Mercury Coding Standard for the Mercury Project


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.

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

Each procedure that is implemented using foreign code should have sufficient documentation about its interface that it can be implemented just by referring to that documentation, 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. Ideally this description should also note any conditions under which the predicate can fail or throw an exception.

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 the appropriate section(s) 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.

Any feature which is left in an incompletely implemented form for a nontrivial length of time should be mentioned in compiler/notes/work_in_progress.html.


Variables should always be given meaningful names, unless they are irrelevant to the code in question.

Different states or different versions of the same entity should be referred to using state variable notation, although if there are only a few versions, and their number is not likely to grow, they may be named Foo0, Foo1, Foo2, ..., Foo.

Predicates which get or set a field of a structure or ADT should be named typename_get_fieldname and typename_set_fieldname respectively.


Your code should reuse existing code as much possible, generalizing if needed. "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.)

Don't use DCG notation for new code.

Use state variables for threading state, such as the IO state. The conventional IO state variable name is !IO.

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 succeeded; 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, directly or indirectly, should always indicate an internal software error, not merely incorrect inputs from the user, or failure of some library routine or system call. In the compiler, do not call error/1 directly; call it indirectly using unexpected/2 or sorry/2 from compiler_util.m. Likewise, use expect/3 from compiler_util.m rather than require/2.

Error messages should be constructed using the facilities in error_util.m. Compiler error messages should be complete sentences; they should start with a capital letter and end in a full stop. If including a full explanation of the meaning of an error and/or of its likely causes would be too tedious for everyday use (probably because the error is frequent enough that most non-novice users of Mercury would be expected to be familiar with it) you should include that extra information in a verbose_only part of the error message, which causes it to be printed if the --verbose-errors option is set.

Error messages from the runtime system should begin with the text "Mercury Runtime:", preferably by using the MR_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 MR_strerror(errno, errbuf, sizeof(errbuf)). If it was a function manipulating some file, the error message should include the filename.

Input / output

All new code in the compiler that reads from a stream or writes to a stream should explicitly pass the stream being accessed to the predicates that do the reading or writing. Do not depend on the library's record of what the current input and output streams are; do not read from the current input stream, or write to the current output stream.


Each module should be indented consistently, with 4 spaces per level of indentation. The indentation should be done using only spaces; there should be no tabs. The only exception should be Makefiles, which should use one tab per level of indentation.

All files other than Makefiles should have something like this at the top, even before the copyright line:

	% vim: ts=4 sw=4 expandtab ft=mercury
Makefiles should have something like this instead:
	% vim: ts=8 sw=8 noexpandtab

No line should extend beyond 79 characters. The reason we don't allow 80 character lines is that these lines wrap around in diffs, since diff adds an extra character at the start of each line.

Since "empty" lines that have spaces or tabs on them prevent the proper functioning of paragraph-oriented commands in vi, lines shouldn't have trailing white space. They can be removed with a vi macro such as the following. (Each pair of square brackets contains a space and a tab.)

	map ;x :g/[     ][      ]*$/s///^M

String literals that don't fit on a single line should be split by writing them as two or more strings concatenated using the "++" operator; the compiler will evaluate this at compile time, if --optimize-constant-propagation is enabled (i.e. at -O3 or higher).

Predicates that have only one mode should use predmode declarations rather than having a separate mode declaration.

If-then-elses should always be parenthesized, except when an if-then-else that occurs as the else part of another if-then-else doesn't need to be parenthesized. We prefer the ( if C then T else E ) syntax over the ( C -> T ; E ) syntax.

The condition can either be on the same line as the 'if' keyword:

    ( if test1 then
    else if test2 then
or, if the test is complicated, on separate line(s) between the 'if' and "then' keywords,
    ( if
    else if
    else if
        test3    % would fit one one line, but separate for consistency

Disjunctions should always 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.

Predicates and functions implemented via foreign code should be formatted like this:

    :- pragma foreign_proc("C",
        int.to_float(IntVal::in, FloatVal::out),
        [will_not_call_mercury, promise_pure],
        FloatVal = IntVal;
The predicate name and arguments should be on a line on their own, as should the list of annotations. The foreign code should also be on lines of its own; it shouldn't share lines with the double quote marks surrounding it.

Type definitions should be formatted in one of the following styles:

	:- type my_type
		--->	my_type(
                    % comment explaining field

	:- type some_other_type == int.

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

	:- type complicated
		--->	complicated_f(
                    % long comment explaining first field
                    f1_arg_name     :: int,

                    % long comment explaining second and third fields
                    % this is ok if they are related to each other
                    f2_arg_name     :: float,
                    f3_arg_name     :: float

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, or the next block comment).

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
or an "inline" comment
	blahblah	% This is an "inline" comment
should be used.


Modules should generally be arranged so that their procedures, types, insts etc. are listed in top-down order, not bottom-up.

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:

Ideally such sections should be identified by "section heading" comments identifying the contents of the section, optionally followed by a more detailed description. These should be laid out like this:
% Section title.

% Detailed description of the contents of the section and/or
% general comments about the contents of the section.
% This part may go one for several lines.
% It can even contain several paragraphs.

The actual code starts here.
For example
% Exception handling.

% This section contains all the code that deals with throwing or catching
% exceptions, including saving and restoring the virtual machine registers
% if necessary.
% Note that we need to take care to ensure that this code is thread-safe!

:- type foo ---> ...
Double dashed lines, i.e.
can also be used to indicate divisions into major sections. Note that these dividing lines should not exceed the 79 character limit (see above). The tools/stdlines script will standardize the lengths of these dashed lines.

Module imports

Each group of :- import_module items should list only one module per line, since this makes it much easier to read diffs that change the set of imported modules. In the compiler, when e.g. an interface section imports modules from both the compiler and the standard library, there should be two groups of imports, the imports from the compiler first and then the ones from the library. For the purposes of this rule, consider the modules of mdbcomp to belong to the compiler.

Each group of import_module items should be sorted, since this makes it easier to detect duplicate imports and missing imports. It also groups together the imported modules from the same package. There should be no blank lines between the imports of modules from different packages, since they would make it harder to resort the group with a single editor command.

Standard library predicates

The descriptive comment for any predicate or function that occurs in the interface of a standard library module must be positioned above the predicate or function declaration. It should be formatted as in the following example:
		% foo(Arg1, Arg2, Result, !Acc),
        % Returns the foo of Arg1 and Arg2, while updating !Acc by ...
	:- pred foo(...
	:- mode foo(...
The arguments of the predicate or function should be given descriptive names, although a one-character non-descriptive name such as X is acceptable when the semantics of an argument is obvious anyway. Neither the name of the predicate nor the names of the arguments should be quoted in any way. A group of related predicate, mode and function declarations may be grouped together under a single description, provided that it is formatted as above. If there is a function declaration in such a grouping, then it should be listed before the others. For example:
		% Insert a new key and corresponding value into a map.
		% Fail if the key already exists.
	:- func insert(map(K, V), K, V) = map(K, V).
	:- pred insert(map(K, V)::in, K::in, V::in, map(K, V)::out) is det.
The reason for using this particular style is that the reference manual for the standard library is automatically generated from the module interfaces, and we want to maintain a uniform appearance as much as is possible.

Avoid module qualification in the interface sections of library modules except where necessary to resolve ambiguity.

A predicate or function which throws exceptions under certain conditions should be described as such, using the phrase "throws an exception". A predicate or function that may cause a runtime abort should be described as such, using the phrase "runtime abort".


Every change should be tested before being committed. The level of testing required depends on the nature of the change. If the change does not touch the semantics of the code (e.g. it modifies comments, renames predicates or adds module qualifications), then checking whether recompilation succeeds is enough. If the change replaces an abort with nonaborting code, then just compiling it and running some tests by hand is sufficient. For pretty much everything else, where the change might break the compiler, you should run a bootstrap check (using the `tools/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 follow the procedures described in compiler/notes/bootstrapping.html.

In addition to testing before a change is committed, you need to make sure that the code will not get broken in the future. 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

When committing a nontrivial change, you should get someone else to review your changes. If the risk of breakage is low, you can ask for a review post commit. Otherwise, get a review, and act on its recommendations, before you commit.

The file compiler/notes/reviews.html contains more information on our review policy.