[Ur] common type for different functor instantiations

[Adam Chlipala]

Gergely Buday wrote:
> I have the following problem:
>
> record ::: {Type} is fixed, [data] is dependent on [record] and
> [MspecificFields]
>
> I have a functor, that, among others, contain
>
> functor Render (M: sig
>                                 con MspecificFields :: {Type}
>                                 val renderMspecific : $MspecificFields ->  xbody
>                               end )
> : sig
>      render : data ->  xbody
>    end
>
> I call this for three different M, but now I want to rewrite it so
> that the resulting module has the signature
>
> sig
>    type mRecord
>    getRecord : data ->   mRecord
>    render :  mRecord ->  mRecord ->  xbody
> end
>
> so that the caller can store the previous value of mRecord and the
> render function can compare the fresh and the stored. Since the
> argument structures differ in some fields, I cannot declare a common
> type for mRecord, or, at least, I do not see how I could.
>
> Is there a solution for this in ur/web?
>    

Sure: define [mRecord] outside the functor completely.  It could be an 
abstract type in an enclosing module.  (Modules can contain functors, 
which can contain modules that contain functors, and so on ad infinitum.)