RFC 0344: conventions-galore

libs (convention)

• Start Date: 2014-10-15
• RFC PR: rust-lang/rfcs#344
• Rust Issue: rust-lang/rust#18074

Summary

This is a conventions RFC for settling a number of remaining naming conventions:

• Referring to types in method names
• Iterator type names
• Additional iterator method names
• Getter/setter APIs
• Associated types
• Trait naming
• Lint naming
• Suffix ordering
• Prelude traits

It also proposes to standardize on lower case error messages within the compiler and standard library.

Motivation

As part of the ongoing API stabilization process, we need to settle naming conventions for public APIs. This RFC is a continuation of that process, addressing a number of smaller but still global naming issues.

Detailed design

The RFC includes a number of unrelated naming conventions, broken down into subsections below.

Referring to types in method names

Function names often involve type names, the most common example being conversions like as_slice. If the type has a purely textual name (ignoring parameters), it is straightforward to convert between type conventions and function conventions:

Type name	Text in methods
String	string
Vec<T>	vec
YourType	your_type

Types that involve notation are less clear, so this RFC proposes some standard conventions for referring to these types. There is some overlap on these rules; apply the most specific applicable rule.

Type name	Text in methods
&str	str
&[T]	slice
&mut [T]	mut_slice
&[u8]	bytes
&T	ref
&mut T	mut
*const T	ptr
*mut T	mut_ptr

The only surprise here is the use of mut rather than mut_ref for mutable references. This abbreviation is already a fairly common convention (e.g. as_ref and as_mut methods), and is meant to keep this very common case short.

Iterator type names

The current convention for iterator type names is the following:

Iterators require introducing and exporting new types. These types should use the following naming convention:

• Base name. If the iterator yields something that can be described with a specific noun, the base name should be the pluralization of that noun (e.g. an iterator yielding words is called Words). Generic contains use the base name Items.

• Flavor prefix. Iterators often come in multiple flavors, with the default flavor providing immutable references. Other flavors should prefix their name:

  • Moving iterators have a prefix of Move.
  • If the default iterator yields an immutable reference, an iterator yielding a mutable reference has a prefix Mut.
  • Reverse iterators have a prefix of Rev.

(These conventions were established as part of this PR and later this one.)

These conventions have not yet been updated to reflect the recent change to the iterator method names, in part to allow for a more significant revamp. There are some problems with the current rules:

• They are fairly loose and therefore not mechanical or predictable. In particular, the choice of noun to use for the base name is completely arbitrary.

• They are not always applicable. The iter module, for example, defines a large number of iterator types for use in the adapter methods on Iterator (e.g. Map for map, Filter for filter, etc.) The module does not follow the convention, and it's not clear how it could do so.

This RFC proposes to instead align the convention with the iter module: the name of an iterator type should be the same as the method that produces the iterator.

For example:

• iter would yield an Iter
• iter_mut would yield an IterMut
• into_iter would yield an IntoIter

These type names make the most sense when prefixed with their owning module, e.g. vec::IntoIter.

Advantages:

• The rule is completely mechanical, and therefore highly predictable.

• The convention can be (almost) universally followed: it applies equally well to vec and to iter.

Disadvantages:

• IntoIter is not an ideal name. Note, however, that since we've moved to into_iter as the method name, the existing convention (MoveItems) needs to be updated to match, and it's not clear how to do better than IntoItems in any case.

• This naming scheme can result in clashes if multiple containers are defined in the same module. Note that this is already the case with today's conventions. In most cases, this situation should be taken as an indication that a more refined module hierarchy is called for.

Additional iterator method names

An earlier RFC settled the conventions for the "standard" iterator methods: iter, iter_mut, into_iter.

However, there are many cases where you also want "nonstandard" iterator methods: bytes and chars for strings, keys and values for maps, the various adapters for iterators.

This RFC proposes the following convention:

• Use iter (and variants) for data types that can be viewed as containers, and where the iterator provides the "obvious" sequence of contained items.

• If there is no single "obvious" sequence of contained items, or if there are multiple desired views on the container, provide separate methods for these that do not use iter in their name. The name should instead directly reflect the view/item type being iterated (like bytes).

• Likewise, for iterator adapters (filter, map and so on) or other iterator-producing operations (intersection), use the clearest name to describe the adapter/operation directly, and do not mention iter.

• If not otherwise qualified, an iterator-producing method should provide an iterator over immutable references. Use the _mut suffix for variants producing mutable references, and the into_ prefix for variants consuming the data in order to produce owned values.

Getter/setter APIs

Some data structures do not wish to provide direct access to their fields, but instead offer "getter" and "setter" methods for manipulating the field state (often providing checking or other functionality).

The proposed convention for a field foo: T is:

• A method foo(&self) -> &T for getting the current value of the field.
• A method set_foo(&self, val: T) for setting the field. (The val argument here may take &T or some other type, depending on the context.)

Note that this convention is about getters/setters on ordinary data types, not on builder objects. The naming conventions for builder methods are still open.

Associated types

Unlike type parameters, the names of associated types for a trait are a meaningful part of its public API.

Associated types should be given concise, but meaningful names, generally following the convention for type names rather than generic. For example, use Err rather than E, and Item rather than T.

Trait naming

The wiki guidelines have long suggested naming traits as follows:

Prefer (transitive) verbs, nouns, and then adjectives; avoid grammatical suffixes (like able)

Trait names like Copy, Clone and Show follow this convention. The convention avoids grammatical verbosity and gives Rust code a distinctive flavor (similar to its short keywords).

This RFC proposes to amend the convention to further say: if there is a single method that is the dominant functionality of the trait, consider using the same name for the trait itself. This is already the case for Clone and ToCStr, for example.

According to these rules, Encodable should probably be Encode.

There are some open questions about these rules; see Unresolved Questions below.

Lints

Our lint names are not consistent. While this may seem like a minor concern, when we hit 1.0 the lint names will be locked down, so it's worth trying to clean them up now.

The basic rule is: the lint name should make sense when read as "allow lint-name" or "allow lint-name items". For example, "allow deprecated items" and "allow dead_code" makes sense, while "allow unsafe_block" is ungrammatical (should be plural).

Specifically, this RFC proposes that:

• Lint names should state the bad thing being checked for, e.g. deprecated, so that #[allow(deprecated)] (items) reads correctly. Thus ctypes is not an appropriate name; improper_ctypes is.

• Lints that apply to arbitrary items (like the stability lints) should just mention what they check for: use deprecated rather than deprecated_items. This keeps lint names short. (Again, think "allow lint-name items".)

• If a lint applies to a specific grammatical class, mention that class and use the plural form: use unused_variables rather than unused_variable. This makes #[allow(unused_variables)] read correctly.

• Lints that catch unnecessary, unused, or useless aspects of code should use the term unused, e.g. unused_imports, unused_typecasts.

• Use snake case in the same way you would for function names.

Suffix ordering

Very occasionally, conventions will require a method to have multiple suffixes, for example get_unchecked_mut. When feasible, design APIs so that this situation does not arise.

Because it is so rare, it does not make sense to lay out a complete convention for the order in which various suffixes should appear; no one would be able to remember it.

However, the mut suffix is so common, and is now entrenched as showing up in final position, that this RFC does propose one simple rule: if there are multiple suffixes including mut, place mut last.

Prelude traits

It is not currently possible to define inherent methods directly on basic data types like char or slices. Consequently, libcore and other basic crates provide one-off traits (like ImmutableSlice or Char) that are intended to be implemented solely by these primitive types, and which are included in the prelude.

These traits are generally not designed to be used for generic programming, but the fact that they appear in core libraries with such basic names makes it easy to draw the wrong conclusion.

This RFC proposes to use a Prelude suffix for these basic traits. Since the traits are, in fact, included in the prelude their names do not generally appear in Rust programs. Therefore, choosing a longer and clearer name will help avoid confusion about the intent of these traits, and will avoid namespace pollution.

(There is one important drawback in today's Rust: associated functions in these traits cannot yet be called directly on the types implementing the traits. These functions are the one case where you would need to mention the trait by name, today. Hopefully, this situation will change before 1.0; otherwise we may need a separate plan for dealing with associated functions.)

Error messages

Error messages -- including those produced by fail! and those placed in the desc or detail fields of e.g. IoError -- should in general be in all lower case. This applies to both rustc and std.

This is already the predominant convention, but there are some inconsistencies.

Alternatives

Iterator type names

The iterator type name convention could instead basically stick with today's convention, but using suffixes instead of prefixes, and IntoItems rather than MoveItems.

Unresolved questions

How far should the rules for trait names go? Should we avoid "-er" suffixes, e.g. have Read rather than Reader?