[Ur] Set / Update values within a structure

Sat Nov 11 12:22:50 EST 2017

The original question smells like a potential XY problem (
http://xyproblem.info/). What's the actual goal? There is probably a
functional pattern that solves the problem in a nicer way than the
imperative mutation of a callback function.

Nitin, if you specifically want to call a mutable Ur/Web callback from
JavaScript, it's probably easiest to pass values you need (sources,
callback functions, or anything else) through a custom JavaScript FFI
function (below called passToJs) that you call from inside main. You can
set global JavaScript variables or do whatever else from inside passToJs.

val main =
    src <- source (fn () => return ());
    return <xml>
      <active code={passToJs (fn () => f <- get src; f ())}/>
      (* XML that may mutate src *)
    </xml>

But I still suspect that this isn't the best solution to whatever the real
problem is!

Ziv

On Fri, Nov 10, 2017 at 17:54 Nitin Surana <nsurana at usc.edu> wrote:

> Hi Adam
>
> I tried to create a sample from what I understand about your suggestion,
> but of course I got it wrong.
> Following is the code (which doesn't compile), which would demonstrate
> what I'm trying to do (look for 4 forward slashes ////)
>
> fun debugMe a =
>         alert a;
>         ////can't read from source n here, because it's defined within main(), below doesn't compile
>         fnn <- get n;
>         fnn()
>
>
>
> structure AB = Mymaths.Make(struct
>                                val callback = debugMe
>                            end)
>
> fun main()=
>
>   n <- source Nil;
>   lss <- source Nil;
>
>
> let
>     fun callMe () =
>         x <- AB.simple "test";
>         debug x
>
>     fun anotherCallback()
>         //iterate over source lss and do something
>
> in
>     //Some code to change the callback debugMe to anotherCallback() but what ?!
>     set n anotherCallback   ////this doesn't work
>
>
>     return <xml>
>         <body>
>             <h1> Sample </h1>
>             <button value="Click Me" onclick={fn _ => callMe()}></button>
>         </body>
>         </xml>
> end
>
> Thanks
>
>
> On Fri, Nov 10, 2017 at 1:45 PM, Adam Chlipala <adamc at csail.mit.edu>
> wrote:
>
>> You can register a callback that, when called, reads another callback out
>> of a [source] and calls it.  Is that sufficient?
>> On 11/10/2017 04:19 PM, Nitin Surana wrote:
>>
>> Hi Adam
>>
>> Thanks for responding. Is there any alternative to change the callback ?
>> I want to trigger a urweb function defined within `let` block upon a
>> javascript event.
>> Since, the function is within `let` block (because it needs access to
>> certain variables within the parent function), I'm unable to use execF(...)
>> from javascript.
>>
>> Also, is `ref` not supported in urweb ?
>>
>> Thanks
>>
>> On Fri, Nov 10, 2017 at 4:48 AM, Adam Chlipala <adamc at csail.mit.edu>
>> wrote:
>>
>>> On 11/09/2017 10:42 PM, Harshita Kasera wrote:
>>>
>>> What I ultimately want to achieve is to be able to change the callback
>>> function *debugMe* at a later point of time. Is it possible to set or
>>> update values within a structure at a later point of time?
>>>
>>>
>>> No, module (structure) fields can't be modified at runtime in Ur/Web or
>>> ML.  Therefore, the module system is not the right tool for runtime
>>> rewriting of callbacks.
>>>
>>
>>
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>
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>>
>>
>
>
> --
> Regards
> Nitin Surana
> MS Computer Science
> 323-690-6529
>
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