12.5 Formal Types
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[A generic formal subtype can be used to pass to a generic unit a subtype
whose type is in a certain category of types.]
Reason: We considered having intermediate
syntactic categories
formal_integer_type_definition,
formal_real_type_definition, and
formal_fixed_point_definition,
to be more uniform with the syntax rules for non-generic-formal types.
However, that would make the rules for formal types slightly more complicated,
and it would cause confusion, since
formal_discrete_type_definition
would not fit into the scheme very well.
Syntax
Legality Rules
{generic actual subtype}
{actual subtype}
{generic actual type}
{actual type}
For a generic formal subtype, the actual shall be
a
subtype_mark;
it denotes the
(generic) actual subtype.
Ramification: When we say simply “formal”
or “actual” (for a generic formal that denotes a subtype)
we're talking about the subtype, not the type, since a name that denotes
a
formal_type_declaration
denotes a subtype, and the corresponding actual also denotes a subtype.
Static Semantics
{generic formal type}
{formal type}
{generic formal subtype}
{formal subtype}
A
formal_type_declaration
declares a
(generic) formal type, and its first subtype, the
(generic)
formal subtype.
Ramification: A subtype (other than the
first subtype) of a generic formal type is not a generic formal subtype.
Reason: This rule is clearer with the
flat syntax rule for
formal_type_definition
given above. Adding
formal_integer_type_definition
and others would make this rule harder to state clearly.
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We use “category’ rather than “class” above,
because the requirement that classes are closed under derivation is not
important here. Moreover, there are interesting categories that are not
closed under derivation. For instance, limited and interface are categories
that do not form classes.
Legality Rules
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The actual type shall be in the category determined for the formal.
Ramification: {
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For example, if the category determined for the formal is the category
of all discrete types, then the actual has to be discrete.
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Note that this rule does not require the actual to belong to every category
to which the formal belongs. For example, formal private types are in
the category of composite types, but the actual need not be composite.
Furthermore, one can imagine an infinite number of categories that are
just arbitrary sets of types (even though we don't give them names, since
they are uninteresting). We don't want this rule to apply to
those
categories.
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“Limited” is not an “interesting” category, but
“nonlimited” is; it is legal to pass a nonlimited type to
a limited formal type, but not the other way around. The reserved word
limited really represents a category containing both limited and
nonlimited types. “Private” is not a category for this purpose;
a generic formal private type accepts both private and nonprivate actual
types.
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It is legal to pass a class-wide subtype as the actual if it is in the
right category, so long as the formal has unknown discriminants.
Static Semantics
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[The formal type also belongs to each category that contains the determined
category.] The primitive subprograms of the type are as for any type
in the determined category. For a formal type other than a formal derived
type, these are the predefined operators of the type. For an elementary
formal type, the predefined operators are implicitly declared immediately
after the declaration of the formal type. For a composite formal type,
the predefined operators are implicitly declared either immediately after
the declaration of the formal type, or later immediately within the declarative
region in which the type is declared according to the rules of
7.3.1.
In an instance, the copy of such an implicit declaration declares a view
of the predefined operator of the actual type, even if this operator
has been overridden for the actual type. [The rules specific to formal
derived types are given in
12.5.1.]
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All properties of the type are as for any type in the category. Some
examples: The primitive operations available are as defined by the language
for each category. The form of
constraint
applicable to a formal type in a
subtype_indication
depends on the category of the type as for a nonformal type. The formal
type is tagged if and only if it is declared as a tagged private type,
or as a type derived from a (visibly) tagged type. (Note that the actual
type might be tagged even if the formal type is not.)
7 Generic formal types, like all types,
are not named. Instead, a
name
can denote a generic formal subtype. Within a generic unit, a generic
formal type is considered as being distinct from all other (formal or
nonformal) types.
8 A
discriminant_part
is allowed only for certain kinds of types, and therefore only for certain
kinds of generic formal types. See
3.7.
Examples
Examples of generic
formal types:
type Item is private;
type Buffer(Length : Natural) is limited private;
type Enum is (<>);
type Int is range <>;
type Angle is delta <>;
type Mass is digits <>;
type Table is array (Enum) of Item;
Example of a generic
formal part declaring a formal integer type:
generic
type Rank is range <>;
First : Rank := Rank'First;
Second : Rank := First + 1; -- the operator "+" of the type Rank
Wording Changes from Ada 83
RM83 has separate sections “Generic Formal
Xs” and “Matching Rules for Formal Xs” (for various
X's) with most of the text redundant between the two. We have combined
the two in order to reduce the redundancy. In RM83, there is no “Matching
Rules for Formal Types” section; nor is there a “Generic
Formal Y Types” section (for Y = Private, Scalar, Array, and Access).
This causes, for example, the duplication across all the “Matching
Rules for Y Types” sections of the rule that the actual passed
to a formal type shall be a subtype; the new organization avoids that
problem.
The matching rules are stated more concisely.
We no longer consider the multiplying operators
that deliver a result of type universal_fixed to be predefined
for the various types; there is only one of each in package Standard.
Therefore, we need not mention them here as RM83 had to.
Wording Changes from Ada 95
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Corrigendum 1 corrected the wording to properly define the location where
operators are defined for formal array types. The wording here was inconsistent
with that in
7.3.1, “
Private
Operations”. For the Amendment, this wording was corrected
again, because it didn't reflect the Corrigendum 1 revisions in
7.3.1.
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We use “determines a category” rather than class, since not
all interesting properties form a class.
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