NOTE: this library has been archived and is superceded by the Plato language

Platonic.CSharp

Platonic C# is a set of style and coding guidelines for cross-platform C# that encourage the usage of functional programming, referential transparency, and immutable data structures.

Image by Johnson Cameraface licensed usage under CC BY-NC-SA 2.0

The purpose of these rules are to:

1. Make it easier for tools to analyze, transform, translate, and optimize C# code
2. Make it simpler and faster to develop high quality software

Platonic C# code can be converted automatically into a pure functional language called Plato which in turn has an optimizer and can be easily compiled or translated to different targets (including JavaScript, and back to C#).

Uniqueness Typing for Controlled Mutability

The key idea of Platonic C# is to restrict mutable data types so that they have at most one reference at any time. This idea is known as a unique type which is a restricted form of linear type.

This allows functions to retain referential transparency even when using mutable types.

The Rules

Language Rules

• Only a subset of C# 7.3 features are supported
• No unsafe code is allowed
• A limited subset of external libraries are supported

Types

• Structs are not supported (use classes instead)
• Classes many declare only properties and methods
• No abstract types
• No delegates

Mutable and Immutable Types

• Data types are immutable or mutable marked by the [Immutable] and [Mutable] attribute respectively.
• Immutable classes and both mutable and immutable interfaces, do not have settable properties
• Only mutable classes may have settable properties
• Static types must be immutable
• Immutable classes can only implement immutable interfaces and derive from immutable classes
• Immutable classes may not have a mutable class as a property

Members

• Only classes may have properties with setters
• All property setters must be private.
• Classes with property setters
• No abstract members
• No events

Methods

• Methods are either
  • [Pure] - meaning it has no side effects
  • [ImpureWrite] - writes to one of the fields
  • [ImpureEffect] - uses a mutable type
• Only [Mutable] types may have [ImpureWrite] functions
• Both [Mutable] and [Immutable] types may have [ImpureEffect] functions
• No async methods

Additional Rules

• No generator methods (e.g., yield return and yield break statements)
• No goto statements
• No nested types

Birth of Plato

Platonic C# came from the idea of embedding a new language in C# that was pure functional and cross-platform. Pure functional languages offer opportunities for tools to rewrite and optimize code that cannot be safely applied to a language with side effects.

The challenge with embedding in C# directly was the requirement for a lot of boilerplate that does not make sense in a pure functional language with an advanced compiler.

Eventually this idea gave rise to a separate language called Plato that uses a subset of C# syntax, but has pure-functional semantics.

The Plato tool-chain and core libraries are being written in C# following the Platonic C# guidelines.

History

I have been using functional programming in C# for several years now, largely in the domain of real-time 3D geometry processing. The idea of Platonic C# has evolved from a set of coding guidelines that I follow, and have enforced with development teams that I lead.

Even though C# has released a rich set of syntactic constructs over recent years for writing code in a functional style, performance of functional programming patterns remains very poor due to how the C# and .NET compilers process and handles lambdas. This is less a short-coming of the compiler and more of an issue with the language design, which allows unconstrained side-effects and reflection. These provide a barrier to a whole family of interesting rewriting optimizations.

The other challenge with C# is that there is a split in terms of what language features can be used where. Many popular plug-in APIs (including Unity and Visual Studio) require using older versions of the language which have fewer features.

Platonic Libraries under Development

• Plato.Collections
• Plato.Math
• Parakeet

Tooling Under Development

• A Roslyn code analyzer that enforces the Platonic code style
• An optimizer that converts Platonic C# into efficient byte code
• A Platonic C# => Plato translator S