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Allow Doc.Element to take seq<#Doc> children
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Now that HTML element construction functions (div, divAttr, etc.) return an Elt, rather than a Doc, it would be convenient if they took a seq<#Doc> as their children, so we don't have to use upcasts.

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It's a choice we've been struggling with for a long time, because we already had the same problem in Html.Client choosing between seq<Pagelet> and seq<#Pagelet>. The problem is that in both cases upcasts are needed, just in different places.

  • If we take seq<Doc>, as is currently the case, then a cast is needed in situations such as:

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    let elt =
    div (
        items
        |> Seq.map (fun item ->
            // without the cast below, type error: expected Doc, got Elt
            span [text item] :> _)
    )

  • If we take seq<#Doc>, then a cast is needed in situations such as:

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    let elt =
    div [
        span [text "this is an Elt"] :> Doc
        // without the cast above, type error below: expected Elt, got Doc
        text "this is a Doc"
    ]

Unfortunately there will be no catch-all solution until F# either supports type parameter covariance (which would make seq<Doc> work), or avoids freezing the type of a seq<#T> to the first element in the list/seq/array literal (which would make seq<#Doc> work). I think I'll submit a language feature proposal for the latter (the former has already been asked for a lot).

By on 8/27/2015 9:03 AM ()

Yes, I see the problem.

Here is what I'm doing to make life nice for me. It addresses this issue at the same time:

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[<JavaScript>]
module Alternative =
    // these are also useful, but nothing to do with this topic
    type attr with
        static member cls = attr.``class``
        static member typ = attr.``type``

    // I use these most of the time
    let inline div atts els = divAttr atts els :> Doc
    let inline a atts els = aAttr atts els :> Doc
    let inline ul atts els = ulAttr atts els :> Doc
    let inline li atts els = liAttr atts els :> Doc

    // I use these if I need to do something imperative
    let divElt = divAttr
    let canvasElt = canvasAttr

Most of the time, I only need a Doc, so the plain HTML names like div return a Doc. On the rare occasions when I need an Elt (e.g. I need to assign to the Text field), I use divElt, which returns an Elt.

By on 8/28/2015 1:42 AM ()

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