3.1.1 Use of Underscores

guideline

Use underscores to separate words in a compound name.

When an identifier consists of more than one word, it is much easier to read if the words are separated by underscores. Indeed, there is precedent in English in which compound words are separated by a hyphen or a space. In addition to promoting readability of the code, if underscores are used in names, a code formatter has more control over altering capitalization. See Guideline 3.1.3.

3.1.2 Numbers

guideline

Represent numbers in a consistent fashion.

Represent literals in a radix appropriate to the problem.

Use underscores to separate digits the same way commas or periods (or spaces for nondecimal bases) would be used in normal text.

When using scientific notation, make the E consistently either uppercase or lowercase.

In an alternate base, represent the alphabetic characters in either all uppercase or all lowercase.

3.1.3 Capitalization

guideline

Make reserved words and other elements of the program visually distinct from each other.

Use lowercase for all reserved words (when used as reserved words).
- Use mixed case for all other identifiers, a capital letter beginning every word separated by underscores.
- Use uppercase for abbreviations and acronyms (see automation notes).

example

...
type Second_Of_Day      is range 0 .. 86_400;
type Noon_Relative_Time is (Before_Noon, After_Noon, High_Noon);
subtype Morning   is Second_Of_Day range 0 .. 86_400 / 2 - 1;
subtype Afternoon is Second_Of_Day range Morning'Last + 2 .. 86_400;
...
Current_Time := Second_Of_Day(Calendar.Seconds(Calendar.Clock));
if Current_Time in Morning then
   Time_Of_Day := Before_Noon;
elsif Current_Time in Afternoon then
   Time_Of_Day := After_Noon;
else
   Time_Of_Day := High_Noon;
end if;
case Time_Of_Day is
   when Before_Noon =>   Get_Ready_For_Lunch;
   when High_Noon   =>   Eat_Lunch;
   when After_Noon  =>   Get_To_Work;
end case;
...

rationale

Visually distinguishing reserved words allows you to focus on program structure alone, if desired, and also aids scanning for particular identifiers.

The instantiation chosen here is meant to be more readable for the experienced Ada programmer, who does not need reserved words to leap off the page. Beginners to any language often find that reserved words should be emphasized to help them find the control structures more easily. Because of this, instructors in the classroom and books introducing the Ada language may want to consider an alternative instantiation. The Ada Reference Manual (1995) chose bold lowercase for all reserved words.

automation notes

Ada names are not case sensitive. Therefore, the names max_limit, MAX_LIMIT, and Max_Limit denote the same object or entity. A good code formatter should be able to automatically convert from one style to another as long as the words are delimited by underscores.

As recommended in Guideline 3.1.4, abbreviations should be project-wide. An automated tool should allow a project to specify those abbreviations and format them accordingly.

3.1.4 Abbreviations

guideline

Do not use an abbreviation of a long word as an identifier where a shorter synonym exists.

Use a consistent abbreviation strategy.

Do not use ambiguous abbreviations.

To justify its use, an abbreviation must save many characters over the full word.

Use abbreviations that are well-accepted in the application domain.

Maintain a list of accepted abbreviations, and use only abbreviations on that list.

Many abbreviations are ambiguous or unintelligible unless taken in context. As an example, Temp could indicate either temporary or temperature. For this reason, you should choose abbreviations carefully when you use them. The rationale in Guideline 8.1.2 provides a more thorough discussion of how context should influence the use of abbreviations.

Because very long variable names can obscure the structure of the program, especially in deeply nested (indented) control structures, it is a good idea to try to keep identifiers short and meaningful. Use short unabbreviated names whenever possible. If there is no short word that will serve as an identifier, then a well-known unambiguous abbreviation is the next best choice, especially if it comes from a list of standard abbreviations used throughout the project.

You can establish an abbreviated format for a fully qualified name using the renames clause. This capability is useful when a very long, fully qualified name would otherwise occur many times in a localized section of code (see Guideline 5.7.2).

A list of accepted abbreviations for a project provides a standard context for using each abbreviation.

3.2.1 Names

guideline

Choose names that are as self-documenting as possible.

Use a short synonym instead of an abbreviation (see Guideline 3.1.4).

Use names given by the application, but do not use obscure jargon.

Avoid using the same name to declare different kinds of identifiers.

In a tree-walker, using the name Left instead of Left_Branch is sufficient to convey the full meaning given the context. However, use Time_Of_Day instead of TOD.

Mathematical formulas are often given using single-letter names for variables. Continue this convention for mathematical equations where they would recall the formula, for example:

   A*(X**2) + B*X + C.

With the use of child packages, a poor choice of package, subunit, and identifier names can lead to a visibility clash with subunits. See the Rationale (1995, §8.1) for a sample of the resulting, rather obscure code.

rationale

A program that follows these guidelines can be more easily comprehended. Self-documenting names require fewer explanatory comments. Empirical studies have shown that you can further improve comprehension if your variable names are not excessively long (Schneiderman 1986, 7). The context and application can help greatly. The unit of measure for numeric entities can be a source of subtype names.

You should try not to use the same name as an identifier for different declarations, such as an object and a child package. Overusing an identifier in seemingly different name spaces can, in fact, lead to visibility clashes if the enclosing program units are intended to work together.

notes

See Guideline 8.1.2 for a discussion on how to use the application domain as a guideline for selecting abbreviations.

3.2.2 Subtype Names

guideline

Use singular, general nouns as subtype identifiers.

Choose identifiers that describe one of the subtype's values.

Consider using suffixes for subtype identifiers that define visible access types, visible subranges, or visible array types.

For private types, do not use identifier constructions (e.g., suffixes) that are unique to subtype identifiers.

Do not use the subtype names from predefined packages.

When this style and the suggested style for object identifiers are used, program code more closely resembles English (see Guideline 3.2.3). Furthermore, this style is consistent with the names of the language's predefined identifiers. They are not named Integers, Booleans, Integer_Type, or Boolean_Type.

However, using the name of a subtype from the predefined packages is sure to confuse a programmer when that subtype appears somewhere without a package qualification.

notes

This style guide tries to be consistent with the Ada Reference Manual (1995) in use of the terms "type" and "subtype" name. In general, a "type" refers to the abstract concept, as in a type declaration, while the "subtype" refers to the name given to that abstract concept in an actual declaration. Thus, what was called a type name in Ada 83 (Ada Reference Manual 1983) is now called a subtype name.

3.2.3 Object Names

guideline

Use predicate clauses or adjectives for Boolean objects.

Use singular, specific nouns as object identifiers.

Choose identifiers that describe the object's value during execution.

Use singular, general nouns as identifiers for record components.

example

Non-Boolean objects:

Today           : Day;
Yesterday       : Day;
Retirement_Date : Date;

Boolean objects:

User_Is_Available : Boolean;        -- predicate clause
List_Is_Empty     : Boolean;        -- predicate clause
Empty             : Boolean;        -- adjective
Bright            : Boolean;        -- adjective

rationale

Using specific nouns for objects establishes a context for understanding the object's value, which is one of the general values described by the subtype's name (see Guideline 3.2.2). Object declarations become very English-like with this style. For example, the first declaration above is read as "Today is a Day."

General nouns, rather than specific, are used for record components because a record object's name will supply the context for understanding the component. Thus, the following component is understood as "the year of retirement":

Retirement_Date.Year

Following conventions that relate object types and parts of speech makes code read more like text. For example, because of the names chosen, the following code segment needs no comments:

if List_Is_Empty then
   Number_Of_Elements := 0;
else
   Number_Of_Elements := Length_Of_List;
end if;

notes

If it is difficult to find a specific noun that describes an object's value during the entire execution of a program, the object is probably serving multiple purposes. Multiple objects should be used in such a case.

3.2.4 Naming of Tagged Types and Associated Packages

guideline

Use a consistent naming convention for tagged types and associated packages.

instantiation

Naming conventions spark "religious wars"; therefore, two different instantiations are presented. The first instantiation integrates the use of object-oriented features. Except for two special cases, it applies the same naming conventions to declarations, independent of whether they use an object-oriented feature:

- Name tagged types no differently than subtype names (see Guideline 3.2.2).
- Use the prefix Abstract_ for packages that export an abstraction for which you intend to provide multiple implementations (see Guideline 9.2.4).
- Use the suffix _Mixin for packages that provide units of functionality that can be "mixed in" to core abstractions.

The second instantiation highlights the use of object-oriented features through special names or suffixes:

- Name class packages after the object they represent, without a suffix (Rosen 1995).
- Name mixin packages after the facet they represent, appending the suffix _Facet (Rosen 1995).
- Name the main tagged type Instance (Rosen 1995).
- Follow the declaration of the specific type with a subtype named Class for the corresponding class-wide type (Rosen 1995).

example

The following two-part example from the Rationale (1995, §§4.4.4 and 4.6.2) applies the naming conventions of the first instantiation.

For the first part of this example, assume the type Set_Element was declared elsewhere:

package Abstract_Sets is
   type Set is abstract tagged private;
   -- empty set
   function Empty return Set is abstract;
   -- build set with 1 element
   function Unit (Element: Set_Element) return Set is abstract;
   -- union of two sets
   function Union (Left, Right: Set) return Set is abstract;
   -- intersection of two sets
   function Intersection (Left, Right: Set) return Set is abstract;
   -- remove an element from a set
   procedure Take (From    : in out Set;
                   Element :    out set_Element) is abstract;
   Element_Too_Large : exception;
private
   type Set is abstract tagged null record;
end Abstract_Sets;
with Abstract_Sets;
package Bit_Vector_Sets is   -- one implementation of set abstraction
   type Bit_Set is new Abstract_Sets.Set with private;
   ...
private
   Bit_Set_Size : constant := 64;
   type Bit_Vector is ...
   type Bit_Set is new Abstract_Sets.Set with
      record
         Data : Bit_Vector;
      end record;
end Bit_Vector_Sets;
with Abstract_Sets;
package Sparse_Sets  -- alternate implementation of set abstraction
   type Sparse_Set is new Abstract_Sets.Set with private;
   ...
private
   ...
end Bit_Vector_Sets;

The second part of this example applies the naming convention to mixin packages that support a windowing system:

-- assume you have type Basic_Window is tagged limited private;
generic
   type Some_Window is abstract new Basic_Window with private;
package Label_Mixin is 
   type Window_With_Label is abstract new Some_Window with private;
   ...
private
   ...
end Label_Mixin;
generic
   type Some_Window is abstract new Basic_Window with private;
package Border_Mixin is 
   type Window_With_Label is abstract new Some_Window with private;
   ...
private
   ...
end Border_Mixin;

The following example applies the naming conventions of the second instantiation, as discussed in Rosen (1995):

package Shape is
   subtype Side_Count is range 0 .. 100;
   type Instance (Sides: Side_Count) is tagged private;
   subtype Class is Instance'Class;
   . . .
   -- operations on Shape.Instance
private
   . . .
end Shape;
with Shape; use Shape;
package Line is
   type Instance is new Shape.Instance with private;
   subtype Class is Instance'Class;
   . . .
   -- Overridden or new operations
private
   . . .
end Line;
with Shape; use Shape;
generic
   type Origin is new Shape.Instance;
package With_Color_Facet is
   type Instance is new Origin with private;
   subtype Class is Instance'Class;
   -- operations for colored shapes
private
   . . .
end With_Color_Facet;
with Line; use Line;
with With_Color_Facet;
package Colored_Line is new With_Color_Facet (Line.Instance);

Sample declarations might look like:

Red_Line : Colored_Line.Instance;
procedure Draw (What : Shape.Instance);

The above scheme works whether you use full names or a use clause. As long as you use the same name for all the specific types (i.e., type Instance) and class-wide types, the unqualified names will always hide one another. Thus, the compiler will insist you use full name qualification to resolve the ambiguity introduced by the use clause (Rosen 1995).

rationale

You want to use a naming scheme that is consistent and readable and conveys the intent of the abstraction. Ideally, the naming scheme should be uniform in how it handles the different ways in which tagged types are used to create classes. If the naming convention is too rigid, however, you will write code fragments that appear stilted from a readability point of view. By using a similar naming convention for type extension through derivation and through generic mixin (see also Guideline 9.5.1), you achieve readable declarations of objects and procedures.

notes

A naming convention for classes draws a hard line between object-oriented abstractions and other kinds of abstractions. Given that engineers have been defining abstract data types in Ada 83 (Ada Reference Manual 1983) for over 10 years, you may not want to change the naming convention just for the sake of using type extension with a type. You must consider how important it is to call out uses of inheritance in the overall use of abstractions in your program. If you prefer to emphasize abstraction, in general, over the mechanism used to implement the abstraction (i.e., inheritance, type-extension, and polymorphism), you may not want to impose such a stringent naming convention. You do not hamper quality by favoring a smoother transition in naming conventions from abstractions developed without inheritance to those developed with inheritance.

If you choose a naming convention that highlights the use of object-oriented features and later decide to change the declaration to one that does not use an object-oriented feature, the change may be expensive. You must naturally change all occurrences of the names and must be careful not to introduce errors as you update the names. If you choose a naming convention that prohibits the use of suffixes or prefixes to characterize the declaration, you lose the opportunity to convey the intended usage of the declared item.

3.2.5 Program Unit Names

guideline

Use action verbs for procedures and entries.

Use predicate clauses for Boolean functions.

Use nouns for non-Boolean functions.

Give packages names that imply a higher level of organization than subprograms. Generally, these are noun phrases that describe the abstraction provided.

Give tasks names that imply an active entity.

Use nouns descriptive of the data being protected for protected units.

Consider naming generic subprograms as if they were nongeneric subprograms.

Consider naming generic packages as if they were nongeneric packages.

Make the generic names more general than the instantiated names.

example

The following are sample names for elements that compose an Ada program:

Sample procedure names:

procedure Get_Next_Token          -- get is a transitive verb
procedure Create                  -- create is a transitive verb

Sample function names for Boolean-valued functions:

function Is_Last_Item             -- predicate clause
function Is_Empty                 -- predicate clause

Sample function names for non-Boolean-valued functions:

function Successor                -- common noun
function Length                   -- attribute
function Top                      -- component

Sample package names:

package Terminals is               -- common noun
package Text_Routines is           -- common noun

Sample protected objects:

protected Current_Location is      -- data being protected
protected type Guardian is         -- noun implying protection

Sample task names:

task Terminal_Resource_Manager is  -- common noun that shows action

The following sample piece of code shows the clarity that results from using the parts-of-speech naming conventions:

Get_Next_Token(Current_Token);
case Current_Token is
   when Identifier =>         Process_Identifier;
   when Numeric    =>         Process_Numeric;
end case;  -- Current_Token
if Is_Empty(Current_List) then
   Number_Of_Elements := 0;
else
   Number_Of_Elements := Length(Current_List);
end if;

When packages and their subprograms are named together, the resulting code is very descriptive:

if Stack.Is_Empty(Current_List) then
   Current_Token := Stack.Top(Current_List);
end if;

rationale

Using these naming conventions creates understandable code that reads much like natural language. When verbs are used for actions, such as subprograms, and nouns are used for objects, such as the data that the subprogram manipulates, code is easier to read and understand. This models a medium of communication already familiar to a reader. Where the pieces of a program model a real-life situation, using these conventions reduces the number of translation steps involved in reading and understanding the program. In a sense, your choice of names reflects the level of abstraction from computer hardware toward application requirements.

See also Guideline 3.2.4 for the use of special-purpose suffixes in packages associated with tagged types.

notes

There are some conflicting conventions in current use for task entries. Some programmers and designers advocate naming task entries with the same conventions used for subprograms to blur the fact that a task is involved. Their reasoning is that if the task is reimplemented as a package, or vice versa, the names need not change. Others prefer to make the fact of a task entry as explicit as possible to ensure that the existence of a task with its presumed overhead is recognizable. Project-specific priorities may be useful in choosing between these conventions.

3.2.6 Constants and Named Numbers

guideline

Use symbolic values instead of literals, wherever possible.

Use the predefined constants Ada.Numerics.Pi and Ada.Numerics.e for the mathematical constants Pi and e.

Use constants instead of variables for constant values.

Use a constant when the value is specific to a type or when the value must be static.

Use named numbers instead of constants, whenever possible.

Use named numbers to replace numeric literals whose type or context is truly universal.

Use constants for objects whose values cannot change after elaboration (United Technologies 1987).

Show relationships between symbolic values by defining them with static expressions.

Use linearly independent sets of literals.

Use attributes like 'First and 'Last instead of literals, wherever possible.

example

3.14159_26535_89793                                 -- literal
Max_Entries : constant Integer       := 400;        -- constant
Avogadros_Number  : constant := 6.022137 * 10**23;  -- named number
Avogadros_Number / 2                                -- static expression
Avogadros_Number                                    -- symbolic value

Declaring Pi as a named number (assuming a with clause for the predefined package Ada.Numerics in the Ada Reference Manual [1995, §A.5] allows it to be referenced symbolically in the assignment statement below:

Area :=       Pi * Radius**2;       -- if radius is known.

instead of:

Area := 3.14159 * Radius**2;        -- Needs explanatory comment.

Also, Ada.Characters.Latin_1.Bel is more expressive than Character'Val(8#007#).

Clarity of constant and named number declarations can be improved by using other constant and named numbers. For example:

Bytes_Per_Page   : constant := 512;
Pages_Per_Buffer : constant := 10;
Buffer_Size      : constant := Pages_Per_Buffer * Bytes_Per_Page;

is more self-explanatory and easier to maintain than:

Buffer_Size : constant := 5_120;   -- ten pages

The following literals should be constants:

if New_Character  = '$' then  -- "constant" that may change
...
if Current_Column = 7 then    -- "constant" that may change

rationale

Using identifiers instead of literals makes the purpose of expressions clear, reducing the need for comments. Constant declarations consisting of expressions of numeric literals are safer because they do not need to be computed by hand. They are also more enlightening than a single numeric literal because there is more opportunity for embedding explanatory names. Clarity of constant declarations can be improved further by using other related constants in static expressions defining new constants. This is not less efficient because static expressions of named numbers are computed at compile time.

A constant has a type. A named number can only be a universal type: universal_integer or universal_real. Strong typing is enforced for constants but not for named numbers or literals. Named numbers allow compilers to generate more efficient code than for constants and to perform more complete error checking at compile time. If the literal contains a large number of digits (as Pi in the example above), the use of an identifier reduces keystroke errors. If keystroke errors occur, they are easier to locate either by inspection or at compile time.

Independence of literals means that the few literals that are used do not depend on one another and that any relationship between constant or named values is shown in the static expressions. Linear independence of literal values gives the property that if one literal value changes, all of the named numbers of values dependent on that literal are automatically changed.

See Guideline 4.1.4 for additional guidelines on choosing a parameterless function versus a constant.

notes

There are some gray areas where the literal is actually more self-documenting than a name. These are application-specific and generally occur with universally familiar, unchangeable values such as the following relationship:

Fahrenheit := 32.0 + (9.0 * Celsius) / 5.0;

3.2.7 Exceptions

guideline

Use a name that indicates the kind of problem the exception represents.

example

Invalid_Name: exception;
Stack_Overflow: exception;

rationale

Naming exceptions according to the kind of problem they are detecting enhances the readability of the code. You should name your exceptions as precisely as you can so that the maintainer of the code understands why the exception might be raised. A well-named exception should be meaningful to the clients of the package declaring the exception.

3.2.8 Constructors

guideline

• Include a prefix like New, Make, or Create in naming constructors (in this sense, operations to create and/or initialize an object).
  
• Use names indicative of their content for child packages containing constructors.

For information regarding the use of Ada constructors, refer to Guideline 9.3.3.

Maintenance programmers need to know the causal interaction of noncontiguous pieces of code to get a global, more or less complete sense of the program. They typically acquire this kind of information from mental simulation of parts of the code. Comments should be sufficient enough to support this process (Soloway et al. 1986).

This section presents general guidelines about how to write good comments. It then defines several different classes of comments with guidelines for the use of each. The classes are file headers, program unit specification headers, program unit body headers, data comments, statement comments, and marker comments.

3.3.1 General Comments

guideline

Make the code as clear as possible to reduce the need for comments.

Never repeat information in a comment that is readily available in the code.

Where a comment is required, make it concise and complete.

Use proper grammar and spelling in comments.

Make comments visually distinct from the code.

Structure comments in header so that information can be automatically extracted by a tool.

3.3.2 File Headers

guideline

Put a file header on each source file.

Place ownership, responsibility, and history information for the file in the file header.