How do I write a computation expression builder that accumulates a value and also allows standard language constructs?

Question

I have a computation expression builder that builds up a value as you go, and has many custom operations. However, it does not allow for standard F# language constructs, and I\'

Answer 1

Following a similar struggle to Joel's (and not finding §6.3.10 of the spec that helpful) my issue with getting the For construct to generate a list came down to getting types to line up properly (no special attributes required). In particular I was slow to realise that For would build a list of lists, and therefore need flattening, despite the best efforts of the compiler to put me right. Examples that I found on the web were always wrappers around seq{}, using the yield keyword, repeated use of which invokes a call to Combine, which does the flattening. In case a concrete example helps, the following excerpt uses for to build a list of integers - my ultimate aim being to create lists of components for rendering in a GUI (with some additional laziness thrown in). Also In depth talk on CE here which elaborates on kvb's points above.

module scratch

    type Dispatcher = unit -> unit
    type viewElement = int
    type lazyViews = Lazy>

    type ViewElementsBuilder() =                
        member x.Return(views: lazyViews) : list = views.Value        
        member x.Yield(v: viewElement) : list = [v]
        member x.ReturnFrom(viewElements: list) = viewElements        
        member x.Zero() = list.Empty
        member x.Combine(listA:list, listB: list) =  List.concat [listA; listB]
        member x.Delay(f) = f()
        member x.For(coll:seq<'a>, forBody: 'a -> list) : list  =         
            // seq {for v in coll do yield! f v} |> List.ofSeq                       
            Seq.map forBody coll |> Seq.collect id  |> List.ofSeq

    let ve = new ViewElementsBuilder()
    let makeComponent(m: int, dispatch: Dispatcher) : viewElement = m
    let makeComponents() : list = [77; 33]

    let makeViewElements() : list =         
        let model = {| Scores = [33;23;22;43;] |> Seq.ofList; Trainer = "John" |}
        let d:Dispatcher = fun() -> () // Does nothing here, but will be used to raise messages from UI
        ve {                        
            for score in model.Scores do
                yield makeComponent (score, d)
                yield makeComponent (score * 100 / 50 , d)

            if model.Trainer = "John" then
                return lazy 
                [ makeComponent (12, d)
                  makeComponent (13, d)
                ]
            else 
                return lazy 
                [ makeComponent (14, d)
                  makeComponent (15, d)
                ]

            yield makeComponent (33, d)        
            return! makeComponents()            
        }