Pipe forwards in C#


Question

Continuing my investigation of expressing F# ideas in C#, I wanted a pipe forward operator. For anything wrapped in a IEnumerable, we already have it, as you can .NextFunc() to your heart's content. But for example if you have any fold-like reduction at the end, you can't feed the result of that into a function.

Here are two extension methods, I wondered if anyone else had tried this, and if it's a good idea or not (EDIT: now with Earwicker's Maybe included):

public static void Pipe<T>(this T val, Action<T> action) where T : class
{ if (val!=null) action(val); }

public static R Pipe<T, R>(this T val, Func<T, R> func) where T : class where R : class
{ return val!=null?func(val):null; }

You can then write something like:

Func<string, string[]> readlines = (f) => File.ReadAllLines(f);
Action<string, string> writefile = (f, s) => File.WriteAllText(f, s);

Action<string, string> RemoveLinesContaining = (file, text) =>
    {
        file.Pipe(readlines)
            .Filter(s => !s.Contains(text))
            .Fold((val, sb) => sb.AppendLine(val), new StringBuilder())
            .Pipe((o) => o.ToString())
            .Pipe((s) => writefile(file, s));
    };

(I know, Filter == Where in C#, and Fold==Aggregate, but I wanted to roll my own, and I could have done WriteAllLines, but that's not the point)

EDIT: corrections as per Earwicker's comment (if I've understood correctly).

1
22
5/23/2017 12:09:23 PM

Accepted Answer

I haven't bothered with a raw pipe, but I have tried making all references into the Maybe monad:

public static class ReferenceExtensions
{
    public static TOut IfNotNull<TIn, TOut>(this TIn v, Func<TIn, TOut> f)
                                    where TIn : class 
                                    where TOut: class
    {
        if (v == null)
            return null;

        return f(v);
    }
}

Then suppose you have an object model that lets you lookup a RecordCompany by name, and then lookup a Band within that RecordCompany, a Member of the Band, and any of these might return null, so this might throw a NullReferenceException:

var pixiesDrummer = Music.GetCompany("4ad.com")
                         .GetBand("Pixes")
                         .GetMember("David");

We can fix that:

var pixiesDrummer = Music.GetCompany("4ad.com")
                         .IfNotNull(rc => rc.GetBand("Pixes"))
                         .IfNotNull(band => band.GetMember("David"));

Hey presto, if any of those transitions return null, pixiesDrummer will be null.

Wouldn't it be great if we could do extension methods that are operator overloads?

public static TOut operator| <TIn, TOut>(TIn v, Func<TIn, TOut> f)

Then I could pipe together my transition lambdas like this:

var pixiesDrummer = Music.GetCompany("4ad.com")     
                     | rc => rc.GetBand("Pixes")
                     | band => band.GetMember("David");

Also wouldn't it be great if System.Void was defined as a type and Action was really just Func<..., Void>?

Update: I blogged a little about the theory behind this.

Update 2: An alternative answer to the original question, which is roughly "How would you express the F# pipe-forward operator in C#?"

Pipe-forward is:

let (|>) x f = f x

In other words, it lets you write a function and its first argument in the opposite order: argument followed by function. It's just a syntactic helper that assists with readability, allowing you to make use of infix notation with any function.

This is exactly what extension methods are for in C#. Without them, we would have to write:

var n = Enumerable.Select(numbers, m => m * 2);

With them, we can write:

var n = numbers.Select(m => m * 2);

(Ignore the fact that they also let us omit the class name - that's a bonus but could also be made available for non-extension methods as it is in Java).

So C# already solves the same problem in a different way.

23
1/14/2010 9:57:47 AM

So for Piping I don't think there is expectation to check for null and not call the piped function. The function argument in many cases could easy take a null and have it handled by the function.

Here is my implementation. I have Pipe and PipeR. Be forewarned, the PipeR is not pipe right, but just for the cases in which the target is in the opposite position for currying, because the alternate overloads allow limited fake currying of parameters.

The nice thing about the fake currying is that you can pipe in the method name after providing the parameters, thus producing less nesting than you would with a lambda.

new [] { "Joe", "Jane", "Janet" }.Pipe(", ", String.Join)

String.Join has the IEnumerable in the last position so this works.

"One car red car blue Car".PipeR(@"(\w+)\s+(car)",RegexOptions.IgnoreCase, Regex.IsMatch)

Regex.IsMatch has the target in the first Position so PipeR works.

Here's my example implementaion:

public static TR Pipe<T,TR>(this T target, Func<T, TR> func)
{
    return func(target);
}

public static TR Pipe<T,T1, TR>(this T target, T1 arg1, Func<T1, T, TR> func)
{
    return func(arg1, target);
}

public static TR Pipe<T, T1, T2, TR>(this T target, T1 arg1, T2 arg2, Func<T1, T2, T, TR> func)
{
    return func(arg1, arg2, target);
}

public static TR PipeR<T, T1, TR>(this T target, T1 arg1, Func<T, T1, TR> func)
{
    return func(target, arg1);
}

public static TR PipeR<T, T1, T2, TR>(this T target, T1 arg1, T2 arg2, Func<T, T1, T2, TR> func)
{
    return func(target, arg1, arg2);
}

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