Design Notes for the PragmAda Reusable Components

The primary consideration in the development of the PragmARCs is that they be
useful in real software. While some component libraries have been developed
around theoretical taxonomies, the vast majority of the PragmARCs were developed
to meet the needs of real projects developing real software. These are
industrial-strength components that may be used with confidence.

The typical PragmARC data-structure component has a generic formal part of

generic
   type Element is limited private;

   with procedure Assign (To : in out Element; From : in Element) is <>;
package ...

This is sometimes perceived as being unnecessarily complex by users who only
need to store unlimited types. "Why not make Element simply private?" they
ask.

This formal part is used to meet a number of different goals simultaneously.

* Precision of Specification

Generic formal parameters, like the formal parameters of a subprogram, are a
specification. They specify what the client must supply the generic for the
generic to do its job. A generic formal private type specifies that the
generic must have ":=" and "=" operations for the type, in addition to any
other generic formal operations specified. A generic formal limited private type
specifies that the generic will only use explicitly specified generic formal
operations for the type.

Most of the data structures in the PragmARCs only use the assignment operation.
Specifying the type as private would indicate that the generic requires "=",
which is incorrect. The only way to specify that only assignment is required
in Ada is a limited private type and an Assign procedure.

* Generality of use

The generic formal part given above allows structures to be created with any
Element type that could be used with a generic formal private type, and with
other types that a private type would not allow. Such types have been needed
on real projects. One example involves building structures of structures.

* Assignment of different-sized bounded structures

A type with a Max_Size discriminant provides maximum client flexibility in the
use of bounded structures. The only way to assign values of such types with
different maximum sizes is with an Assign procedure. Such assignment is
meaningful, since the logical value contained in such a structure may be
smaller than the structure's maximum size. To allow structures of bounded
structures, the generic formal part must import an Assign procedure.

* Ease of use

This seems to be the easiest generic signature to instantiate that meets the
constraints given above. Other libraries have taken other approaches, but this
is the best approach that meets these constraints in terms of ease of use.
When using a data structure with a type that is not limited, generic procedure
PragmARC.Assignment helps keep the instantiation simple.