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Sun May 17 10:04:27 CEST 2009

removed front page stuff again..

I keep on doing this.. Why?  Anyways, the new introduction says almost
the same as this, but with examples.


<h2>Basic Elements</h2>

 <p>
   Metaprogramming is about manipulating programs as data.  A key
   element here is the representation of programs.  Being a compiler,
   Staapl has two languages to deal with: a high-level input language
   and a low-level output language. Both languages are
   <a href="http://en.wikipedia.org/wiki/Concatenative_programming_language">concatenative</a>.
   <ul>
   <li> Output language programs are represented as lists of
   instructions
   <code>[op]</code>.

   <li> Input language programs are represented as output language
   program transformer functions <code>[op]->[op]</code>.
   </ul>

<p> A meaning can be attached to high-level concatenative source
  code using the following compilation algorithm:
   <ol>
     <li> Read the tokens from the input program source, and
     associate each of them with an <code>[op]->[op]</code>
     transformer.
     <li> Compose all <code>[op]->[op]</code> transformers using
     function composition, in the order their tokens appear in the
     input program source.
     <li> Apply this function to the empty program <code>[]</code> to
     obtain the final output program.
   </ol>
   From this one could infer that each <code>[op]->[op]</code>
   function appends a small machine code fragment to the code
   accumulator, essentially behaving as
   an <a href="http://en.wikipedia.org/wiki/Assembly_language#Macros">assembler
   macro</a>.  However, the function is free to modify the accumulated code in
   its entirety, performing any optimization it sees fit.

<p> Using this representation the task of building code generators can
be split in these parts
  <ul>
    <li> High level: Composing generators/transformers is expressed
      using concatenation.  I.e. the Scheme form <code>(macro: 123
      +)</code> creates a composite code transformer, built from the
      code transformers <code>123</code> and <code>+</code>.
      The <code>macro:</code> form behaves as quasiquote, facilitating
      template programming. I.e. the previous transformer is
      equivalent to <code>(let ((x 123)) (macro: ',x +))</code>.

    <li> Low level: Creating language primitives as
      <code>[op]</code> processors, and possibly defining a machine
      instruction set <code>op</code> that has the double function of
      representing target semantics and serving as a representation of
      compile time data.
  </ul>


<h2>Macro Forth</h2>

<p> By interpreting the code list <code>[op]</code> as a stack and
  adding an <code>op</code> instruction <code>(QW value)</code> that
  represents loading of a literal value on the
   <a href="http://en.wikipedia.org/wiki/Forth_programming_language#The_stacks">parameter
   stack</a> at run time, it is possible to implement Forth with eager
   partial evaluation.
<p> I.e. the function associated to the token word <code>+</code>
  would normally only compile machine code that pops 2 numbers from
  the run-time stack, adds them and pushes the result to the stack.
  Instead it could be made to inspect the code it is passed for any
  instructions that load literal values on the runtime stack, remove
  those, perform the addition at compile time, and generate code for a
  constant instead.

<p> Note that the basic structure of <code>[op]</code> lists
  and <code>[op]->[op]</code> transformers is more general than stack
  languages.  In fact, it could be used to implement partially
  evaluating macro languages for any kind of state machine.

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