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Sat Nov 19 09:30:57 EST 2011

Meta Introduction

( This is an expansion of the blog header. )

Metaprogramming is about dealing with programs as data.  This is a
general principle that hides under the following names:

  - code generation
  - model based design
  - model driven development
  - active libraries
  - multi-stage programming
  - macro programming
  - language towers
  - domain specific languages
  - ...

There is a continuum of different techniques and not really a standard
one-size-fits-all approach.  I focus on formal languages (programming
languages) as models.  In this approach the main problems are:

   - Design of a Domain Specific Language (DSL) that is used to
     express a model.  A DSL lives in the trade-off space between:

       * Simplicity: Make analysis and compilation practically
         feasible.

       * Expressivity: Allow full specification of a solution in the
         problem domain.

   - Translation in the form of interpretation and compilation of DSL
     programs.

       * Static analysis: determine properties of the model without
         executing it in its final destination environment.

       * Code generation.  Transfer from high level form that allows
         static analysis, to low level form that allows execution in
         the field, i.e. as machine (or C) code executing on a
         microcontroller.

     Translation can involve a wide spectrum of techniques, but the
     basic trade-off seems to be about how to add implementation hints
     to a model.  This is a trade-off between:

       * A black-box compiler that does not need hints.  This is
         usually catalogued under the name ``partial evaluation''.

       * A white-box compiler that uses implementation strategies
         explicitly provided by the programmer, next to the model
         specification.  This is called ``staging''.

     Ideally, you'd want a black-box compiler to just solve the
     problem for you.  However due to a usually astronomically huge
     implementation space this is not feasible.  Aiming at a
     well-designed approach to providing implementation knowledge
     seems a second best approach.
       



Concerning the tool set I focus on two approaches, both based on the
idea that side-effect-free functional programs are easier to
manipulate (transform, analyze, verify).

  * Hygienic macros/modules combined with first class units.  Racketc
    (PLT Scheme) is a framework for building language towers based on
    the Scheme programming language.  See Staapl[1].

  * Polymorphism and types in typed functional programming.  Haskell's
    type class based polymorphism, (Meta)OCaml modules and MetaOCaml
    multi-stage code types.

[1] http://zwizwa.be/staapl
[2] http://zwizwa.be/-/libprim



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