Neteril.ComputationExpression

A generic re-implementation of F# computation expressions in C# by (ab)using async/await.

TL;DR lifting the tricks of my previous attempt at borrowing F# computation expression concepts in C# into a generic form that can be reused for other builder types.

This code utilizes the F# definition of computation expression builders (or more precisely the subset of Bind/Return/Zero/Combine members) for a given monad type and plug it into a customized async/await method builder and awaiter system.

Basically the idea is to go from this kind of F#:

let divideBy bottom top =
  if bottom = 0
  then None
  else Some(top/bottom)

type MaybeBuilder() =
  member this.Bind(m, f) = Option.bind f m
  member this.Return(x) = Some x

let maybe = new MaybeBuilder()

let divideByWorkflow =
  maybe {
    let! a = 120 |> divideBy 2
    let! b = a |> divideBy 2
    let! c = b |> divideBy 2
    return c
  }

To this kind of C#:

Option<int> TryDivide (int up, int down)
  => down == 0 ? None<int>.Value : Some.Of (up / down);

class MaybeBuilder : IMonadExpressionBuilder
{
  IMonad<T> IMonadExpressionBuilder.Bind<U, T> (IMonad<U> m, Func<U, IMonad<T>> f)
  {
    switch ((Option<U>)m) {
      case Some<U> some: return f (some.Item);
      case None<U> none:
      default: return None<T>.Value;
    }
  }
  public IMonad<T> Return<T> (T v) => Some.Of (v);
  public IMonad<T> Zero<T> () => None<T>.Value;
  // We don't have optional interface methods in C# quite yet
  public IMonad<T> Combine<T> (IMonad<T> m, IMonad<T> n) => throw new NotSupportedException ();
}

ComputationExpression.Run<int, Option<int>> (new OptionExpressionBuilder (), async () => {
  var val1 = await TryDivide (120, 2);
  var val2 = await TryDivide (val1, 2);
  var val3 = await TryDivide (val2, 2);
  return val3;
})

In this example the code is very similar with C#'s await becoming the equivalent of F#'s let!/do!. The library also supports the yield keyword via an extra API call.

In addition to the plumbing, the library provides some monads and their expression builder already. They can be found in the Instances folder. See the included Workbook Examples for API usage and full running samples.