Something to note about event objects in javascript is that they can go out
of scope when you carry them away from the event handler, consequently, elm
does all decoding on the same stack as the event, and you're likely to get
a runtime error if you store the raw event value.
I decided to write a bit about the topic of hairy decoders:
https://medium.com/@prozacchiwawa/the-im-stupid-elm-language-nugget-16-295f201eb458
Using a Dict to enumerate all keys can work as you see, but you're also
exposed to strange values (such as the "item" function, and elm doesn't
like function values much).
Kind of wrote this as well because, despite that there's nothing super
wrong with it in the case of touch events (hopefully it only goes to 10),
my programmer sense goes off matching length to constants as in Max
Goldstein's reply.
TLDR for here: you can easily capture a Json.Decode.Value to use as part of
a decoder, as long as you take your info from it in the same stack, and
from there, you can convert code that uses map, andThen and Ok as results
to Json.Decode.Decoder.
On Saturday, May 6, 2017 at 9:12:32 AM UTC-7, Max Goldstein wrote:
>
> What about something like this?
>
> decodeTouches : Decoder (List Touch)
> decodeTouches =
> Decode.field "length" Decode.int
> |> Decode.andThen
> (\len ->
> case len of
> 1 ->
> Decode.map (\a -> [ a ])
> (Decode.field "1" touchDecoder)
>
> 2 ->
> Decode.map2 (\a b -> [ a, b ])
> (Decode.field "1" touchDecoder)
> (Decode.field "2" touchDecoder)
>
> 3 ->
> Decode.map3 (\a b c -> [ a, b, c ])
> (Decode.field "1" touchDecoder)
> (Decode.field "2" touchDecoder)
> (Decode.field "3" touchDecoder)
>
> _ ->
> Decode.fail "Unexpected length"
> )
>
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