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  - Authors:
  -   Christian Schulte <schulte@ps.uni-sb.de>
  -
  - Copyright:
  -   Christian Schulte, 1998
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  -   $Date: 1999/01/21 09:52:36 $ by $Author: schulte $
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  - of Oz 3
  -    http://www.mozart-oz.org
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<chapter id="chapter.mof">
  <title/More On Functors/

  <section id="section.mof.computed">
    <title/Computed Functors/

    <p>
      We distinguish between <def/compiled functors/ and <def/computed
      functors/.  A compiled functor is obtained by compilation of a
      functor definition.  Computed functors are obtained by executing
      compiled functors whose definitions contain nested functor
      definitions. Compiled functors can only have lexical bindings to
      the data structures of the base environment. Computed functors
      can have lexical bindings to all data structures that the
      creating compiled functors supply to their definitions.

    <p>
      Pickled computed functors can carry computed data structures
      with them.  This matters since 
      <list enum>
        <item> 
          a computed data structure can now be loaded together
          with a functor rather than being computed  a new for each
          virtual machine using it. 
        <item>
          the functors needed to compute the carried with data
          structure are not needed by the virtual machine using it.
      </list>

    <p>
      Computed functors are syntactically supported by a <<prepare>>
      and <<require>> section. For example, the root functor
      definition in the file <file/LMF.oz/ can be rewritten using a
      <<prepare>> section as
      follows:
    <p>
<chunk class=anonymous><title/Computed Functor/
functor
import
   DB Form            % User defined
   System Application % System
prepare
   Flights = <chunk.ref/Sample flights/ 
define
   %% Enter some flights
   {ForAll Flights DB.add}
   ...
end
</chunk>

    <p>
      Here the difference between the compiled functor and the
      computed functor is that the compiled functor contains the code
      to create the list of sample flights. The computed functor just
      contains the list itself.

    <p>
      All variable identifiers that are introduced in the <<prepare>>
      section are visible in the <<define>> section. The variables
      introduced by the <<import>> section are of course only visible
      in the <<define>> section.

    <p>
      The <<require>> section of a computed functors relates to the
      <<prepare>> section as does the <<import>> section to the
      <<define>> section: modules imported in the <<require>> section
      are available in the <<prepare>> section.

  <section id="section.mof.import">
    <title/Imports/

    <p>
      <<import>> and <<require>> specifications support features and
      fields. For example, in the main functor for our last minute
      flight booking system, we could have written the import clause
      for <<DB>> as follows:
      <<<DB(add getAll remove)>>>
    
    <p>
      Besides of the documentational advantage of explicitly listing
      the features, the compiler tries to enforce that only listed
      features are used for dot access. For example, given the above
      import clause, the following access
      <<<DB.addd>>>
      raises an error during compilation.

    <p>
      In addition, also variables can be given as fields in the import
      specification as follows:
      <<<DB(add:Add getAll:GetAll remove)>>>
      The variables introduced for the fields interact with dynamic
      linking as follows: The module is requested as soon as the value
      for one of the variables is requested.
  
  
</chapter>
