NOTE: this library has been archived and is superceded by the Plato language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

Make it easier for tools to analyze, transform, translate, and optimize C# code
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

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

License

cdiggins/Platonic.CSharp

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