How do I Aggregate multiple IEnumerables of T

Question

Given....

Public MasterList as IEnumerable(Of MasterItem)
Public Class MasterItem(Of T) 
    Public SubItems as IEnumerable(Of T)
End Class 

I

Answer 1

Enumerable.SelectMany is the key to the IEnumerable monad, just as its Haskell equivalent, concatMap, is the key to Haskell's list monad.

As it turns out, your question goes right to the heart of a deep aspect of computer science.

You will want to be careful with your phrasing, because Aggregate means something very different from SelectMany - even the opposite. Aggregate combines an IEnumerable of values into a single value (of possibly another type), while SelectMany uncombines an IEnumerable of values into even more values (of possibly another type).