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@c %**start of header
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@c                                                                            o
@c                           GNAT DOCUMENTATION                               o
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@c                      G N A T   C O D I N G   S T Y L E                     o
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@c                               $Revision: 1.5 $
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@c          Copyright (C) 1992-2001 Ada Core Technologies, Inc.               o
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@setfilename gnat-style.info
@settitle GNAT Coding Style
@dircategory Programming
@direntry 
* gnat-style: (gnat-style).      GNAT Coding Style
@end direntry

@macro syntax{element}
@t{\element\}
@end macro
@c %**end of header

@ifinfo
     @center GNAT Coding Style

     @center A guide for GNAT developers
     Copyright (C) 1992-2001 Ada Core Technologies, Inc.
@end ifinfo

@titlepage
@sp 10
@title GNAT Coding Style
@subtitle A guide for GNAT developers
@subtitle Document revision level $Revision: 1.5 $
@author Ada Core Technologies, Inc.
@end titlepage
@raisesections

@node    Top,       General,         , (dir)
@comment node-name, next,    previous, up

@menu
* General::
* Lexical Elements::
* Declarations and Types::
* Expressions and Names::
* Statements::
* Subprograms::
* Packages::
* Program Structure::
@end menu

@c  -------------------------------------------------------------------------
@node    General, Lexical Elements, Top, Top
@section General
@c  -------------------------------------------------------------------------

@noindent
Most of GNAT is written in Ada using a consistent style to ensure
readability of the code.  This document has been written to help
maintain this consistent style, while having a large group of developers 
work on the compiler.

@noindent
For the coding style in the C parts of the compiler and run time,
see the GNU Coding Guidelines.

@noindent
This document is structured after the Ada Reference manual. 
Those familiar with that document should be able to quickly 
lookup style rules for particular constructs.

@c  -------------------------------------------------------------------------
@node    Lexical Elements, Declarations and Types, General, Top
@section Lexical Elements
@c  -------------------------------------------------------------------------

@subsection Character Set and Separators
@c  -------------------------------------------------------------------------

@itemize @bullet
@item
The character set used should be plain 7-bit ASCII@.
The only separators allowed are space and the end-of-line sequence. 
No other control character or format effector (such as HT, VT, FF) 
should be used.
The normal end-of-line sequence is used, which may be LF, CR/LF or CR,
depending on the host system.  An optional SUB (16#1A#) may be present as the 
last character in the file on hosts using that character as file terminator.

@item
Files that are checked in or distributed should be in host format.

@item
A line should never be longer than 79 characters, not counting the line
separator.

@item
Lines must not have trailing blanks.

@item
Indentation is 3 characters per level for if statements, loops, case
statements.  For exact information on required spacing between lexical
elements, see file @file{style.adb}.

@end itemize

@subsection Identifiers
@c  -------------------------------------------------------------------------
@itemize @bullet
@item
Identifiers will start with an upper case letter, and each letter following
an underscore will be upper case.  Short acronyms may be all upper case. 
All other letters are lower case.  
An exception is for identifiers matching a foreign language.  In particular,
we use all lower case where appropriate for C@.

@item
Use underscores to separate words in an identifier.  

@item Try to limit your use of abbreviations in identifiers. 
It is ok to make a few abbreviations, explain what they mean, and then
use them frequently, but don't use lots of obscure abbreviations.  An
example is the @code{ALI} word which stands for Ada Library
Information and is by convention always written in upper-case when
used in entity names.

@smallexample
       procedure Find_ALI_Files;
@end smallexample

@item
Don't use the variable @samp{I}, use @samp{J} instead, @samp{I} is too
easily mixed up with @samp{1} in some fonts.  Similarly don't use the
variable @samp{O}, which is too easily mixed up with @samp{0}.
@end itemize

@subsection Numeric Literals
@c  -------------------------------------------------------------------------
@itemize @bullet
@item
Numeric literals should include underscores where helpful for
readability.

@smallexample
      1_000_000
      16#8000_000#
      3.14159_26535_89793_23846
@end smallexample
@end itemize

@subsection Reserved Words
@c  -------------------------------------------------------------------------
@itemize @bullet
@item
Reserved words use all lower case.

@smallexample
       return else
@end smallexample

@item
The words @samp{Access}, @samp{Delta} and @samp{Digits} are
capitalized when used as @syntax{attribute_designator}.
@end itemize

@subsection Comments
@c  -------------------------------------------------------------------------

@itemize @bullet
@item
Comment start with @samp{--  } (i.e.@: @samp{--} followed by two spaces).
The only exception to this rule (i.e.@: one space is tolerated) is when the
comment ends with @samp{ --}.
It also accepted to have only one space between @samp{--} and the start 
of the comment when the comment is at the end of a line, 
after some Ada code.

@item
Every sentence in a comment should start with an upper-case letter (including
the first letter of the comment).

@item
When declarations are commented with ``hanging'' comments, i.e.@:
comments after the declaration, there is no blank line before the
comment, and if it is absolutely necessary to have blank lines within
the comments these blank lines @emph{do} have a @samp{--} (unlike the
normal rule, which is to use entirely blank lines for separating
comment paragraphs).  The comment start at same level of indentation
as code they are commenting.

@smallexample
       z : Integer;
       --  @r{Integer value for storing value of} z
       --
       --  @r{The previous line was a blank line.}
@end smallexample

@item
Comments that are dubious or incomplete or comment on possibly
wrong or incomplete code should be preceded or followed by @samp{???}@.

@item
Comments in a subprogram body must generally be surrounded by blank lines,
except after a @samp{begin}:

@smallexample
       begin
          --  @r{Comment for the next statement}

          A := 5;

          --  @r{Comment for the B statement}

          B := 6;
@end smallexample

@item
In sequences of statements, comments at the end of the lines should be
aligned.

@smallexample
        My_Identifier := 5;      --  @r{First comment}
        Other_Id := 6;           --  @r{Second comment}
@end smallexample

@item
Short comments that fit on a single line are @emph{not} ended with a
period.  Comments taking more than a line are punctuated in the normal
manner.

@item
Comments should focus on why instead of what.
Descriptions of what subprograms do go with the specification.

@item
Comments describing a subprogram spec should specifically mention the
formal argument names.  General rule: write a comment that does not
depend on the names of things.  The names are supplementary, not
sufficient, as comments.

@item
Do NOT put two spaces after periods in comments.
@end itemize

@c  -------------------------------------------------------------------------
@node    Declarations and Types, Expressions and Names, Lexical Elements,Top
@section Declarations and Types
@c  -------------------------------------------------------------------------

@itemize @bullet
@item
In entity declarations, colons must be surrounded by spaces.  Colons
should be aligned.

@smallexample
        Entity1   : Integer;
        My_Entity : Integer;
@end smallexample

@item
Declarations should be grouped in a logical order. 
Related groups of declarations may be preceded by a header comment.

@item
All local subprograms in a subprogram or package body should be declared
before the first local subprogram body. 

@item
Don't declare local entities that hide global entities.

@item
Don't declare multiple variables in one declaration that spans lines. 
Start a new declaration on each line, instead.

@item
The @syntax{defining_identifier}s of global declarations serve as
comments of a sort.  So don't choose terse names, but look for names
that give useful information instead.

@item
Local names can be shorter, because they are used only within 
one context, where comments explain their purpose.

@end itemize


@c  -------------------------------------------------------------------------
@node    Expressions and Names, Statements, Declarations and Types, Top
@section Expressions and Names
@c  -------------------------------------------------------------------------

@itemize @bullet

@item
Every operator must be surrounded by spaces, except for the 
exponentiation operator.

@smallexample
       E := A * B**2 + 3 * (C - D);
@end smallexample

@item
When folding a long line, fold before an operator, not after.

@item
Use parentheses where they make the intended order of evaluation clearer:
@smallexample
       (A / B) * C
@end smallexample
@end itemize

@c  -------------------------------------------------------------------------
@node    Statements, Subprograms, Expressions and Names, Top
@section Statements
@c  -------------------------------------------------------------------------

@subsection Simple and Compound Statements
@c  -------------------------------------------------------------------------
@itemize @bullet
@item
Use only one statement or label per line.
@item
A longer @syntax{sequence_of_statements} may be divided in logical
groups or separated from surrounding code using a blank line.
@end itemize

@subsection If Statements
@c  -------------------------------------------------------------------------
@itemize @bullet
@item
When the @samp{if}, @samp{elsif} or @samp{else} keywords fit on the
same line with the condition and the @samp{then} keyword, then the
statement is formatted as follows:

@smallexample
        if @var{condition} then
           ...
        elsif @var{condition} then
           ...
        else
           ...
        end if;
@end smallexample

@noindent
When the above layout is not possible, @samp{then} should be aligned
with @samp{if}, and conditions should preferably be split before an
@samp{and} or @samp{or} keyword a follows:

@smallexample
        if @var{long_condition_that_has_to_be_split}
          and then @var{continued_on_the_next_line}
        then
           ...
        end if;
@end smallexample

@noindent
The @samp{elsif}, @samp{else} and @samp{end if} always line up with
the @samp{if} keyword.  The preferred location for splitting the line
is before @samp{and} or @samp{or}.  The continuation of a condition is
indented with two spaces or as many as needed to make nesting clear.
As exception, if conditions are closely related either of the
following is allowed:

@smallexample
     if x = lakdsjfhlkashfdlkflkdsalkhfsalkdhflkjdsahf
          or else
        x = asldkjhalkdsjfhhfd
          or else
        x = asdfadsfadsf
     then

     if x = lakdsjfhlkashfdlkflkdsalkhfsalkdhflkjdsahf or else
        x = asldkjhalkdsjfhhfd                         or else
        x = asdfadsfadsf
     then
@end smallexample

@item
Conditions should use short-circuit forms (@samp{and then}, 
@samp{or else}).

@item
Complex conditions in if statements are indented two characters:

@smallexample
      if @var{this_complex_condition}
        and then @var{that_other_one}
        and then @var{one_last_one}
      then
         ...
@end smallexample

@item
Every @samp{if} block is preceded and followed by a blank line, except
where it begins or ends a @syntax{sequence_of_statements}.

@smallexample
        A := 5;

        if A = 5 then
           null;
        end if;

        A := 6;
@end smallexample
@end itemize

@subsection Case Statements
@itemize @bullet

@item
Layout is as below.  For long case statements, the extra indentation
can be saved by aligning the when clauses with the opening case.

@smallexample
       case @var{expression} is
          when @var{condition} =>
             ...
          when @var{condition} =>
             ...
       end case;
@end smallexample
@end itemize

@subsection Loop Statements
@itemize @bullet

@noindent
When possible, have @samp{for} or @samp{while} on one line with the
condition and the @samp{loop} keyword.

@smallexample
       for J in S'Range loop
          ...
       end loop;
@end smallexample

@noindent
If the condition is too long, split the condition (see ``If
statements'' above) and align @samp{loop} with the @samp{for} or
@samp{while} keyword.

@smallexample
      while @var{long_condition_that_has_to_be_split}
        and then @var{continued_on_the_next_line}
      loop
         ...
      end loop;
@end smallexample

@noindent
If the @syntax{loop_statement} has an identifier, it is laid out as follows:

@smallexample
      Outer : while not @var{condition} loop
         ...
      end Outer;
@end smallexample
@end itemize

@subsection Block Statements
@itemize @bullet

@item
The @samp{declare} (optional), @samp{begin} and @samp{end} statements
are aligned, except when the @syntax{block_statement} is named.  There
is a blank line before the @samp{begin} keyword:

@smallexample
      Some_Block : declare
         ...

      begin
         ...
      end Some_Block;
@end smallexample

@end itemize

@c  -------------------------------------------------------------------------
@node    Subprograms, Packages, Statements, Top
@section Subprograms
@c  -------------------------------------------------------------------------


@subsection Subprogram Declarations
@c  -------------------------------------------------------------------------
@itemize @bullet

@item
Do not write the @samp{in} for parameters, especially in functions:

@smallexample
      function Length (S : String) return Integer;
@end smallexample

@item
When the declaration line for a procedure or a function is too long, fold it.
In this case, align the colons, and, for functions, the result type.

@smallexample
      function Head
        (Source : String;
         Count  : Natural;
         Pad    : Character := Space)
         return   String;
@end smallexample

@item
The parameter list for a subprogram is preceded by a space:

@smallexample
        procedure Func (A : Integer);
@end smallexample

@end itemize

@subsection Subprogram Bodies
@c  -------------------------------------------------------------------------
@itemize @bullet

@item
The functions and procedures should always be sorted alphabetically in
a compilation unit.

@item
All subprograms have a header giving the function name, with the following
format:

@smallexample
      -----------------
      -- My_Function --
      -----------------

      procedure My_Function is
      begin
@end smallexample

Note that the name in the header is preceded by a single space, 
not two spaces as for other comments.

@item
Every subprogram body must have a preceding @syntax{subprogram_declaration}.

@item
If there any declarations in a subprogram, the @samp{begin} keyword is
preceded by a blank line.

@item
If the declarations in a subprogram contain at least one nested
subprogram body, then just before the of the enclosing subprogram
@samp{begin}, there is a line:

@smallexample
        --  @r{Start of processing for @var{Enclosing_Subprogram}}

	begin
@end smallexample

@end itemize

@c  -------------------------------------------------------------------------
@node    Packages, Program Structure, Subprograms, Top
@section Packages and Visibility Rules
@c  -------------------------------------------------------------------------

@itemize @bullet

@item
All program units and subprograms have their name at the end:

@smallexample
      package P is
         ...
      end P;
@end smallexample

@item
We will use the style of @samp{use}-ing @samp{with}-ed packages, with
the context clauses looking like:

@smallexample
      with A; use A;
      with B; use B;
@end smallexample

@item
Names declared in the visible part of packages should be
unique, to prevent name clashes when the packages are @samp{use}d.

@smallexample
      package Entity is
         type Entity_Kind is ...;
	 ...
      end Entity;
@end smallexample

@item
After the file header comment, the context clause and unit specification
should be the first thing in a @syntax{program_unit}.
@end itemize

@c  -------------------------------------------------------------------------
@node    Program Structure,, Packages, Top
@section Program Structure and Compilation Issues
@c  -------------------------------------------------------------------------

@itemize @bullet
@item
Every GNAT source file must be compiled with the @option{-gnatg}
switch to check the coding style (Note that you should look at
@file{style.adb} to see the lexical rules enforced by
@option{-gnatg}).

@item
Each source file should contain only one compilation unit.

@item
Filenames should be 8 characters or less followed by the @samp{.adb}
extension for a body or @samp{.ads} for a spec.

@item
Unit names should be distinct when krunched to 8 characters 
(see @file{krunch.ads}) and the filenames should match the unit name, 
except that they are all lower case.
@end itemize

@bye
