3.2.4 Style Checking

The -gnatyx switch causes the compiler to enforce specified style rules. A limited set of style rules has been used in writing the GNAT sources themselves. This switch allows user programs to activate all or some of these checks. If the source program fails a specified style check, an appropriate warning message is given, preceded by the character sequence "(style)". The string x is a sequence of letters or digits indicating the particular style checks to be performed. The following checks are defined:

1-9 (specify indentation level)

If a digit from 1-9 appears in the string after -gnaty then proper indentation is checked, with the digit indicating the indentation level required. The general style of required indentation is as specified by the examples in the Ada Reference Manual. Full line comments must be aligned with the -- starting on a column that is a multiple of the alignment level.

a (check attribute casing)

If the letter a appears in the string after -gnaty then attribute names, including the case of keywords such as digits used as attributes names, must be written in mixed case, that is, the initial letter and any letter following an underscore must be uppercase. All other letters must be lowercase.

b (blanks not allowed at statement end)

If the letter b appears in the string after -gnaty then trailing blanks are not allowed at the end of statements. The purpose of this rule, together with h (no horizontal tabs), is to enforce a canonical format for the use of blanks to separate source tokens.

c (check comments)

If the letter c appears in the string after -gnaty then comments must meet the following set of rules:

• The "–" that starts the column must either start in column one, or else at least one blank must precede this sequence.
• Comments that follow other tokens on a line must have at least one blank following the "–" at the start of the comment.
• Full line comments must have two blanks following the "–" that starts the comment, with the following exceptions.
• A line consisting only of the "–" characters, possibly preceded by blanks is permitted.
• A comment starting with "–x" where x is a special character is permitted. This alows proper processing of the output generated by specialized tools including gnatprep (where –! is used) and the SPARK annnotation language (where –# is used). For the purposes of this rule, a special character is defined as being in one of the ASCII ranges 16#21#..16#2F# or 16#3A#..16#3F#.
• A line consisting entirely of minus signs, possibly preceded by blanks, is permitted. This allows the construction of box comments where lines of minus signs are used to form the top and bottom of the box.
• If a comment starts and ends with "–" is permitted as long as at least one blank follows the initial "–". Together with the preceding rule, this allows the construction of box comments, as shown in the following example:

                 ---------------------------
                 -- This is a box comment --
                 -- with two text lines.  --
                 ---------------------------
            

e (check end/exit labels)

If the letter e appears in the string after -gnaty then optional labels on end statements ending subprograms and on exit statements exiting named loops, are required to be present.

f (no form feeds or vertical tabs)

If the letter f appears in the string after -gnaty then neither form feeds nor vertical tab characters are not permitted in the source text.

h (no horizontal tabs)

If the letter h appears in the string after -gnaty then horizontal tab characters are not permitted in the source text. Together with the b (no blanks at end of line) check, this enforces a canonical form for the use of blanks to separate source tokens.

i (check if-then layout)

If the letter i appears in the string after -gnaty, then the keyword then must appear either on the same line as corresponding if, or on a line on its own, lined up under the if with at least one non-blank line in between containing all or part of the condition to be tested.

k (check keyword casing)

If the letter k appears in the string after -gnaty then all keywords must be in lower case (with the exception of keywords such as digits used as attribute names to which this check does not apply).

l (check layout)

If the letter l appears in the string after -gnaty then layout of statement and declaration constructs must follow the recommendations in the Ada Reference Manual, as indicated by the form of the syntax rules. For example an else keyword must be lined up with the corresponding if keyword.

There are two respects in which the style rule enforced by this check option are more liberal than those in the Ada Reference Manual. First in the case of record declarations, it is permissible to put the record keyword on the same line as the type keyword, and then the end in end record must line up under type. For example, either of the following two layouts is acceptable:

          

          type q is record
             a : integer;
             b : integer;
          end record;
          
          type q is
             record
                a : integer;
                b : integer;
             end record;
          
     

Second, in the case of a block statement, a permitted alternative is to put the block label on the same line as the declare or begin keyword, and then line the end keyword up under the block label. For example both the following are permitted:

          

          Block : declare
             A : Integer := 3;
          begin
             Proc (A, A);
          end Block;
          
          Block :
             declare
                A : Integer := 3;
             begin
                Proc (A, A);
             end Block;
          
     

The same alternative format is allowed for loops. For example, both of the following are permitted:

          

          Clear : while J < 10 loop
             A (J) := 0;
          end loop Clear;
          
          Clear :
             while J < 10 loop
                A (J) := 0;
             end loop Clear;
          
     

m (check maximum line length)

If the letter m appears in the string after -gnaty then the length of source lines must not exceed 79 characters, including any trailing blanks. The value of 79 allows convenient display on an 80 character wide device or window, allowing for possible special treatment of 80 character lines.

Mnnn (set maximum line length)

If the sequence Mnnn, where nnn is a decimal number, appears in the string after -gnaty then the length of lines must not exceed the given value.

n (check casing of entities in Standard)

If the letter n appears in the string after -gnaty then any identifier from Standard must be cased to match the presentation in the Ada Reference Manual (for example, Integer and ASCII.NUL).

o (check order of subprogram bodies)

If the letter o appears in the string after -gnaty then all subprogram bodies in a given scope (e.g. a package body) must be in alphabetical order. The ordering rule uses normal Ada rules for comparing strings, ignoring casing of letters, except that if there is a trailing numeric suffix, then the value of this suffix is used in the ordering (e.g. Junk2 comes before Junk10).

p (check pragma casing)

If the letter p appears in the string after -gnaty then pragma names must be written in mixed case, that is, the initial letter and any letter following an underscore must be uppercase. All other letters must be lowercase.

r (check references)

If the letter r appears in the string after -gnaty then all identifier references must be cased in the same way as the corresponding declaration. No specific casing style is imposed on identifiers. The only requirement is for consistency of references with declarations.

s (check separate specs)

If the letter s appears in the string after -gnaty then separate declarations ("specs") are required for subprograms (a body is not allowed to serve as its own declaration). The only exception is that parameterless library level procedures are not required to have a separate declaration. This exception covers the most frequent form of main program procedures.

t (check token spacing)

If the letter t appears in the string after -gnaty then the following token spacing rules are enforced:

• The keywords abs and not must be followed by a space.
• The token => must be surrounded by spaces.
• The token <> must be preceded by a space or a left parenthesis.
• Binary operators other than ** must be surrounded by spaces. There is no restriction on the layout of the ** binary operator.
• Colon must be surrounded by spaces.
• Colon-equal (assignment) must be surrounded by spaces.
• Comma must be the first non-blank character on the line, or be immediately preceded by a non-blank character, and must be followed by a space.
• If the token preceding a left paren ends with a letter or digit, then a space must separate the two tokens.
• A right parenthesis must either be the first non-blank character on a line, or it must be preceded by a non-blank character.
• A semicolon must not be preceded by a space, and must not be followed by a non-blank character.
• A unary plus or minus may not be followed by a space.
• A vertical bar must be surrounded by spaces.

In the above rules, appearing in column one is always permitted, that is, counts as meeting either a requirement for a required preceding space, or as meeting a requirement for no preceding space.

Appearing at the end of a line is also always permitted, that is, counts as meeting either a requirement for a following space, or as meeting a requirement for no following space.

If any of these style rules is violated, a message is generated giving details on the violation. The initial characters of such messages are always "(style)". Note that these messages are treated as warning messages, so they normally do not prevent the generation of an object file. The -gnatwe switch can be used to treat warning messages, including style messages, as fatal errors.

The switch -gnaty on its own (that is not followed by any letters or digits), is equivalent to gnaty3abcefhiklmprst, that is all checking options are enabled with the exception of -gnatyo, with an indentation level of 3. This is the standard checking option that is used for the GNAT sources.