[Ur] Structure within let

Thu Nov 9 06:12:04 EST 2017

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
>
>
>
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