Seq.groupWith

groupwith.fsx

module Seq = 
    let groupWith grouper items = 
        seq {
            let mutable currentKey = Unchecked.defaultof<'key>
            let mutable currentSet = null
            for item in items do
                let thisKey = grouper item
                if currentKey = Unchecked.defaultof<'key> 
                then
                    // new grouping
                    currentKey <- thisKey
                    currentSet <- ResizeArray()
                    currentSet.Add item
                else 
                    // existing grouping
                    if thisKey = currentKey then
                        currentSet.Add item
                    else
                        // beginning of new grouping
                        yield currentSet :> seq<_>
                        currentKey <- thisKey
                        currentSet <- ResizeArray()
                        currentSet.Add item
            // don't forget to emit last group
            yield currentSet
        }