[Ur] Structure within let

Thu Nov 9 08:19:12 EST 2017

Matt's response is longer than I had planned!  My answer was:
1) Ur/Web doesn't support module definitions anywhere inside expressions.
2) It's actually quite intricate to get the typing and run-time behavior 
right if adding such a feature.  OCaml only added support for it in the 
last few years, I believe.

On 11/09/2017 06:12 AM, Matt Rice wrote:
> Hi, i'm certainly no good at urweb, but hopefully i can shed some light on this
> First a little bit of history:
>
> traditionally, e.g. in SML "let" works on expressions, e.g. let expr
> in expr end,
> while structure, is a form of declaration.
>
> There exists in SML, a let-esque variation which works on declarations, "local",
> where the syntax is "local dec in dec end".
>
> however with the advent of the module system, this has largely gone
> out of favor.
> with the advent of "ascription", (transparent, or opaque), There is a
> chapter on this in a sml book here:
> http://www.cs.cmu.edu/~rwh/isml/book.pdf#chapter.20
> ascription (also sometimes called sealing, or structural subtyping)
>
> from what I can tell ur/web drops support for the old "local" syntax,
> But still we can use signature ascription to hide structures like the
> "structure AB" within another structure.
> E.g, we have to flip it around somewhat from hiding with
> expressions/scoping to hiding within declarations.
> This appeared to compile, hopefully it works.
>
> structure Mymaths = struct
>    functor Make (M : sig val callback : string -> transaction {} end)
>       : sig val simple : string -> transaction string end
>       = struct fun simple x = M.callback x; return x end
> end
>
> signature CALLABLE =
> sig
>    val callMe : unit -> transaction unit
> end
>
> structure Callable : CALLABLE = struct
>    fun debugMe a =
>          alert a
>
>    structure AB = Mymaths.Make(struct val callback = debugMe end)
>
>    fun callMe() =
>          x <- AB.simple "test";
>          debug x
> end
>
> fun main()=
>      return <xml>
>          <body>
>              <h1> Sample </h1>
>              <button value="Click Me" onclick={fn _ =>
> Callable.callMe()}></button>
>          </body>
>          </xml>
>
> On Thu, Nov 9, 2017 at 1:10 AM, Nitin Surana <nsurana at usc.edu> wrote:
>> Hi
>>
>> I'm trying to define a structure within let block but it fails. Having the
>> structure outside works perfectly fine.
>>
>> sample.ur
>>
>> fun main()=
>> let
>>
>>      fun debugMe a =
>>              alert a
>>
>>
>>      structure AB = Mymaths.Make(struct
>>                                     val callback = debugMe
>>                                 end)
>>
>>      fun callMe () =
>>          x <- AB.simple "test";
>>          debug x
>>
>> in
>>      return <xml>
>>          <body>
>>              <h1> Sample </h1>
>>              <button value="Click Me" onclick={fn _ => callMe()}></button>
>>          </body>
>>          </xml>
>> end
>>
>>
>> Above code throws compile error, where as the following code compiles & runs
>> fine :
>>
>> fun debugMe a =
>>              alert a
>>
>>      structure AB = Mymaths.Make(struct
>>                                     val callback = debugMe
>>                                 end)
>>
>> fun main()=
>> let
>>      fun callMe () =
>>          x <- AB.simple "test";
>>          debug x
>>
>> in
>>      return <xml>
>>          <body>
>>              <h1> Sample </h1>
>>              <button value="Click Me" onclick={fn _ => callMe()}></button>
>>          </body>
>>          </xml>
>> end
>>
>>
>> Any idea what's going wrong ? Can't a structure be created within let ? If
>> no, then is there an alternative ?
>>
>>
>> --
>> Regards
>> Nitin Surana
>> MS Computer Science