interface Elem
    exposes [Elem, PressEvent, row, col, text, button, none, translate, list]
    imports [Action.{ Action }]

Elem state : [
    # PERFORMANCE NOTE:
    # If there are 8 or fewer tags here, then on a 64-bit system, the tag can be stored
    # in the pointer - for massive memory savings. Try extremely hard to always limit the number
    # of tags in this union to 8 or fewer!
    Button (ButtonConfig state) (Elem state),
    Text Str,
    Col (List (Elem state)),
    Row (List (Elem state)),
    Lazy (Result { state, elem : Elem state } [NotCached] -> { state, elem : Elem state }),
    # TODO FIXME: using this definition of Lazy causes a stack overflow in the compiler!
    # Lazy (Result (Cached state) [NotCached] -> Cached state),
    None,
]

## Used internally in the type definition of Lazy
Cached state : { state, elem : Elem state }

ButtonConfig state : { onPress : state, PressEvent -> Action state }

PressEvent : { button : [Touch, Mouse [Left, Right, Middle]] }

text : Str -> Elem *
text = \str ->
    Text str

button : { onPress : state, PressEvent -> Action state }, Elem state -> Elem state
button = \config, label ->
    Button config label

row : List (Elem state) -> Elem state
row = \children ->
    Row children

col : List (Elem state) -> Elem state
col = \children ->
    Col children

lazy : state, (state -> Elem state) -> Elem state
lazy = \state, render ->
    # This function gets called by the host during rendering. It will
    # receive the cached state and element (wrapped in Ok) if we've
    # ever rendered this before, and Err otherwise.
    Lazy
        \result ->
            when result is
                Ok cached if cached.state == state ->
                    # If we have a cached value, and the new state is the
                    # same as the cached one, then we can return exactly
                    # what we had cached.
                    cached

                _ ->
                    # Either the state changed or else we didn't have a
                    # cached value to use. Either way, we need to render
                    # with the new state and store that for future use.
                    { state, elem: render state }

none : Elem *
none = None # I've often wanted this in elm/html. Usually end up resorting to (Html.text "") - this seems nicer.
## Change an element's state type.
##
## TODO: indent the following once https://github.com/roc-lang/roc/issues/2585 is fixed.
## State : { photo : Photo }
##
## render : State -> Elem State
## render = \state ->
## child : Elem State
## child =
## Photo.render state.photo
## |> Elem.translate .photo &photo
##
## col {} [child, otherElems]
##
translate = \child, toChild, toParent ->
    when child is
        Text str ->
            Text str

        Col elems ->
            Col (List.map elems \elem -> translate elem toChild toParent)

        Row elems ->
            Row (List.map elems \elem -> translate elem toChild toParent)

        Button config label ->
            onPress = \parentState, event ->
                toChild parentState
                |> config.onPress event
                |> Action.map \c -> toParent parentState c

            Button { onPress } (translate label toChild toParent)

        Lazy renderChild ->
            Lazy
                \parentState ->
                    { elem, state } = renderChild (toChild parentState)

                    {
                        elem: translate toChild toParent newChild,
                        state: toParent parentState state,
                    }

        None ->
            None

## Render a list of elements, using [Elem.translate] on each of them.
##
## Convenient when you have a [List] in your state and want to make
## a [List] of child elements out of it.
##
## TODO: indent the following once https://github.com/roc-lang/roc/issues/2585 is fixed.
## State : { photos : List Photo }
##
## render : State -> Elem State
## render = \state ->
## children : List (Elem State)
## children =
## Elem.list Photo.render state .photos &photos
##
## col {} children
## TODO: format as multiline type annotation once https://github.com/roc-lang/roc/issues/2586 is fixed
list : (child -> Elem child), parent, (parent -> List child), (parent, List child -> parent) -> List (Elem parent)
list = \renderChild, parent, toChildren, toParent ->
    List.mapWithIndex
        (toChildren parent)
        \index, child ->
            toChild = \par -> List.get (toChildren par) index

            newChild = translateOrDrop
                child
                toChild
                \par, ch ->
                    toChildren par
                    |> List.set ch index
                    |> toParent

            renderChild newChild

## Internal helper function for Elem.list
##
## Tries to translate a child to a parent, but
## if the child has been removed from the parent,
## drops it.
##
## TODO: format as multiline type annotation once https://github.com/roc-lang/roc/issues/2586 is fixed
translateOrDrop : Elem child, (parent -> Result child *), (parent, child -> parent) -> Elem parent
translateOrDrop = \child, toChild, toParent ->
    when child is
        Text str ->
            Text str

        Col elems ->
            Col (List.map elems \elem -> translateOrDrop elem toChild toParent)

        Row elems ->
            Row (List.map elems \elem -> translateOrDrop elem toChild toParent)

        Button config label ->
            onPress = \parentState, event ->
                when toChild parentState is
                    Ok newChild ->
                        newChild
                        |> config.onPress event
                        |> Action.map \c -> toParent parentState c

                    Err _ ->
                        # The child was removed from the list before this onPress handler resolved.
                        # (For example, by a previous event handler that fired simultaneously.)
                        Action.none

            Button { onPress } (translateOrDrop label toChild toParent)

        Lazy childState renderChild ->
            Lazy
                (toParent childState)
                \parentState ->
                    when toChild parentState is
                        Ok newChild ->
                            renderChild newChild
                            |> translateOrDrop toChild toParent

                        Err _ ->
                            None

        # I don't think this should ever happen in practice.
        None ->
            None