Re: Modules and Extensions

[Robert Bruce Findler <robby@cs.rice.edu>]

At 8:31 PM -0600 7/17/01, Matthew Flatt wrote:
>In general, they are to be used together. Modules can replace units
>where units are used merely for namespace control, but `unit' still has
>its own place:
>
>  * Implementing parameterized components: The `unit' form separates
>    interface from implementation, unlike `module'. For example, the web
>    browser is still a unit, so that its base GUI classes and ".plt"
>    handler can be parameterized.
>
>  * Encasulating a program that has "global" state: A unitized program
>    can be instantiated multiple times. For example, the compiler part
>    of mzc is still a unit so that it can be run easily by other tools.

DrScheme still uses units for two other reasons:

   * Mutually recursive linking: modules don't allow this.

   * Dynamic linking: the tool-set is not determined at compile
     time, so DrScheme dynamically loads and links tools.
     Modules handle some of this with `dynamic-require',
     but I wanted to provide imports to the dynamically
     linked component and those imports come from units
     (which you use, because they were linked recursively).
     I found the easiest thing was to use units.

DrScheme also uses multiply-invoked units. I'm not sure how mzc is 
ever invoked multiple times in one mzscheme or mred, but teachpacks 
certainly are (I'm still working on that, actually -- teachpacks may 
change...).

Robby