This library uses and abuses the features of C# to provide a pure functional-programming 'Base Class Library' that, if you squint, can look like extensions to the language itself. The desire here is to make programming in C# much more robust by helping the engineer's inertia flow in the direction of declarative and pure functional code rather than imperative. Using these techniques for large code-bases can bring tangible benefits to long-term maintenance by removing hidden complexity and by easing the engineer's cognitive load.
Author on...
- Blog: Notes from a Small Functional Island
- Twitter: @paullouth
- Mastodon: @louthy@4four.org
- Github ReadME project: 'Functional programming is finally going mainstream'
Nu-get package | Description |
---|---|
LanguageExt.Core | All of the core types and functional 'prelude'. This is all that's needed to get started. |
LanguageExt.FSharp | F# to C# interop package. Provides interop between the LanguageExt.Core types (like Option , List and Map ) to the F# equivalents, as well as interop between core BCL types and F# |
LanguageExt.Parsec | Port of the Haskell parsec library |
LanguageExt.Rx | Reactive Extensions support for various types within the Core |
LanguageExt.Sys | Provides an effects wrapper around the .NET System namespace making common IO operations pure and unit-testable |
To use this library, simply include LanguageExt.Core.dll
in your project or grab it from NuGet. It is also worth setting up some global using
for your project. This is the full list that will cover the key functionality and bring it into scope:
global using LanguageExt;
global using LanguageExt.Common;
global using static LanguageExt.Prelude;
global using LanguageExt.Traits;
global using LanguageExt.Effects;
global using LanguageExt.Pipes;
global using LanguageExt.Pretty;
global using LanguageExt.Traits.Domain;
A minimum, might be:
global using LanguageExt;
global using LanguageExt.Common;
global using static LanguageExt.Prelude;
global using LanguageExt.Traits;
The namespace LanguageExt
contains most of the core types; LanguageExt.Prelude
contains the functions that bring into scope the prelude functions that behave like standalone functions in ML style functional programming languages; LanguageExt.Traits
brings in the higher-kinded trait-types and many extensions; LanguageExt.Common
brings in the Error
type and predefined Errors
.
From C# 6 onwards we got the ability to treat static classes like namespaces. This means that we can use static methods without qualifying them first. That instantly gives us access to single term method names that look exactly like functions in ML-style functional languages. i.e.
using static System.Console;
WriteLine("Hello, World");
This library tries to bring some of the functional world into C#. It won't always sit well with the seasoned C# OO programmer,
especially the choice of camelCase
names for a lot of functions and the seeming 'globalness' of a lot of the library.
I can understand that much of this library is non-idiomatic, but when you think of the journey C# has been on, is "idiomatic" necessarily right? A lot of C#'s idioms are inherited from Java and C# 1.0. Since then we've had generics, closures, Func, LINQ, async... C# as a language is becoming more and more like a functional language on every release. In fact, the bulk of the new features are either inspired by or directly taken from features in functional languages. So perhaps it's time to move the C# idioms closer to the functional world's idioms?
My goal with this library is very much to create a whole new community within the larger C# community. This community is not constrained by the dogma of the past or by the norms of C#. It understands that the OOP approach to programming has some problems and tries to address them head-on.
And for those that say "just use F#" or "just use Haskell", sure, go do that. But it's important to remember that C# has a lot going for it:
- Incredible investment into a state-of-the art compiler
- Incredible tooling (Visual Studio and Rider)
- A large ecosystem of open-source libraries
- A large community of developers already using it
- This is also very important for companies that hire engineers
- It is a functional programming language! It has first-class functions, lambdas, etc.
- And with this library it has a functional-first Base Class Library
One of the areas that's likely to get seasoned C# heads worked up is my choice of naming style. The intent is to try and make something that feels like a functional language rather than following rules of naming conventions (mostly set out by the BCL).
There is, however, a naming guide that will keep you in good stead while reading through this documentation:
- Type names are
PascalCase
in the normal way - The types all have constructor functions rather than public constructors that you instantiate with
new
. They will always bePascalCase
:
Option<int> x = Some(123);
Option<int> y = None;
Seq<int> items = Seq(1,2,3,4,5);
List<int> items = List(1,2,3,4,5);
HashMap<int, string> dict = HashMap((1, "Hello"), (2, "World"));
Map<int, string> dict = Map((1, "Hello"), (2, "World"));
- Any (non-type constructor) static function that can be used on its own by
using static LanguageExt.Prelude
arecamelCase
.
var x = map(opt, v => v * 2);
- Any extension methods, or anything "fluent" are
PascalCase
in the normal way
var x = opt.Map(v => v * 2);
Even if you disagree with this non-idiomatic approach, all of the camelCase
static functions have fluent variants, so you never actually have to see the non-standard stuff.
Location | Feature | Description |
---|---|---|
Core |
Atom<A> |
A lock-free atomically mutable reference for working with shared state |
Core |
Ref<A> |
An atomic reference to be used in the transactional memory system |
Core |
AtomHashMap<K, V> |
An immutable HashMap with a lock-free atomically mutable reference |
Core |
AtomSeq<A> |
An immutable Seq with a lock-free atomically mutable reference |
Core |
VectorClock<A> |
Understand distributed causality |
Core |
VersionVector<A> |
A vector clock with some versioned data |
Core |
VersionHashMap <ConflictV, K, V> |
Distrubuted atomic versioning of keys in a hash-map |
Location | Feature | Description |
---|---|---|
Core |
Arr<A> |
Immutable array |
Core |
Seq<A> |
Lazy immutable list, evaluate at-most-once - very, very fast! |
Core |
Iterable<A> |
Wrapper around IEnumerable with support for traits - enables the higher-kinded traits to work with enumerables. |
Core |
Lst<A> |
Immutable list - use Seq over Lst unless you need InsertAt |
Core |
Map<K, V> |
Immutable map |
Core |
Map<OrdK, K, V> |
Immutable map with Ord constraint on K |
Core |
HashMap<K, V> |
Immutable hash-map |
Core |
HashMap<EqK, K, V> |
Immutable hash-map with Eq constraint on K |
Core |
Set<A> |
Immutable set |
Core |
Set<OrdA, A> |
Immutable set with Ord constraint on A |
Core |
HashSet<A> |
Immutable hash-set |
Core |
HashSet<EqA, A> |
Immutable hash-set with Eq constraint on A |
Core |
Que<A> |
Immutable queue |
Core |
Stck<A> |
Immutable stack |
Location | Feature | Description |
---|---|---|
Core |
Option<A> |
Option monad |
Core |
OptionT<M, A> |
Option monad-transformer |
Core |
Either<L,R> |
Right/Left choice monad |
Core |
EitherT<L, M, R> |
Right/Left choice monad-transformer |
Core |
Fin<A> |
Error handling monad, like Either<Error, A> |
Core |
FinT<M, A> |
Error handling monad-transformer |
Core |
Try<A> |
Exception handling monad |
Core |
TryT<M, A> |
Exception handling monad-transformer |
Core |
Validation<FAIL ,SUCCESS> |
Validation applicative and monad for collecting multiple errors before aborting an operation |
Core |
ValidationT<FAIL, M, SUCCESS> |
Validation applicative and monad-transformer |
Location | Feature | Description |
---|---|---|
Core |
Reader<E, A> |
Reader monad |
Core |
ReaderT<E, M, A> |
Reader monad-transformer |
Core |
Writer<W, A> |
Writer monad that logs to a W constrained to be a Monoid |
Core |
WriterT<W, M, A> |
Writer monad-transformer |
Core |
State<S, A> |
State monad |
Core |
StateT<S, M, A> |
State monad-transformer |
Location | Feature | Description |
---|---|---|
Parsec |
Parser<A> |
String parser monad and full parser combinators library |
Parsec |
Parser<I, O> |
Parser monad that can work with any input stream type |
Location | Feature | Description |
---|---|---|
Core |
Doc<A> |
Produce nicely formatted text with smart layouts |
Location | Feature | Description |
---|---|---|
Core |
Patch<EqA, A> |
Uses patch-theory to efficiently calculate the difference (Patch.diff(list1, list2) ) between two collections of A and build a patch which can be applied (Patch.apply(patch, list) ) to one to make the other (think git diff). |
The traits are major feature of v5
+ language-ext that makes generic programming with higher-kinds a reality. Check out Paul's series on Higher Kinds to get a deeper insight.
These work a little like NewType
but they impart semantic meaning and some common operators for the underlying value.
Location | Feature | Description |
---|---|---|
Core |
DomainType<SELF, REPR> |
Provides a mapping from SELF to an underlying representation: REPR |
Core |
Identifier <SELF> |
Identifiers (like IDs in databases: PersonId for example), they are equivalent to DomaintType with equality. |
Core |
VectorSpace<SELF, SCALAR> |
Scalable values; can add and subtract self, but can only multiply and divide by a scalar. Can also negate. |
Core |
Amount <SELF, SCALAR> |
Quantities, such as the amount of money in USD on a bank account or a file size in bytes. Derives VectorSpace , IdentifierLike , DomainType , and is orderable (comparable). |
Core |
LocusLike <SELF, SCALAR, DISTANCE> |
Works with space-like structures. Spaces have absolute and relative distances. Has an origin/zero point and derives DomainType , IdentifierLike , AmountLike and VectorSpace . DISTANCE must also be an AmountLike<SELF, REPR, SCALAR> . |
These features are still a little in-flux as of 17th May 2024 - they may evolve, be renamed, or removed - but I like the idea!
For some non-reference like documentation:
- Paul's blog: Notes from a Small Functional Island does deep dives into the philosophy of FP and the inner-workings of language-ext.
- The wiki has some additional documentation, some might be a little out of date since the big
v5
refactor, but should give some good insights.
If you would like to get involved with this project, please first read the Contribution Guidelines and the Code of Conduct.