How to map an IGrouping<Key, ValueA> to a IGrouping<Key, ValueB>?

Question

Given a function f:ValueA -> ValueB, how could I map an IGrouping of type IGrouping<Key, ValueA> to IGrouping<Key, ValueB>?

Problem instance:

Say you have this type:

TaggedItem = { Tag:Tag ; Item:Item }

and this query:

query {
    for i in taggedItems
    groupBy i.Tag into g
    select g
}

This would give you a seq of type: IGrouping<Tag, TaggedItem>, but I really want a seq of type: IGrouping<Tag, Item>. The mapping function is: fun taggedItem -> taggedItem.Item

Solution

The solution is to avoid the mapping of groupings and instead do the transformation while doing the group, using groupValBy, as pointed by the selected answer. The selected answer also shows how to do the mapping from one type of grouping to the other, if you insist.

query {
    for i in taggedItems
    groupValBy i.Tag i.Item into g
    select g
}

Answer 1

How about this?

let mapGrouping f (xs : IGrouping<_,_>) =
    let projection = xs |> Seq.map (fun x -> xs.Key, f x)
    (projection.GroupBy (fst, snd)).First()

From your code example, I think you want this:

query {
    for i in taggedItems do
    groupValBy i.Item i.Tag into g
    select g
    }

At https://msdn.microsoft.com/en-us/library/hh225374.aspx, we learn that groupValBy "selects a value for each element selected so far and groups the elements by the given key."