[Ur] RPC call line throwing error

[Adam Chlipala]

I like Benjamin's solution.  To me, it clearly leads to better code, 
even independently of this issue.  I always try to avoid querying SQL 
row data that won't be used.

The problem has to do with embedding of server-side values in 
client-side code.  The type [client] should only appear in server-side 
code, so it's proper for the compiler to signal an error when 
[client]-containing variable [row] is mentioned in a GUI event handler.  
The error message arises because this faulty embedding prevents the 
compiler from translating code to JavaScript, and therefore the code to 
get the mouse-event info remains in server-side code, which isn't 
allowed, because only client code can call that function.

Clearly not a nice error message, but I don't plan to fix it, per se.  I 
have longer-term plans to reimplement Ur/Web or a similar system in a 
nicer way that avoids the problem with static typing.

On 10/25/2017 10:33 PM, Benjamin Barenblat wrote:
> On Tue, Oct 24, 2017 at 7:56 PM, Nitin Surana <nsurana at usc.edu> wrote:
>> I'll really appreciate if someone can help us. It shouldn't take more
>> than a few minutes - https://github.com/urweb/urweb/issues/94
> This took a bit more than a few minutes, and I'm still not entirely sure
> what's going on, but I eventually figured out how to fix your particular
> problem. I ran the compiler with the `-explainEmbed` flag, which gives
> more information about errors like this, and got
>
>      Can't embed
>      loc:  peers.ur:20:20-21:3
>        e:  UNBOUND_1.Peers.A
>        t:  FFI(Basis.client)
>      peers.ur:20:20: (to 21:3) Server-side code uses client-side-only identifier "Basis.mouseEvent"
>
> So I tried changing your `onclick` handler to avoid capturing
> `row.Peers.A` - i.e., I changed your `SELECT` statement to
>
>      SELECT Peers.C FROM peers WHERE peers.A > {[me]} OR peers.A < {[me]}
>
> and it compiled.
>
> This is definitely a case of a bad error message, but there may be more
> in play here. It sounds like the compiler is trying to evaluate the body
> of the `onclick` handler on the server instead of on the client, which
> seems wrong to me. This may also be related to Ur/Web's apparent
> inability to handle client IDs on the client side [1]. In any case,
> though, if anybody else has experienced issues like this, please speak
> up.
>
> As an unrelated aside, you can simplify your `SELECT` statement further
> - Ur uses `<>` as the inequality operator, so you can say
>
>      SELECT Peers.C FROM peers WHERE peers.A <> {[me]}
>
> with the same semantics.
>
> Best,
> Benjamin
>
>
> [1] https://github.com/urweb/urweb/issues/96
>
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