[Ur] Ur/Web -> JavaScript objects

[Mark Clements]

Adam: that is very cool -- thank you.

All: as a noob question, I can update a record using:
{A=1,B=2,C=3} --- [A=_,B=_] ++ {A=2,B=1}
or
{A=1,B=2,C=3} -- #A -- #B ++ {A=2,B=1}
==
{A=2, B=1, C=3}

How can I write a general record update function (assuming all fields in
the update record are in the base record), such that
let
   val base = {A=1,B=2,C=3}
   val update = {A=2,B=1}
in
   updater base update
end
==
{A=2, B=1, C=3}

I got as far as:

fun updater [tbase ::: {Type}] [tupdate ::: {Type}]
            (fl : folder tupdate) (base : $tbase) (update : $tupdate) :
$tbase =
     @foldR (* ?? *)
      (fn [nm ::_] [t ::_] [rest ::_] [[nm] ~ rest] t acc =>
          acc -- nm ++ {nm = t})
      base fl update

Any guidance would be appreciated.

Kindly, Mark.

On 8/10/19 10:57 pm, Adam Chlipala wrote:
> On 10/6/19 5:45 PM, Mark Clements wrote:
>> In defining an FFI to a JavaScript library such as Chart.js, config
>> options may be defined in terms of (nested) JavaScript objects with a
>> range of potential keys/fields. Analogous to the xml tags, is there
>> any way to define a record type in Ur/Web that potentially includes a
>> subset of fields?
>
> In my own FFI wrapping so far, I have just given fields [option] types
> when they are optional, dealing with writing in [None] values for
> unused fields manually.  I don't think it's so bad!
>
> Since I never miss an opportunity to show off metaprogramming in
> Ur/Web, here's a function you can use to cast a record to a wider type
> where all the new fields have [option] types.
>
> fun expand [original ::: {Type}] [additional ::: {Type}] [original ~
> additional]
>            (fl : folder additional) (r : $original)
>     : $(original ++ map option additional) =
>     r ++ @map0 [option] (fn [t ::_] => None) fl
>
> fun foo (x : {A : int, B : option string, C : float, D : option bool,
> E : string}) =
>     42
>
> val example = foo (expand {A = 3, C = 1.234, D = Some True, E = "hi"})
>
>
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