Is there idiomatic C# equivalent to C's comma operator?

Question

I'm using some functional stuff in C# and keep getting stuck on the fact that List.Add doesn't return the updated list.

In general, I'd like to call a function on an object and then return the updated object.

For example it would be great if C# had a comma operator:

((accum, data) => accum.Add(data), accum)

I could write my own "comma operator" like this:

static T comma(Action a, Func<T> result) {
    a();
    return result();
}

It looks like it would work but the call site would ugly. My first example would be something like:

((accum, data) => comma(accum.Add(data), ()=>accum))

Enough examples! What's the cleanest way to do this without another developer coming along later and wrinkling his or her nose at the code smell?

Answer 1

I know this as Fluent .

A Fluent example of a List.Add using Extension Methods

static List<T> MyAdd<T>(this List<T> list, T element)
{
    list.Add(element);
    return list;
}

Answer 2

I know that this thread is very old, but I want to append the following information for future users:

There isn't currently such an operator. During the C# 6 development cycle a semicolon operator was added, as:

int square = (int x = int.Parse(Console.ReadLine()); Console.WriteLine(x - 2); x * x);

which can be translated as follows:

int square = compiler_generated_Function();

[MethodImpl(MethodImplOptions.AggressiveInlining)]
private int compiler_generated_Function()
{
    int x = int.Parse(Console.ReadLine());

    Console.WriteLine(x - 2);

    return x * x;
}

However, this feature was dropped before the final C# release .

Answer 3

This is what Concat http://msdn.microsoft.com/en-us/library/vstudio/bb302894%28v=vs.100%29.aspx is for. Just wrap a single item in an array. Functional code should not mutate the original data. If performance is a concern, and this isn't good enough, then you'll no longer be using the functional paradigm.

((accum, data) => accum.Concat(new[]{data}))

Answer 4

您自然可以在C＃3.0中使用代码块来完成几乎第一个示例。

((accum, data) => { accum.Add(data); return accum; })

Answer 5

The extension method is arguably the best solution, but for completeness' sake, don't forget the obvious alternative: a wrapper class.

public class FList<T> : List<T>
{
    public new FList<T> Add(T item)
    {
        base.Add(item);
        return this;
    }

    public new FList<T> RemoveAt(int index)
    {
        base.RemoveAt(index);
        return this;
    }

    // etc...
}

{
     var list = new FList<string>();
     list.Add("foo").Add("remove me").Add("bar").RemoveAt(1);
}

Answer 6

I thought it would be interesting to make a version of my wrapper class answer that doesn't require you write the wrapper methods.

public class FList<T> : List<T>
{
    public FList<T> Do(string method, params object[] args)
    {
        var methodInfo = GetType().GetMethod(method);

        if (methodInfo == null)
            throw new InvalidOperationException("I have no " + method + " method.");

        if (methodInfo.ReturnType != typeof(void))
            throw new InvalidOperationException("I'm only meant for void methods.");

        methodInfo.Invoke(this, args);
        return this;
    }
}

{
    var list = new FList<string>();

    list.Do("Add", "foo")
        .Do("Add", "remove me")
        .Do("Add", "bar")
        .Do("RemoveAt", 1)
        .Do("Insert", 1, "replacement");

    foreach (var item in list)
        Console.WriteLine(item);    
}

Output:

foo 
replacement 
bar

EDIT

You can slim down the syntax by exploiting C# indexed properties.

Simply add this method:

public FList<T> this[string method, params object[] args]
{
    get { return Do(method, args); }
}

And the call now looks like:

list = list["Add", "foo"]
           ["Add", "remove me"]
           ["Add", "bar"]
           ["RemoveAt", 1]
           ["Insert", 1, "replacement"];

With the linebreaks being optional, of course.

Just a bit of fun hacking the syntax.

Answer 7

Another technique, straight from functional programming, is as follows. Define an IO struct like this:

/// <summary>TODO</summary>
public struct IO<TSource> : IEquatable<IO<TSource>> {
    /// <summary>Create a new instance of the class.</summary>
    public IO(Func<TSource> functor) : this() { _functor = functor; }

    /// <summary>Invokes the internal functor, returning the result.</summary>
    public TSource Invoke() => (_functor | Default)();

    /// <summary>Returns true exactly when the contained functor is not null.</summary>
    public bool HasValue => _functor != null;

    X<Func<TSource>> _functor { get; }

    static Func<TSource> Default => null;
}

and make it a LINQ-able monad with these extension methods:

[SuppressMessage("Microsoft.Naming", "CA1724:TypeNamesShouldNotMatchNamespaces")]
public static class IO {
    public static IO<TSource> ToIO<TSource>( this Func<TSource> source) {
        source.ContractedNotNull(nameof(source));
        return new IO<TSource>(source);
    }

    public static IO<TResult> Select<TSource,TResult>(this IO<TSource> @this,
        Func<TSource,TResult> projector
    ) =>
        @this.HasValue && projector!=null
             ? New(() => projector(@this.Invoke()))
             : Null<TResult>();

    public static IO<TResult> SelectMany<TSource,TResult>(this IO<TSource> @this,
        Func<TSource,IO<TResult>> selector
    ) =>
        @this.HasValue && selector!=null
             ? New(() => selector(@this.Invoke()).Invoke())
             : Null<TResult>();

    public static IO<TResult> SelectMany<TSource,T,TResult>(this IO<TSource> @this,
        Func<TSource, IO<T>> selector,
        Func<TSource,T,TResult> projector
    ) =>
        @this.HasValue && selector!=null && projector!=null
             ? New(() => { var s = @this.Invoke(); return projector(s, selector(s).Invoke()); } )
             : Null<TResult>();

    public static IO<TResult> New<TResult> (Func<TResult> functor) => new IO<TResult>(functor);

    private static IO<TResult> Null<TResult>() => new IO<TResult>(null);
}

and now you can use the LINQ comprehensive syntax thus:

using Xunit;
[Fact]
public static void IOTest() {
    bool isExecuted1 = false;
    bool isExecuted2 = false;
    bool isExecuted3 = false;
    bool isExecuted4 = false;
    IO<int> one = new IO<int>( () => { isExecuted1 = true; return 1; });
    IO<int> two = new IO<int>( () => { isExecuted2 = true; return 2; });
    Func<int, IO<int>> addOne = x => { isExecuted3 = true; return (x + 1).ToIO(); };
    Func<int, Func<int, IO<int>>> add = x => y => { isExecuted4 = true; return (x + y).ToIO(); };

    var query1 = ( from x in one
                   from y in two
                   from z in addOne(y)
                   from _ in "abc".ToIO()
                   let addOne2 = add(x)
                   select addOne2(z)
                 );
    Assert.False(isExecuted1); // Laziness.
    Assert.False(isExecuted2); // Laziness.
    Assert.False(isExecuted3); // Laziness.
    Assert.False(isExecuted4); // Laziness.
    int lhs = 1 + 2 + 1;
    int rhs = query1.Invoke().Invoke();
    Assert.Equal(lhs, rhs); // Execution.

    Assert.True(isExecuted1);
    Assert.True(isExecuted2);
    Assert.True(isExecuted3);
    Assert.True(isExecuted4);
}

When one desires an IO monad that composes but returns only void , define this struct and dependent methods:

public struct Unit : IEquatable<Unit>, IComparable<Unit> {
    [CLSCompliant(false)]
    public static Unit _ { get { return _this; } } static Unit _this = new Unit();
}

public static IO<Unit> ConsoleWrite(object arg) =>
    ReturnIOUnit(() => Write(arg));

public static IO<Unit> ConsoleWriteLine(string value) =>
    ReturnIOUnit(() => WriteLine(value));

public static IO<ConsoleKeyInfo> ConsoleReadKey() => new IO<ConsoleKeyInfo>(() => ReadKey());

which readily allow the writing of code fragments like this:

from pass  in Enumerable.Range(0, int.MaxValue)
let counter = Readers.Counter(0)
select ( from state in gcdStartStates
         where _predicate(pass, counter())
         select state )
into enumerable
where ( from _   in Gcd.Run(enumerable.ToList()).ToIO()
        from __  in ConsoleWrite(Prompt(mode))
        from c   in ConsoleReadKey()
        from ___ in ConsoleWriteLine()
        select c.KeyChar.ToUpper() == 'Q' 
      ).Invoke()
select 0;

where the old C comma operator is readily recognized for what it is: a monadic compose operation.

The true merit of the comprehension syntax is apparent when one attempts to write that fragment in the flunt style:

( Enumerable.Range(0,int.MaxValue)
            .Select(pass => new {pass, counter = Readers.Counter(0)})
            .Select(_    => gcdStartStates.Where(state => _predicate(_.pass,_.counter()))
                                          .Select(state => state)
                   )
).Where(enumerable => 
   ( (Gcd.Run(enumerable.ToList()) ).ToIO()
        .SelectMany(_ => ConsoleWrite(Prompt(mode)),(_,__) => new {})
        .SelectMany(_ => ConsoleReadKey(),          (_, c) => new {c})
        .SelectMany(_ => ConsoleWriteLine(),        (_,__) => _.c.KeyChar.ToUpper() == 'Q')
    ).Invoke()
).Select(list => 0);