open Microsoft.FSharp.Linq

​

type QueryBuilder with

  [<CustomOperation("where2",MaintainsVariableSpace=true,AllowIntoPattern=true)>] 

  member this.Where2 (source, [<ProjectionParameter>] predicate) = this.Where(source, predicate)

  [<CustomOperation("countAsync")>] 

  member __.CountAsync (source:QuerySource<'T,'Q>) = async { return System.Linq.Enumerable.Count source.Source }

​

let x = query { 

    for i in 1 .. 10 do

    where2 (i % 2 = 0)

    countAsync

}

​

printfn "%A" (Async.RunSynchronously x)