guideline

Use records to group heterogeneous but related data.

Consider records to map to I/O device data.

type Propulsion_Method is (Sail, Diesel, Nuclear);
type Craft is
   record
      Name   : Common_Name;
      Plant  : Propulsion_Method;
      Length : Feet;
      Beam   : Feet;
      Draft  : Feet;
   end record;
type Fleet is array (1 .. Fleet_Size) of Craft;

rationale

You help the maintainer find all of the related data by gathering it into the same construct, simplifying any modifications that apply to all rather than part. This, in turn, increases reliability. Neither you nor an unknown maintainer is liable to forget to deal with all the pieces of information in the executable statements, especially if updates are done with aggregate assignments whenever possible.

The idea is to put the information a maintainer needs to know where it can be found with the minimum of effort. For example, if all information relating to a given Craft is in the same place, the relationship is clear both in the declarations and especially in the code accessing and updating that information. But, if it is scattered among several data structures, it is less obvious that this is an intended relationship as opposed to a coincidental one. In the latter case, the declarations may be grouped together to imply intent, but it may not be possible to group the accessing and updating code that way. Ensuring the use of the same index to access the corresponding element in each of several parallel arrays is difficult if the accesses are at all scattered.

If the application must interface directly to hardware, the use of records, especially in conjunction with record representation clauses, could be useful to map onto the layout of the hardware in question.

notes

It may seem desirable to store heterogeneous data in parallel arrays in what amounts to a FORTRAN-like style. This style is an artifact of FORTRAN's data structuring limitations. FORTRAN only has facilities for constructing homogeneous arrays.

exceptions

If the application must interface directly to hardware, and the hardware requires that information be distributed among various locations, then it may not be possible to use records.