# GNAT Coding Style A Guide for GNAT Developers ¶

> GNAT Coding Style: A Guide for GNAT Developers , Jan 03, 2025

> AdaCore

> Copyright © 2008-2025, Free Software Foundation

## Table of Contents

## 1 General ¶

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.

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

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.

## 2 Lexical Elements ¶

### 2.1 Character Set and Separators ¶

- 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.
- Files that are checked in or distributed should be in host format.
- A line should never be longer than 79 characters, not counting the line separator.
- Lines must not have trailing blanks.
- Indentation is 3 characters per level for `if` statements, loops, and `case` statements. For exact information on required spacing between lexical elements, see file style.adb.

### 2.2 Identifiers ¶

- 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.

- Use underscores to separate words in an identifier.
- 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 `ALI` word which stands for Ada Library Information and is by convention always written in upper-case when used in entity names.

      procedure Find_ALI_Files;

- Don’t use the variable name `I`, use `J` instead; `I` is too easily confused with `1` in some fonts. Similarly don’t use the variable `O`, which is too easily mistaken for the number `0`.

### 2.3 Numeric Literals ¶

- Numeric literals should include underscores where helpful for readability.

      1_000_000
      16#8000_0000#
      3.14159_26535_89793_23846

### 2.4 Reserved Words ¶

- Reserved words use all lower case.

      return else

- The words `Access`, `Delta` and `Digits` are capitalized when used as attribute_designator.

### 2.5 Comments ¶

- A comment starts with `--` followed by two spaces. The only exception to this rule (i.e. one space is tolerated) is when the comment ends with a single space followed by `--`. It is also acceptable to have only one space between `--` and the start of the comment when the comment is at the end of a line, after some Ada code.
- Every sentence in a comment should start with an upper-case letter (including the first letter of the comment).
- 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, e.g. to make paragraph separations within a single comment, these blank lines ‘do’ have a `--` (unlike the normal rule, which is to use entirely blank lines for separating comment paragraphs). The comment starts at same level of indentation as code it is commenting.

      z : Integer;
      --  Integer value for storing value of z
      --
      --  The previous line was a blank line.

- Comments that are dubious or incomplete, or that comment on possibly wrong or incomplete code, should be preceded or followed by `???`.
- Comments in a subprogram body must generally be surrounded by blank lines. An exception is a comment that follows a line containing a single keyword ( `begin`, `else`, `loop` ):

      begin
         --  Comment for the next statement

         A := 5;

         --  Comment for the B statement

         B := 6;
      end;

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

      My_Identifier := 5;      --  First comment
      Other_Id := 6;           --  Second comment

- Short comments that fit on a single line are ‘not’ ended with a period. Comments taking more than a line are punctuated in the normal manner.
- Comments should focus on ‘why’ instead of ‘what’. Descriptions of what subprograms do go with the specification.
- 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.
- ‘Do not’ put two spaces after periods in comments.

## 3 Declarations and Types ¶

- In entity declarations, colons must be surrounded by spaces. Colons should be aligned.

      Entity1   : Integer;
      My_Entity : Integer;

- Declarations should be grouped in a logical order. Related groups of declarations may be preceded by a header comment.
- All local subprograms in a subprogram or package body should be declared before the first local subprogram body.
- Do not declare local entities that hide global entities.
- Do not declare multiple variables in one declaration that spans lines. Start a new declaration on each line, instead.
- The defining_identifiers of global declarations serve as comments of a sort. So don’t choose terse names, but look for names that give useful information instead.
- Local names can be shorter, because they are used only within one context, where comments explain their purpose.
- When starting an initialization or default expression on the line that follows the declaration line, use 2 characters for indentation.

      Entity1 : Integer :=
        Function_Name (Parameters, For_Call);

- If an initialization or default expression needs to be continued on subsequent lines, the continuations should be indented from the start of the expression.

      Entity1 : Integer := Long_Function_Name
                             (parameters for call);

## 4 Expressions and Names ¶

- Every operator must be surrounded by spaces. An exception is that this rule does not apply to the exponentiation operator, for which there are no specific layout rules. The reason for this exception is that sometimes it makes clearer reading to leave out the spaces around exponentiation.

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

- Use parentheses where they clarify the intended association of operands with operators:

      (A / B) * C

## 5 Statements ¶

### 5.1 Simple and Compound Statements ¶

- Use only one statement or label per line.
- A longer sequence_of_statements may be divided in logical groups or separated from surrounding code using a blank line.

### 5.2 If Statements ¶

- When the `if`, `elsif` or `else` keywords fit on the same line with the condition and the `then` keyword, then the statement is formatted as follows:

      if condition then
         ...
      elsif condition then
         ...
      else
         ...
      end if;

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

      if long_condition_that_has_to_be_split
        and then continued_on_the_next_line
      then
         ...
      end if;

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

      if x = lakdsjfhlkashfdlkflkdsalkhfsalkdhflkjdsahf
           or else
         x = asldkjhalkdsjfhhfd
           or else
         x = asdfadsfadsf
      then
        ...
      end if;

      if x = lakdsjfhlkashfdlkflkdsalkhfsalkdhflkjdsahf or else
         x = asldkjhalkdsjfhhfd                         or else
         x = asdfadsfadsf
      then
        ...
      end if;

- Conditions should use short-circuit forms ( `and then`, `or else` ), except when the operands are boolean variables or boolean constants.
- Complex conditions in `if` statements are indented two characters:

      if this_complex_condition
        and then that_other_one
        and then one_last_one
      then
         ...
      end if;

  There are some cases where complex conditionals can be laid out in manners that do not follow these rules to preserve better parallelism between branches, e.g.

      if xyz.abc (gef) = 'c'
           or else
         xyz.abc (gef) = 'x'
      then
         ...
      end if;

- Every `if` block is preceded and followed by a blank line, except where it begins or ends a sequence_of_statements.

      A := 5;

      if A = 5 then
         null;
      end if;

      A := 6;

### 5.3 Case Statements ¶

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

      case expression is
         when condition =>
            ...
         when condition =>
            ...
      end case;

### 5.4 Loop Statements ¶

- When possible, have `for` or `while` on one line with the condition and the `loop` keyword.

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

  If the condition is too long, split the condition (see ‘If statements’ above) and align `loop` with the `for` or `while` keyword.

      while long_condition_that_has_to_be_split
        and then continued_on_the_next_line
      loop
         ...
      end loop;

  If the loop_statement has an identifier, it is laid out as follows:

      Outer : while not condition loop
         ...
      end Outer;

### 5.5 Block Statements ¶

- The `declare` (optional), `begin` and `end` words are aligned, except when the block_statement is named. There is a blank line before the `begin` keyword:

      Some_Block : declare
         ...

      begin
         ...
      end Some_Block;

## 6 Subprograms ¶

### 6.1 Subprogram Declarations ¶

- Do not write the `in` for parameters.

      function Length (S : String) return Integer;

- When the declaration line for a procedure or a function is too long to fit the entire declaration (including the keyword procedure or function) on a single line, then fold it, putting a single parameter on a line, aligning the colons, as in:

      procedure Set_Heading
        (Source : String;
         Count  : Natural;
         Pad    : Character := Space;
         Fill   : Boolean   := True);

  In the case of a function, if the entire spec does not fit on one line, then the return may appear after the last parameter, as in:

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

  Or it may appear on its own as a separate line. This form is preferred when putting the return on the same line as the last parameter would result in an overlong line. The return type may optionally be aligned with the types of the parameters (usually we do this aligning if it results only in a small number of extra spaces, and otherwise we don’t attempt to align). So two alternative forms for the above spec are:

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

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

### 6.2 Subprogram Bodies ¶

- Function and procedure bodies should usually be sorted alphabetically. Do not attempt to sort them in some logical order by functionality. For a sequence of subprogram specs, a general alphabetical sorting is also usually appropriate, but occasionally it makes sense to group by major function, with appropriate headers.
- All subprograms have a header giving the function name, with the following format:

      -----------------
      -- My_Function --
      -----------------

      procedure My_Function is
      begin
        ...
      end My_Function;

  Note that the name in the header is preceded by a single space, not two spaces as for other comments. These headers are used on nested subprograms as well as outer level subprograms. They may also be used as headers for sections of comments, or collections of declarations that are related.

- Every subprogram body must have a preceding subprogram_declaration, which includes proper client documentation so that you do not need to read the subprogram body in order to understand what the subprogram does and how to call it. All subprograms should be documented, without exceptions.
- A sequence of declarations may optionally be separated from the following begin by a blank line. Just as we optionally allow blank lines in general between declarations, this blank line should be present only if it improves readability. Generally we avoid this blank line if the declarative part is small (one or two lines) and the body has no blank lines, and we include it if the declarative part is long or if the body has blank lines.
- If the declarations in a subprogram contain at least one nested subprogram body, then just before the `begin` of the enclosing subprogram, there is a comment line and a blank line:

      --  Start of processing for Enclosing_Subprogram

      begin
        ...
      end Enclosing_Subprogram;

- When nested subprograms are present, variables that are referenced by any nested subprogram should precede the nested subprogram specs. For variables that are not referenced by nested procedures, the declarations can either also be before any of the nested subprogram specs (this is the old style, more generally used). Or then can come just before the begin, with a header. The following example shows the two possible styles:

      procedure Style1 is
         Var_Referenced_In_Nested      : Integer;
         Var_Referenced_Only_In_Style1 : Integer;

         proc Nested;
         --  Comments ...

         ------------
         -- Nested --
         ------------

         procedure Nested is
         begin
            ...
         end Nested;

      --  Start of processing for Style1

      begin
         ...
      end Style1;

      procedure Style2 is
         Var_Referenced_In_Nested : Integer;

         proc Nested;
         --  Comments ...

         ------------
         -- Nested --
         ------------

         procedure Nested is
         begin
            ...
         end Nested;

         --  Local variables

         Var_Referenced_Only_In_Style2 : Integer;

      --  Start of processing for Style2

      begin
         ...
      end Style2;

  For new code, we generally prefer Style2, but we do not insist on modifying all legacy occurrences of Style1, which is still much more common in the sources.

## 7 Packages and Visibility Rules ¶

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

      package P is
         ...
      end P;

- We will use the style of `use` -ing `with` -ed packages, with the context clauses looking like:

      with A; use A;
      with B; use B;

- Names declared in the visible part of packages should be unique, to prevent name clashes when the packages are `use` d.

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

- After the file header comment, the context clause and unit specification should be the first thing in a program_unit.
- Preelaborate, Pure and Elaborate_Body pragmas should be added right after the package name, indented an extra level and using the parameterless form:

      package Preelaborate_Package is
         pragma Preelaborate;
         ...
      end Preelaborate_Package;

## 8 Program Structure and Compilation Issues ¶

- Every GNAT source file must be compiled with the `-gnatg` switch to check the coding style. (Note that you should look at style.adb to see the lexical rules enforced by `-gnatg` ).
- Each source file should contain only one compilation unit.
- Filenames should be 8 or fewer characters, followed by the `.adb` extension for a body or `.ads` for a spec.
- Unit names should be distinct when ‘krunch’ed to 8 characters (see krunch.ads) and the filenames should match the unit name, except that they are all lower case.

### 8.1 GNU Free Documentation License ¶

Version 1.3, 3 November 2008

Copyright 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc ‘`https://fsf.org/`’

Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.

‘Preamble’

The purpose of this License is to make a manual, textbook, or other functional and useful document “free” in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

‘1. APPLICABILITY AND DEFINITIONS’

This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The ‘Document’, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “‘you’”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.

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‘5. COMBINING DOCUMENTS’

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‘6. COLLECTIONS OF DOCUMENTS’

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You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

‘7. AGGREGATION WITH INDEPENDENT WORKS’

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If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.

‘8. TRANSLATION’

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If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.

‘9. TERMINATION’

You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.

However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.

Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.

Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.

‘10. FUTURE REVISIONS OF THIS LICENSE’

The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See ‘`https://www.gnu.org/copyleft/`’.

Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document.

‘11. RELICENSING’

“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.

“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.

“Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.

An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.

‘ADDENDUM: How to use this License for your documents’

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

> Copyright © YEAR YOUR NAME. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with … Texts.” line with this:

> with the Invariant Sections being LIST THEIR TITLES, with the Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

## Index ¶

Jump to: | - A B C E F H I K L N O P S U

| Index Entry | Section

-
| -gnatg option (to gcc) | Program Structure and Compilation Issues

A
| Alignment (in a block statement) | Block Statements
| Alignment (in a loop statement) | Loop Statements
| Alignment (in an if statement) | If Statements
| Alignment (in comments) | Comments
| Alignment (in declarations) | Declarations and Types
| ASCII | Character Set and Separators

B
| Blank lines (in an if statement) | If Statements
| Blank lines (in comments) | Comments
| Blank lines (in subprogram bodies) | Subprogram Bodies

C
| Casing (for identifiers) | Identifiers
| Casing (for reserved words) | Reserved Words
| Casing (in comments) | Comments
| Character set | Character Set and Separators

E
| End-of-line | Character Set and Separators

F
| File name length | Program Structure and Compilation Issues

H
| Hiding of outer entities | Declarations and Types

I
| Indentation | Character Set and Separators
| Indentation | Comments
| Indentation (in if statements) | If Statements

K
| krunch.ads file | Program Structure and Compilation Issues

L
| Line length | Character Set and Separators

N
| Name clash avoidance | Packages and Visibility Rules

O
| Operators | Expressions and Names

P
| Parenthesization of expressions | Expressions and Names

S
| Separators | Character Set and Separators
| Short-circuit forms | If Statements
| style.adb file | Character Set and Separators
| style.adb file | Program Structure and Compilation Issues

U
| Underscores | Identifiers
| Underscores | Numeric Literals
| use clauses | Packages and Visibility Rules