Holy goodness, this series got a lot longer than I expected. I was hoping to get to some actual category theory, but let’s just talk a bit about LINQ and then wrap up. Maybe I’ll do a series on the basics of category theory in the future.

I’ve said several times that `SelectMany`

is the `bind`

operation on the sequence monad, but of course there are several overloads of the `SelectMany`

extension method of `IEnumerable<T>`

. The one that is `bind`

is as we previously discussed: [1. Error checking removed for clarity.]

public static IEnumerable<R> SelectMany<A, R>(
this IEnumerable<A> items,
Func<A, IEnumerable<R>> function)
{
foreach(A outer in items)
foreach(R inner in function(outer))
yield return inner;
}

Now you might think that when you say

from outer in items
from inner in function(outer)
select inner

this translates directly into a call to

items.SelectMany(outer=>function(outer))

**This is not what actually happens.**

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