enso/doc/scala-style-guide.md

Scala Style Guide

Like many style guides, this Scala style guide exists for two primary reasons. The first is to provide guidelines that result in a consistent code style across all of the Enso codebases, while the second is to guide people towards a style that is expressive while still easy to read and understand.

In general, it aims to create a set of 'zero-thought' rules in order to ease the programmer burden; there is usually only one way to lay out code correctly.

• Code Formatting
• Naming
• Package Structure and Naming
  • The Public API
• Build Tooling
• Commenting
  • Documentation Comments
  • Source Notes
  • TODO Comments
  • Other Comment Usage
• Program Design
  • Code Complexity
  • Safety
  • Testing and Benchmarking
  • Warnings, and Lints

Code Formatting

This section explains the rules for visually laying out your code. They provide a robust set of guidelines for creating a consistent visual to the code.

Primary formatting is dealt with through use of the Scala formatting tool scalafmt, which enforces rules around whitespace, line-wrapping, and alignment. The Enso repository contains the main .scalafmt.conf configuration file, and this is what should be used for all new Scala projects.

All files must be formatted using scalafmt before commit, and this should be set up as either a precommit hook, or using the integration in IntelliJ. If you use the IntelliJ integration, please note that you need only have the official Scala Plugin installed, and be using IntelliJ 2019.1 or later. You should not use the independent Scalafmt plugin.

Naming

Enso has some fairly simple general naming conventions, though the sections below may provide more rules for use in specific cases.

• Types are written using UpperCamelCase.
• Variables and function names are written using camelCase.
• If a name contains an initialism or acronym, all parts of that initialism should be of the same case: httpRequest or makeHTTPRequest.
• Short variable names such as a and b should only be used in contexts where there is no other appropriate name, and should never be used to refer to temporary data in a function.
• Names should be descriptive, even if this makes them longer.
• Any function that performs an unsafe operation that is not documented in its type (e.g. def head[T](ts: List[T]): T, which fails if the list is empty), must be named using the word 'unsafe' (e.g. unsafeHead). For more information on unsafe function usage, see the section on safety. The one exception to this rule is for functions which fail intentionally on a broken implementation (e.g. "should not happen"-style fatal crashes).

Package Structure and Naming

Enso follows the Java convention for naming packages: package name components may contain only lower case characters and, if necessary, an underscore character. All Enso package names should be prefixed with org.enso. For example, the package for implementation of File Manager project should be named org.enso.filemanager.

When the name of the file in the package is the same as the final component of the package name, the file should be moved one level up. For examples, if File Manager project contains FileManager.scala file, then the file should be placed directly in the org.enso package instead of org.enso.filemanager. This is to avoid repetitious constructs like org.enso.filemanager.FileManager.

The Public API

In order to produce as flexible a codebase as possible, we tend not to make use of access modifiers in our code (public, private, and so on). Instead, we use the concept of Internal modules to separate public from private.

If you are writing code in a package X.Y.MyType and would like to signal that a particular construct (e.g. a function) is for internal use in that package, you should create a X.Y.MyType.Internal package. You can then write the relevant language construct in that package instead of the source package.

Using Access Modifiers

There are, however, a few notable exceptions to the above:

• Safety: Privacy modifiers (e.g. private and private[this]) should be used to enforce an API contract around safety. An example of this is Scala's immutable List, which contains a private mutable buffer for performance reasons.
• Reducing Overhead: As the Internal module is a separate module, there can (under some circumstances) be some overhead for its use. If you are writing code on a performance-critical path, you may instead make use of access modifiers.
• Enabling Optimisations: The JVM is capable of performing optimisations by making use of visibility information (e.g. in the interpreter). If you are writing performance-critical code, you may use access modifiers to provide the JVM with additional information.

Build Tooling

All Scala projects in the Enso organisation should manage their dependencies and build setup using SBT.

If you are using IntelliJ, please ensure that you select to use the SBT shell for both imports and builds.

Commenting

Comments in code are a tricky area to get right as we have found that comments often expire quickly, and in absence of a way to validate them, remain incorrect for long periods of time. In order to best deal with this problem, we make the keeping of comments up-to-date into an integral part of our programming practice while also limiting the types and kinds of comments we allow.

Comments across the Enso codebases fall into three main types:

• Documentation Comments: API documentation for all appropriate language constructs.
• Source Notes: Detailed explorations of design reasoning that avoid cluttering the code itself.
• Tasks: Things that need doing or fixing in the codebase.

When we write comments, we try to follow one general guideline. A comment should explain what and why, without mentioning how. The how should be self-explanatory from reading the code, and if you find that it is not, that is a sign that the code in question needs refactoring.

Code should be written in such a way that it guides you over what it does, and comments should not be used as a crutch for badly-designed code.

Documentation Comments

One of the primary forms of comment that we allow across the Enso codebases is the doc comment. We use these comments to document the public API of a module, as defined in The Public API. For constructs that are part of the public API, the following should be documented:

1. Top-Level Type Definitions: All top-level type definitions must have a doc comment. If a type is defined multiple times (e.g. trait T and object T), you need only document it once. Under these circumstances it is recommended to document the in the order of: trait, class, object.
2. Functions: Function documentation should provide at-a-glance intuition for how to use that function.

An example of a valid set of comments is as follows:

package org.enso.syntax.graph

/** An [[Action]] is a representation of an operation that can be made on a
  * [[SpanTree]].
  */
sealed trait Action
object Action {
  object Insert extends Action
  object Erase  extends Action
  object Set    extends Action
}

/** Values representing sets of [[Action]]s at a given point.
  */
object Actions {
  val All: Set[Action]      = Set(Action.Insert, Action.Erase, Action.Set)
  val Function: Set[Action] = Set(Action.Set)
  val Root: Set[Action]     = Set(Action.Set)

  /** Makes a set from the provided actions.
    *
    * @param actions a variable number of actions
    * @return a set containing the provided actions
    */
  def mkActionSet(actions: Action*): Set[Action] = {
    // ...
  }
}

Documentation comments are intended for consumption by the users of the API, and are written using the standard scaladoc syntax. Doc comments should contain:

1. Summary: A one-line summary of the construct's behaviour. This should be a valid sentence with 'this X' (where X = function, trait, etc) prepended to it.
2. Description (Optional): Any useful information that would be necessary for a consumer of the API to know (that is not encoded in the types). This should be written in grammatically correct English.
3. Parameters and Returns: The return value must always be described. If the parameters are all obvious from their names, you must omit the @param annotations. If one or more parameters require explanation (for things not expressed in their name or type), then all parameters must be annotated.

An example comment that requires a description is as follows (but omits the necessary comment on Tree for brevity):

trait Tree[T}] {
  /** Provides a sequence representation of the tree.
    *
    * The function provides configurable behaviour for the order in which the
    * tree is walked. See [[WalkStrategy]] for the provided options.
    *
    * @param order the strategy by which the tree's elements are traversed
    * @return the elements contained in the tree arranged according to the
    *     provided `order`
    */
  def walkToSequence(order: WalkStrategy[Tree[T]]): Seq[T]
}

A simpler example that does not require a description is as follows (but omits the necessary comment on Tree for brevity):

trait Tree[T] {
  /** Provides a sequence representation of the tree.
    *
    * @return the elements of the tree arranged in preorder-walk sequence
    */
  def toSeq(): Seq[T]
}

Documentation comments should not reference internal implementation details, or be used to explain choices made in the implementation. For this kind of info, you should use Source Notes as described below.

You may document more than what is specified here, but this is the minimum required for acceptance at code-review time.

Source Notes

Source Notes is a mechanism for moving detailed design information about a piece of code out of the code itself. In doing so, it retains the key information about the design while not impeding the flow of the code. They are used in the following circumstances:

• Design Information: Documentation about why something was written in a particular fashion, as well as information on the process that led to it being done this way.
• Explaining Complexity: If an implementation uses complex constructs or any elements that are non-obvious, these should be explained as part of a source note.
• Knowledge Provenance: Explaining where some knowledge (e.g. a mathematical formula or an algorithm) was obtained from. It is also useful to accompany these by some commentary on why the choice was made.
• Safety: Any unsafe usage of a function must be accompanied by a source note that explains what makes this particular usage safe.

Source notes are detailed comments that, like all comments, explain both the what and the why of the code being described. In very rare cases, it may include some how, but only to refer to why a particular method was chosen to achieve the goals in question.

A source note comment is broken into two parts:

1. Referrer: This is a small comment left at the point where the explanation is relevant. It takes the following form: // Note [Note Name], where Note Name is a unique identifier across the codebase. These names should be descriptive, and make sure you search for it before using it, in case it is already in use.
2. Source Note: This is the comment itself, which is a large block comment placed after the first function in which it is referred to in the module. It uses the scala block-comment syntax /* ... */, and the first line names the note using the same referrer as above: /* Note [Note Name]. The name(s) in the note are underlined using a string of the ~ (tilde) character.

A source note may contain sections within it where necessary. These are titled using the following syntax: == Note [Note Name (Section Name)], and can be referred to from a referrer much as the main source note can be.

Sometimes it is necessary to reference a source note in another module, but this should never be done in-line. Instead, a piece of code should reference a source note in the same module that references the other note while providing additional context to that reference.

An example, based on some code in the GHC codebase, can be seen below:

{
def prepRHS (env : SimplEnv, outExpr : OutExpr) : SimplM[SimplEnv, OutExpr] = {
  val (ty1, _ty2) = coercionKind(env) // Note [Float Coercions]

  if (!isUnliftedType(ty1)) {
    val newTy1 = convertTy(ty1) // Note [Float Coercions (Unlifted)]
    ...more expressions defining prepRHS...
  }
}

/* Note [Float Coercions]
 * ~~~~~~~~~~~~~~~~~~~~~~
 * When we find the binding
 *         x = cast(e, co)
 * we'd like to transform it to
 *         x' = e
 *         x = cast(x, co) // A trivial binding
 * There's a chance that e will be a constructor application or function, or
 * something like that, so moving the coercion to the usage site may well cancel
 * the coercions and lead to further optimisation.
 *         ...more stuff about coercion floating...
 *
 * Note [Float Coercions (Unlifted)]
 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 *      ...explanations of floating for unlifted types...
 */
}

TODO Comments

We follow a simple convention for TODO comments in our codebases:

• The line starts with TODO or FIXME.
• It is then followed by the author's initials [ARA], or for multiple people [ARA, MK], in square brackets.
• It is then followed by an explanation of what needs to be done.

For example:

{
// TODO [ARA] This is a bit of a kludge. Instead of X it should to Y, accounting
// for the fact that Z.
}

Other Comment Usage

There are, of course, a few other situations where commenting is very useful:

• Commenting Out: You may comment out code while developing it, but if you commit any commented out code, it should be accompanied by an explanation of why said code can't just be deleted.
• Bugs: You can use comments to indicate bugs in our code, as well as third-party bugs. In both cases, the comment should link to the issue tracker where the bug has been reported.

Program Design

Any good style guide goes beyond purely stylistic rules, and also talks about design styles to use in code.

Code Complexity

While we often have to write complex functionality, we want to ensure that the code itself is kept as simple and easy to read as possible. To do this, please use the following rules:

• Write single-line expressions wherever possible, rather than writing one complex chunk of code.
• Separate intermediate results out to their own variables with appropriate names. Even if they are temporaries, giving them a name is a great aid to code comprehension.

Safety

It is incredibly important that we can trust the code that we use, and hence we tend to disallow the definition of unsafe functions in our public API. When defining an unsafe function, you must account for the following:

• It must be named unsafeX, as mentioned above in naming.
• Unsafe functions should only be used in the minimal scope in which it can be shown correct, not in larger pieces of code.
• Unsafe function definition must be accompanied by a source note explaining why it is not defined safely (e.g. performance).
• Unsafe function usage must be accompanied by a source note explaining why this usage of it is safe.

Furthermore, we do not allow for code containing pattern matches that can fail.

Testing and Benchmarking

New code should always be accompanied by tests. These can be unit, integration, or some combination of the two, and they should always aim to test the new code in a rigorous fashion.

• We tend to use ScalaTest, but also make use of ScalaCheck for property-based testing.
• Tests should be declared in the project configuration so they can be trivially run.
• A test file should be named after the module it tests.

Any performance-critical code should also be accompanied by a set of benchmarks. These are intended to allow us to catch performance regressions as the code evolves, but also ensure that we have some idea of the code's performance in general.

• We use Caliper for our benchmarks.
• We measure time, but also memory usage and CPU time where possible.
• Where relevant, benchmarks may set thresholds which, when surpassed, cause the benchmark to fail. These thresholds should be set for a release build, and not for a development build.

Do not benchmark a development build as the data you get will often be entirely useless.

Warnings, and Lints

In general, we aim for a codebase that is free of warnings and lints, and we do this using the following ideas:

Warnings

New code should introduce no new warnings onto master. You may build with warnings on your own branch, but the code that is submitted as part of a PR should not introduce new warnings. You should also endeavour to fix any warnings that you come across during development.

Sometimes it is impossible to fix a warning (often in situations involving the use of macros). In such cases, you are allowed to suppress the warning locally, but this must be accompanied by a source note explaining why you are doing so.