complex_nested_ce.fs

type IBuilderResult = interface end
type ICombinable = inherit IBuilderResult
type INonCombinable = inherit IBuilderResult

type First =
    | Payload of int 
    | Nested of ICombinable
    interface ICombinable
type Second =
    | Payload of string
    interface INonCombinable
    
and AllBuilder() =
    member __.Yield (xs: First list) = [ for x in xs -> x :> ICombinable]
    member __.Yield (xs: Second list) = [ for x in xs -> x :> INonCombinable]
    member __.Combine (x: ICombinable list, y: INonCombinable list) =
        Seq.cast<IBuilderResult> x
        |> Seq.append (Seq.cast<IBuilderResult> y)
        |> Seq.toList
    member __.Combine (x: ICombinable list, y: ICombinable list) = x @ y
    member __.Combine (x: ICombinable list, y: IBuilderResult list) =
        y @ (Seq.cast<IBuilderResult> x |> Seq.toList)
    member __.Delay f = f()
    member __.Zero() = []
and FirstBuilder() =
    member __.Yield (x: int) = [First.Payload x]
    member __.Yield (x: ICombinable) = [First.Nested x]
    member __.Combine (x, y) = x @ y
    member __.Delay f = f()
    member __.Zero() = []
and SecondBuilder() =
    member __.Yield (x: string) = [Second.Payload x]
    member __.Combine (x, y) = x @ y
    member __.Delay f = f()
    member __.Zero() = []
    
let all = AllBuilder()
let first = FirstBuilder()
let second = SecondBuilder()

let a = first { 1 }
let b = second { "2" }

// allowed to combine with non-combinable once
// IBuilderResult list
let example =
    all {
        a
        b
        //b
    }

// allowed to combine combinable how many times you want
// ICombinable list
let example2 =
    all {
        a
        a
        a
        a        
    }

// not allowed to combine with noncombinable
// INonCombinable list
let example3 =
    all {
        b
        // not allowed to place second b here
        //b 
    }
   
// allowd to combine combinable how many times you want and end with one non-combinable
// IBuilderResult list
let example4 =
    all {
        a
        a
        a
        b
    }