Mercury Coding Standard for the Mercury Project


Documentation

Each module should contain header comments which state the module's name, main author(s), and purpose, and give an overview of what the module does, what are the major algorithms and data structures it uses, etc.

Each type or predicate that is exported from a module should have sufficient documentation that it can be understood without reference to the module's implementation section.

Each predicate other than trivial access predicates should have a short comment describing what the predicate is supposed to do, and what the meaning of the arguments is.

There should be a comment for each field of a structure saying what the field represents.

Any user-visible changes such as new compiler options or new features should be documented in appropriate section of the Mercury documentation (usually the Mercury User's Guide and/or the Mercury Reference Manual). Any major new features should be documented in the NEWS file, as should even small changes to the library interface, or anything else that might cause anyone's existing code to break.

Any new compiler modules or other major design changes should be documented in `compiler/notes/compiler_design.html'.

Naming

Variables should always be given meaningful names, unless they are irrelevant to the code in question. For example, it is OK to use single-character names in an access predicate which just sets a single field of a structure, such as


	bar_set_foo(Foo, bar(A, B, C, _, E), bar(A, B, C, Foo, E)).

Variables which represent different states or different versions of the same entity should be named Foo0, Foo1, Foo2, ..., Foo.

Predicates which get or set a field of a structure or ADT should be named bar_get_foo and bar_set_foo respectively, where bar is the name of the structure or ADT and foo is the name of the field.

Coding

Your code should make as much reuse of existing code as possible. "cut-and-paste" style reuse is highly discouraged.

Your code should be efficient. Performance is a quite serious issue for the Mercury compiler.

No fixed limits please! (If you really must have a fixed limit, include detailed documentation explaining why it was so hard to avoid.)

Error Handling

Code should check for both erroneous inputs from the user and also invalid data being passed from other parts of the Mercury compiler. You should also always check to make sure that the routines that you call have succeed; make sure you don't silently ignore failures. (This last point almost goes without saying in Mercury, but is particularly important to bear in mind if you are writing any C code or shell scripts, or if you are interfacing with the OS.)

Calls to error/1 should always indicate an internal software error, not merely incorrect inputs from the user, or failure of some library routine or system call.

Error messages should follow a consistent format. For compiler error messages, each line should start with the source file name and line line number in "%s:%03d: " format (but use prog_out__write_context -- we may want to change this format later, e.g. to include column numbers). Compiler error messages should be complete sentences; they should start with a capital letter and end in a full stop. For error messages that are spread over more than one line (as are most of them), the second and subsequent lines should be indented two spaces. If the `--verbose-errors' option was set, you should print out additional text explaining in detail what the error message means and what the likely causes are.

Error messages from the runtime system should begin with the text "Mercury Runtime:", preferably by using the fatal_error() routine.

If a system call or C library function that sets errno fails, the error message should be printed with perror() or should contain strerror(errno). If it was a function manipulating some file, the error message should include the filename.

Layout

Code should be indented consistently. Tabs should be every 8 spaces. Indentation should be one tab per level of indentation. except for highly intended code which may be indented at 4 spaces per level of indentation. Each line should not extend beyond 79 characters, unless this is necessary due to the use of long string literals. If-then-elses should always be parenthesized, except that an if-then-else that occurs as the else part of another if-then-else doesn't need to be parenthesized. The semicolon of a disjunction should never be at the end of a line -- put it at the start of the next line instead. The condition of an if-then-else can either be on the same line as the opening parenthesis and the `->',

	( test1 ->
		goal1
	; test2 ->
		goal2
	;
		goal
	)

or, if the test is complicated, it can be on a line of its own:

	(
		very_long_test_that_does_not_fit_on_one_line(VeryLongArgument1,
			VeryLongArgument2)
	->
		goal1
	;
		test2a,
		test2b,
	->
		goal2
	;
		test3	% would fit one one line, but separate for consistency
	->
		goal3
	;
		goal
	).

Type definitions should be formatted in the following style:
	:- type my_type
		--->	my_type(
				some_other_type	% comment explaining it
			).

	:- type some_other_type == int.

	:- type foo
		--->	bar(
				int,		% comment explaining it
				float		% comment explaining it
			)
		;	baz
		;	quux.

If an individual clause is long, it should be broken into sections, and each section should have a "block comment" describing what it does; blank lines should be used to show the separation into sections. Comments should precede the code to which they apply, rather than following it.
	%
	% This is a block comment; it applies to the code in the next
	% section (up to the next blank line).
	%
	blah,
	blah,
	blahblah,
	blah,
If a particular line or two needs explanation, a "line" comment
	% This is a "line" comment; it applies to the next line or two
	% of code
	blahblah
or an "inline" comment
	blahblah	% This is an "inline" comment
should be used. At a higher level, code should be grouped into bunches of related predicates, functions, etc., and sections of code that are conceptually separate should be separated with dashed lines:

%---------------------------------------------------------------------------%

Double-dashed lines, i.e.

%---------------------------------------------------------------------------%
%---------------------------------------------------------------------------%

can also be used to indicate divisions into major sections.

Testing

Every change should be tested before being committed. The level of testing required depends on the nature of the change. If this change fixes an existing bug, and is not likely to introduce any new bugs, then just compiling it and running some tests by hand is sufficient. If the change might break the compiler, you should run a bootstrap check (using the `bootcheck' script) before committing. If the change means that old versions of the compiler will not be able to compile the new version of the compiler, you must notify all the other Mercury developers.

In addition to testing before a change is committed, you need to make sure that the code will not get broken in the future by adding tests to the test suite. Every time you add a new feature, you should add some test cases for that new feature to the test suite. Every time you fix a bug, you should add a regression test to the test suite.

Committing changes

Before committing a change, you should get someone else to review your changes.

The file "REVIEWS" contains more information on review policy.


Last update was $Date: 2000/11/28 05:08:47 $ by $Author: fjh $@cs.mu.oz.au.