palladin

delegate dynamic D(dynamic d);

static async Task Main()
{
    Func<D, D> selfApp = f => f(f);
    var Y = (Func<D, D> f) => selfApp(g => f(x => g(g)(x)));

    Func<D, D> fact = (D f) => n => n <= 1 ? 1 : n * f(n - 1);
    Console.WriteLine(Y(fact)(5));
}

palladin

using System;
using System.Linq;


static class Program
{
    public static IEnumerable<TAcc> ScanBy<T, TKey, TAcc>(this IEnumerable<T> source, Func<T, TKey> keyFunc, Func<TKey, T, TAcc> seedFunc, Func<TAcc, T, TAcc> aggregateFunc)
    {
        TAcc acc = default!;
        TKey key = default!;

palladin

using System.Linq;


public static class Index
{
    public static Index<TKey, TValue> Create<TKey, TValue>()
        where TKey : notnull
    {
        var keyMap = new Dictionary<TKey, HashSet<TValue>>();
        Action<TKey, TValue> add = (key, value) =>

palladin

using Nessos.Effects;
using Nessos.Effects.Handlers;

public class ConsolePrintEffect : Effect
{
    public string? Message { init; get; }
}

palladin

let dropWhile : ('a -> bool) -> list<'a> -> list<'a> = fun pred xs -> 
    let (_, ys) = List.foldBack (fun x (xs, ys) -> let xs' = x :: xs in if pred x then xs', ys else xs', xs') xs ([], [])
    ys
    
// example 1 
[1..10] |> dropWhile (fun x -> x <= 5) // [6..10] 

// example 2
[1..10] |> dropWhile (fun x -> false) // [1..10] 

palladin

type Append<T, TArray> = TArray extends [infer TCurrent,  ...infer Rest] ? 
                           TCurrent extends any[] ? [[T, ...TCurrent], ...Append<T, Rest>] : never
                         : [] 
                         
type PowerSet<TArray extends any[]> = 
    TArray extends [infer TCurrent,  ...infer Rest] ? [...Append<TCurrent, PowerSet<Rest>>, ...PowerSet<Rest>] : [[]]

type Test = PowerSet<[1, 2, 3]> // [[1, 2, 3], [1, 2], [1, 3], [1], [2, 3], [2], [3], []]

palladin

let fact = `(f => n => (n === 0) ? 1 : n * f(n - 1))`
function Y(g : string) : (any) => any {
    let f = `(f => ${g}(x => eval(f)(f)(x)))`
    return eval(f)(f)
}


console.log(Y(fact)(10))

palladin

Expression<Func<Expression, Func<int, int>>> f =
  self => x => ((Expression<Func<Expression, Func<int, int>>>)self).Compile()(self)(x + 1);

var x = f.Compile()(f)(0);

palladin

static async IAsyncEnumerable<T> UnFold<T, TAcc>(Func<TAcc, Task<(bool Next, IAsyncEnumerable<T> Values, TAcc Acc)>> f, TAcc seed)
{
    var acc = seed;
    var result = default((bool Next, IAsyncEnumerable<T> Values, TAcc Acc));
    do
    {
        result = await f(acc);
        await foreach (var value in result.Values)
            yield return value;

palladin

 // https://blog.neteril.org/blog/2017/04/26/maybe-computation-expression-csharp/
 [AsyncMethodBuilder(typeof(MaybeAsyncMethodBuilder<>))]
    interface Option<T> { }

    // Could use the closed type hierarchy Roslyn feature
    // to be an approximation of a discriminated union
    // https://github.com/dotnet/csharplang/issues/485
    sealed class None<T> : Option<T> { public static readonly None<T> Value = new None<T>(); }
    sealed class Some<T> : Option<T>
    {