an object with a different tag or constraint. 
11/3
{AI05-0139-2} {AI05-0292-1} For an array component iterator, the iterable_name is evaluated and the denoted array object becomes the array for the loop. If the array for the loop is a null array, then the execution of the loop_statement is complete. Otherwise, the sequence_of_statements is executed with the loop parameter denoting each component of the array for the loop, using a canonical order of components, which is last dimension varying fastest (unless the array has convention Fortran, in which case it is first dimension varying fastest). For a forward array component iterator, the iteration starts with the component whose index values are each the first in their index range, and continues in the canonical order. For a reverse array component iterator, the iteration starts with the component whose index values are each the last in their index range, and continues in the reverse of the canonical order. The loop iteration proceeds until the sequence_of_statements has been executed for each component of the array for the loop, or until the loop is left as a consequence of a transfer of control.
12/3
{AI05-0139-2} {AI05-0292-1} For a container element iterator, the iterable_name is evaluated and the denoted iterable container object becomes the iterable container object for the loop. The default iterator function for the type of the iterable container object for the loop is called on the iterable container object and the result is the loop iterator. An object of the default cursor subtype is created (the loop cursor).
13/3
{AI05-0139-2} {AI05-0292-1} For a forward container element iterator, the operation First of the iterator type is called on the loop iterator, to produce the initial value for the loop cursor. If the result of calling Has_Element on the initial value is False, then the execution of the loop_statement is complete. Otherwise, the sequence_of_statements is executed with the loop parameter denoting an indexing (see 4.1.6) into the iterable container object for the loop, with the only parameter to the indexing being the current value of the loop cursor; then the Next operation of the iterator type is called with the loop iterator and the loop cursor to produce the next value to be assigned to the loop cursor. This repeats until the result of calling Has_Element on the loop cursor is False, or until the loop is left as a consequence of a transfer of control. For a reverse container element iterator, the operations Last and Previous are called rather than First and Next. If the loop parameter is a constant (see above), then the indexing uses the default constant indexing function for the type of the iterable container object for the loop; otherwise it uses the default variable indexing function.
14/4
{AI12-0120-1} Any exception propagated by the execution of a generalized iterator or container element iterator is propagated by the immediately enclosing loop statement.
14.a/4
Ramification: This text covers exceptions raised by called functions that make up the execution of the iterator as well as exceptions raised by the assignment to the loop parameter or cursor. 

Examples

15/3
{AI05-0269-1} -- Array component iterator example:
for Element of Board loop  -- See 3.6.1.
   Element := Element * 2.0; -- Double each element of Board, a two-dimensional array.
end loop;
16/3
{AI05-0268-1} For examples of use of generalized iterators, see A.18.32 and the corresponding container packages in A.18.2 and A.18.3

Extensions to Ada 2005

16.a/3
{AI05-0139-2} Generalized forms of loop iteration are new. 

Incompatibilities With Ada 2012

16.b/4
{AI12-0047-1} Corrigendum: Added a rule to ensure that the object being iterated cannot be a component that could disappear before the loop completes. This could be incompatible by making a loop that was legal (and worked correctly, so long as the enclosing object is not modified during the loop) from the original Ada 2012 illegal in corrected Ada 2012. Such loops should be pretty rare, especially as these iterator forms are new to Ada 2012.
16.c/4
{AI12-0120-1} Corrigendum: Added rules to reject loops if the call to the default iterator function for a container element iterator is illegal, or if the cursor type of an iterator is limited. These are formally incompatible with original Ada 2012, but as it's unlikely that any Ada 2012 compiler ever allowed the illegal usages in an expansion of a loop (it's much more likely that they would have just caused an internal error in the compiler), this should have no effect in practice.
16.d/4
{AI12-0151-1} Corrigendum: Added a requirement that the given subtype statically match the subtype of the element or component for a component element iterator or array component iterator. Original Ada 2012 text allowed any type that covered the subtype of the element or component, but that led to questions of what the meaning was if they are different. In this case, the element is essentially a renaming of the container element, and it doesn't make sense for the constraints to be different. Ignoring explicitly defined constraints in renames is a mistake that we don't want to continue, thus we require static matching. This means that some programs might be illegal, but those programs were misleading at best, and potentially would raise unexpected exceptions because the element values might have been invalid or abnormal with respect to the declared constraint. 

Wording Changes from Ada 2012

16.e/4
{AI12-0120-1} Corrigendum: Added wording to specify that a loop propagates any exceptions propagated by the execution of an iterator. Since that's what naturally would happen from a macro-style expansion of the parts of an iterator, and no other interpretation makes sense given the way the rest of Ada works, we consider it so unlikely that any Ada 2012 implementation ever did anything else that we don't document this as a possible inconsistency.

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