How would you implement LINQ methods with SelectMany? [closed]

Question

Erik Meijer is fond of pointing out that every LINQ function could actually be implemented by SelectMany; everything else is just a convenience.

This is what Eric Lippert says answering a question about monads, but I've heard Erik Meijer say this in other videos about LINQ and Rx. (Simply put, Erik Meijer is the guy who created LINQ and Rx)

I wonder how you would implement some of the most used LINQ functions with SelectMany? Ignore perfromance for now, let's focus on elegance and succinctness.

• Where
• Select
• First
• Take(n)
• TakeWhile
• GroupBy
• OrderBy
• Zip
• Others...

Answer 1

The main bit to keep in mind is that SelectMany works on an IEnumerable and returns an IEnumerable using lambda expressions that have access to the current item and its index. So anything you could do to transform the result with access to the current item or its index is possible:

• Decide which elements to keep or throw away (Where, First, Take, Skip, TakeWhile)
• Change elements and return new ones (Select)
• Do other stuff? (GroupBy, honestly don't know how I would implement that without some thought)

This simple Where example will make it clear how many of these could be accomplished easily:

SomeList.SelectMany(x =>
    ShouldBeIncluded(x) ?
        Enumerable.Repeat(x, 1) :
        Enumerable.Empty<AClass>();
    );

Edit - Great link posted by Tim Schmelter in the comments proves again that Jon Skeet has already done it cleaner:

return Enumerable.Repeat(x, ShouldBeIncluded(x) ? 1 : 0;