Standard Module Style

This document describes general guidance for style when writing a standard module. Of course, there will be exceptions to this guidance when there is a good reason.

PascalCase and camelCase

Prefer PascalCase or camelCase to separating_with_underscores. The rest of this document will use PascalCase or camelCase to signify the pattern of indicating new words with an upper case letter and starting the identifier; where PascalCase starts the name with an upper case letter and camelCase starts with a lower case letter.

When including acronyms in PascalCase or camelCase, treat the acronym as if it were a regular word. There is one exception - if using PascalCase and the acronym is the entire symbol name, then let the acronym be all uppercase.

Here are a few examples:
  • “rendered HTML DOM” in PascalCase is RenderedHtmlDom

  • “rendered HTML DOM” in camelCase is renderedHtmlDom

  • “HTML” in PascalCase is HTML

  • “HTML” in camelCase is html


The following are exceptions to the above capitalization guidelines that occur in the standard modules or packages:

  • numPUs (num Processor-Units)

Our hope and intention is to update our rules over time so that they cover cases like this, obviating the need to call it out as an exception. As more exceptions accumulate, rules may be amended to cover multiple cases at once.


Module names should be PascalCase.


Chapel does not use a separate namespace for types vs. module names. As a result, it’s generally a goal to avoid using the same name for, say, a type and a module.


Class type names should be PascalCase. The idea is that starting with an uppercase letter is the convention for by-reference types.


Record type names should be camelCase. The idea is that starting with a lowercase letter is the convention for by-value types.


Enum type names should be camelCase. The enum values should also be camelCase.

Functions and Methods

Function and method names should be camelCase.

Generally speaking, it’s desirable to use methods on a value (vs functions that aren’t methods) when there is a clear type responsible for the operation.

Use parentheses-less methods only for returning properties that could be reasonably implemented as fields. However, if such a method is named isXYZ or hasXYZ it should use parentheses (so, use proc isReal() { ...  } rather than proc isReal { ... }). Parentheses-less functions that aren’t methods should be avoided.

Many paren-ful methods take some notable action. Try to make these methods method names be a verb. In particular, a method that modifies an argument in-place should be a verb.

Factory Functions and Methods

Factory functions or methods are procedures where the main purpose of that procedure is to create a new object or value. There are many other procedures that return a value but constructing that value is not the main purpose of the function. For example, ‘open’ and ‘spawn’ are not factory functions – the main action of these methods is to work with the OS to open a file or launch a subprocess.

In many cases factory procedures are not needed. Regular initializers will work just fine to support the pattern of new Something(...). Type conversion is best supported by creating a cast (:) operator overload.

One case in which factory procedures are useful is when it is awkward to distinguish between several ways of creating something based on the initializer arguments alone.

Factory procedures should have a name that begin with ‘create’.

Factory procedures can be functions or type methods. For example, we could have:

  • createStringWithBorrowedBuffer or

  • string.createBorrowingBuffer.


Accessor methods are methods intended to return a distinct piece of information about the type on which they are defined. This piece of information could be reasonably implemented as a field. Typically, such methods are defined without parentheses, allowing them to appear similar to a field access. However, there is no requirement that the contents of the accessor be similarly simple - whether the information returned is calculated based on the internal fields of the type or whether it is actually a field is an implementation detail.

Accessor methods will avoid using “get” in their name. E.g., instead of array.getIdxType, the accessor is named array.idxType.

Methods that are not accessors are still allowed to use “get” in their name.

Other Identifiers

Variables, fields, and argument names should be camelCase or PascalCase.