Naming conventions

proposals/p0861.md

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+# Naming conventions
+
+<!--
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+[Pull request](https://github.com/carbon-language/carbon-lang/pull/861)
+
+<!-- toc -->
+
+## Table of contents
+
+-   [Problem](#problem)
+-   [Background](#background)
+    -   [Cross-language precedent](#cross-language-precedent)
+    -   [Carbon artifacts](#carbon-artifacts)
+-   [Proposal](#proposal)
+-   [Details](#details)
+    -   [Constants](#constants)
+    -   [Properties](#properties)
+    -   [Carbon-provided item naming](#carbon-provided-item-naming)
+-   [Open questions](#open-questions)
+    -   [Should private members have a name augment?](#should-private-members-have-a-name-augment)
+-   [Rationale based on Carbon's goals](#rationale-based-on-carbons-goals)
+-   [Alternatives considered](#alternatives-considered)
+    -   [Other naming conventions](#other-naming-conventions)
+    -   [Other conventions for naming Carbon types](#other-conventions-for-naming-carbon-types)
+
+<!-- tocstop -->
+
+## Problem
+
+The goal of this proposal is to establish naming conventions for:
+
+-   Idiomatic Carbon code.
+-   Carbon-provided features, including:
+    -   Keywords, such as `fn` or `for`.
+    -   Type literals, such as `i32`.
+    -   Types, such as `bool` or `String`.
+
+The reason for resolving this through proposal is to ensure we're headed in a
+reasonably consistent path as we write early documentation and example code.
+
+## Background
+
+### Cross-language precedent
+
+Naming conventions are an issue frequently addressed by style guides. A few
+examples of language-provided guidelines are:
+
+-   [Effective Go](https://golang.org/doc/effective_go#names)
+-   [Python's PEP 8](https://www.python.org/dev/peps/pep-0008/#naming-conventions)
+    -   Note PEP 8 offers a list of
+        [naming styles](https://www.python.org/dev/peps/pep-0008/#descriptive-naming-styles).
+-   [Rust API Guidelines](https://rust-lang.github.io/api-guidelines/naming.html)
+-   [Swift's API Design Guidelines](https://swift.org/documentation/api-design-guidelines/#general-conventions)
+
+### Carbon artifacts
+
+Related issues:
+
+-   Issue
+    [#543: pick names for fixed-size integer types](https://github.com/carbon-language/carbon-lang/issues/543)
+    covers numeric type literal naming.
+-   Issue
+    [#750: Naming conventions for Carbon-provided features](https://github.com/carbon-language/carbon-lang/issues/750)
+    was used for initial naming convention discussion.
+
+Related proposals:
+
+-   Proposal
+    [#720: Property naming in C++](https://github.com/carbon-language/carbon-lang/pull/720)
+    covers nuances of property naming.
+
+## Proposal
+
+Only `UpperCamelCase` and `lower_snake_case` conventions will be used, in order
+to minimize the variation in rules.
+
+-   For idiomatic Carbon code:
+    -   `UpperCamelCase` will be used when the identifier names a specific
+        program entity, such as a function, a namespace, or a compile-time
+        constant value.
+    -   `lower_snake_case` will be used when the identifier's value won't be
+        known until runtime, such as for variables.
+        -   Properties will use this style because they are intended to be
+            interchangeable with member variables.
+        -   Private class members will have a suffix `_`.
+-   For Carbon-provided features:
+    -   Keywords and type literals will use `lower_snake_case`.
+    -   Other code will use the guidelines for idiomatic Carbon code.
+
+In other words:
+
+| Item                      | Convention         | Explanation                                                                                |
+| ------------------------- | ------------------ | ------------------------------------------------------------------------------------------ |
+| Packages                  | `UpperCamelCase`   | Used for compile-time lookup.                                                              |
+| Types                     | `UpperCamelCase`   | Resolved at compile-time.                                                                  |
+| Functions                 | `UpperCamelCase`   | Resolved at compile-time.                                                                  |
+| Methods                   | `UpperCamelCase`   | Methods, including virtual methods, are equivalent to functions.                           |
+| Generic parameters        | `UpperCamelCase`   | May vary based on inputs, but are ultimately resolved at compile-time.                     |
+| Compile-time constants    | `UpperCamelCase`   | Resolved at compile-time. See [constants](#constants) for more remarks.                    |
+| Variables                 | `lower_snake_case` | May be reassigned and thus require runtime information.                                    |
+| Member variables          | `lower_snake_case` | Behave like variables. See [properties](#properties) for more remarks.                     |
+| Properties                | `lower_snake_case` | Behave as wrappers for class members. See [properties](#properties) for more remarks.      |
+| Keywords                  | `lower_snake_case` | Special, and developers can be expected to be comfortable with this casing cross-language. |
+| Type literals             | `lower_snake_case` | Equivalent to keywords.                                                                    |
+| Boolean type and literals | `lower_snake_case` | Equivalent to keywords.                                                                    |
+| Other Carbon types        | `UpperCamelCase`   | Behave like normal types.                                                                  |
+| `Self` and `Super`        | `UpperCamelCase`   | These are similar to type members on a class.                                              |
+
+## Details
+
+### Constants
+
+Supposing `let` might be used to designate a constant, consider the following
+code:
+
+```carbon
+package Example;
+
+let CompileTimeConstant: i32 = 7;
+
+fn RuntimeFunction(runtime_constant: i32);
+```
+
+In this example, `CompileTimeConstant` has a singular value (`7`) which is known
+at compile-time. As such, it uses `UpperCamelCase`.
+
+On the other hand, `runtime_constant` may be constant within the function body,
+but it is assigned at runtime when `RuntimeFunction` is called. Its value is
+only known in a given runtime invocation of `RuntimeFunction`. As such, it uses
+`lower_snake_case`.
+
+### Properties
+
+> NOTE: Property syntax hasn't been established for Carbon, but is expected, so
+> this proposal aims to include it.
+
+The primary purpose of a
+[property](<https://en.wikipedia.org/wiki/Property_(programming)>) is to provide
+a transparent wrapper to a private member variable. This may be done for reasons
+such as guaranteeing lazy initialization of data on first access, or providing a
+helpful breakpoint for debug access. Regardless of the reason, we want it to be
+trivial for properties to replace member variables without affecting clients of
+an API, and so they use the same naming scheme.
+
+### Carbon-provided item naming
+
+Carbon-provided items are split into a few categories:
+
+-   Keywords; for example, `for`, `fn`, and `var`
+-   Type literals; for example, `i<digits>`, `u<digits>`, and `f<digits>`
+-   Boolean type and literals; for example, `bool`, `true`, and `false`
+    -   The separate categorization of booleans should not be taken as a rule
+        that only booleans would use lowercase; it's just the only example right
+        now.
+-   `Self` and `Super`
+-   Other Carbon types; for example, `Int`, `UInt`, and `String`
+
+Note that while other Carbon types currently use `UpperCamelCase`, that should
+not be inferred to mean that future Carbon types will do the same. The leads
+will make decisions on future naming.
+
+## Open questions
+
+### Should private members have a name augment?
+
+In
+[#720: Property naming in C++](https://github.com/carbon-language/carbon-lang/pull/720),
+we added a suffix `_` for private data members in C++. Should Carbon do similar,
+for the same reasons of avoiding property and private data member name
+collisions?
+
+This is currently unresolved because properties haven't been designed.
+
+## Rationale based on Carbon's goals
+
+-   [Code that is easy to read, understand, and write](/docs/project/goals.md#code-that-is-easy-to-read-understand-and-write)
+    -   The intent is that limiting to `UpperCamelCase` and `lower_snake_case`
+        will offer developers a limited number of style rules to learn.
+    -   There is a desire to maintain good ergonomics for developers.
+-   [Interoperability with and migration from existing C++ code](/docs/project/goals.md#interoperability-with-and-migration-from-existing-c-code)
+    -   There is a general desire to maintain naming consistency with C++, and
+        this can be seen with keywords such as `for` as well as booleans.
+
+## Alternatives considered
+
+### Other naming conventions
+
+A couple naming conventions that were discussed briefly are:
+
+-   `lowerCamelCase` names came up, but are hard to distinguish from
+    `lower_snake_case` for single-word identifiers. By contrast,
+    `UpperCamelCase` and `lower_snake_case` are distinct, whether a single word
+    or multiple.
+-   `ALL_CAPS_SNAKE_CASE` is used in C++ code, such as for macros and
+    compile-time constants. With Carbon, we hope the language is simple enough
+    that the readability benefit of an additional naming convention wouldn't
+    outweigh the cost of giving developers more naming conventions to learn.
+
+### Other conventions for naming Carbon types
+
+In detail, individual naming decisions that had alternative patterns or options
+discussed were:
+
+-   Type literals use `lower_snake_case` as described in issue
+    [#543: pick names for fixed-size integer types](https://github.com/carbon-language/carbon-lang/issues/543).
+    -   `i8` is more ergonomic than `Int8` for brevity.
+    -   `i32` and `i64` maintain the same length as C++'s `int`.
+    -   We don't want to use `I32` because `I32` can be hard to visually
+        distinguish with `132` or `l32`, even though it may help screen readers.
+-   Booleans use `bool`, `true`, and `false` mainly because there's a desire not
+    to skew from C++.
+    -   While leads are okay with some things being shadowed, such as `Self` and
+        `Super`, there is a desire not to allow shadowing of booleans.
+-   `Self` and `Super` are `UpperCamelCase` because leads want them to look more
+    like normal types.
+    -   They may be implemented as a hybrid approach, using something similar to
+        lookup in classes so they aren't quite keywords, but somewhat similar to
+        keywords because leads may want to reject them as identifiers in
+        non-class contexts.
+-   `String` and potentially other names follow idiomatic naming conventions.
+    -   Note that, in this case, divergence from C++ is accepted.
+
+Taking that into consideration, and that we are likely to keep
+`lower_snake_case` for keywords and type literals in particular, a couple
+options discussed would have mainly affect `self`, `super`, and `string`:
+
+-   Anything language-provided without needing an `import` is always
+    `lower_snake_case`.
+    -   `i32`, `bool`, `true`, `self`, `super`, `string.is_empty()`
+-   As above, but prelude class members follow standard naming conventions.
+    -   `i32`, `bool`, `true`, `self`, `super`, `string.IsEmpty()`
+
+The understanding is that the difference between `bool` being treated similarly
+to a keyword and `Self` being treated similarly to a type (both regardless of
+implementation) is somewhat arbitrary. The decision not to use
+`lower_snake_case` consistently depends mainly on the leaning of the leads, that
+`self` and `super` felt like they shouldn't strictly be treated as keywords, and
+`string` felt like the point where users should expect something more like an
+idiomatic class. Future borderline cases may need to be decided by leads whether
+they should be treated as more keyword-like or type-like.