Question

I’ve got a project that I’m trying to convert to F#, with a fair amount of linq-to-IEnumerable style queries in it. I’m curious what the most elegant way of doing a join of multiple lists would be in F#. For instance if I have a bunch of C# code like the following

        var joinedList =
            from val1 in list1
            join val2 in list2 on val1.Key equals val2.Parent
            join val3 in list3 on val1.Key equals val3.Parent
            orderby val1
            select new {val1, val2, val3};

what would be the best way to translate this to F#? I’ll leave open the definition of “best”, but I’m looking for something functional rather than imperative, and preferably without converting my lists to seqs and/or importing System.Linq. Both lazy and eager solutions would be useful. And of course reusability is key, since I have queries like this all over. If there’s any way of using workflows to make the syntax more elegant, that would be great too.

Answer 1

Well, in Don’s blog about LINQ and the Powerpack:

Link

it shows off how to do LINQ join calls. But of course that’s on seqs and using LINQ. He also points out that a join is like a conditional. So I think e.g. your example is like

let joinedList = [
    for v1 in l1 do
    for v2 in l2 do
    if v1.Key = v2.Parent then
    for v3 in l3 do
    if v1.Key = v3.Parent then
    yield (v1, v2, v3)] |> List.sort

though I haven’t verified it on real data.