Ada '83 Quality and Style:

Guidelines for Professional Programmers

Copyright 1989, 1991,1992 Software Productivity Consortium, Inc., Herndon, Virginia.

CHAPTER 3: Readability

3.4 Using Types

3.4.1 Declaring Types

guideline

• Limit the range of scalar types as much as possible.
• Seek information about possible values from the application.
• Do not overload any of the type names in package Standard.
• Use subtype declarations to improve program readability (Booch 1987).
• Use derived types and subtypes in concert (see Guideline 5.3.1).

example

subtype Card_Image is String (1 .. 80);

Input_Line : Card_Image := (others => ' ');

-- restricted integer type: 
type    Day_Of_Leap_Year     is                  range 1 .. 366; 
subtype Day_Of_Non_Leap_Year is Day_Of_Leap_Year range 1 .. 365;

By the following declaration, the programmer means, "I haven't the foggiest idea how many," but the actual range will show up buried in the code or as a system parameter:

Employee_Count : Integer;

rationale

Different implementations provide different sets of values for most of the predefined types. A reader cannot determine the intended range from the predefined names. This situation is aggravated when the predefined names are overloaded.

The names of an object and its subtype can clarify their intended use and document low-level design decisions. The example above documents a design decision to restrict the software to devices whose physical parameters are derived from the characteristics of punch cards. This information is easy to find for any later changes, thus enhancing program maintainability.

Section 8.5 of the Ada Language Reference Manual says that declaring a subtype without a constraint is one method for renaming a type.

Types can have highly constrained sets of values without eliminating useful values. Usage as described in Guideline 5.3.1 eliminates many flag variables and type conversions within executable statements. This renders the program more readable while allowing the compiler to enforce strong typing constraints.

note

Recognize that any deviation from this guideline detracts from the advantages of the strong typing facilities of the Ada language.

Language Ref Manual references: 3.3 Types and Subtypes, 3.4 Derived Types, 3.5 Scalar Types, 8.5 Renaming Declarations, C. Predefined Language Environment

3.4.2 Enumeration Types

guideline

• Use enumeration types instead of numeric codes.
• Use representation clauses to match requirements of external devices.

example

type Color is (Blue, Red, Green, Yellow);

rather than

Blue : constant := 1; Red : constant := 2; Green : constant := 3; Yellow : constant := 4;

and add the following if necessary.

for Color use (Blue => 1, Red => 2, Green => 3, Yellow => 4);

rationale

In addition, Ada provides a number of attributes ('Pos, 'Val, 'Succ, 'Pred, 'Image, and 'Value) for enumeration types which, when used, are more reliable than user-written operations on encodings.

A numeric code may at first seem appropriate to match external values. Instead, these situations call for a representation clause on the enumeration type. The representation clause documents the "encoding." If the program is properly structured to isolate and encapsulate hardware dependencies (see Guideline 7.1.5), the numeric code ends up in an interface package where it can be easily found and replaced should the requirements change.

Language Ref Manual references: 3.5.1 Enumeration Types, 13.3 Enumeration Representation Clauses, A. Predefined Language Attributes

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