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[Axiom-developer] Re: FW: data structure vs. mathematical structure

From: Ralf Hemmecke
Subject: [Axiom-developer] Re: FW: data structure vs. mathematical structure
Date: Wed, 15 Nov 2006 12:07:57 +0100
User-agent: Thunderbird (X11/20061025)

And that would match the usual definition of List as the least fixed
point of the functor

     X |-> 1 + X

in CPO.

In case anyone is wondering what Gaby is talking about, I think the
following paper by Dos Reis and Jarvi:

  What is Generic Programming?
  Library-Centric Software Design LCSD'05

provides a good introduction. It defines a List as the least fixed
point of

      X |-> 1 + T x X

Hopefully a future paper will deal more specifically with Axiom. :-)

In Axiom List could be defined something like this

  List(T:Type):ListAggregate == add
    Rep == Union(nil,Record(first:T,rest:%))

(perhaps it is in Aldor?) but in fact the actual definition refers
directly to the underlying Lisp architecture.

I had no need to define List this way, but what about binary trees?
The following code snipped is from Aldor-Combinat and it is running code.

---BEGIN test/

testRecursive1(): () == {
        macro {
                I == EmptySetSpecies;
                X == SingletonSpecies;
                + == Plus;
                * == Times;

        A(L: LabelType): CombinatorialSpecies L == (I + X*A*A)(L) add;

        import from Integer, Array Integer;
            [1, 1, 4, 30, 336, 5040],
            [1, 1, 2, 5, 14, 42]

---END test/

If you want List the line should be simply ...

  MyList(L: LabelType): CombinatorialSpecies L == (I + X*MyList)(L) add;

Well, but in LibAldor List is not defined as a real Union. It rather relies on the fact that an actual record can never be the nil pointer.

Oh, by the way, the above recursive definition of binary trees not only defines the data structure but also the exponential and ordinary generating series. So one could ask how many trees are there with 7 nodes (labelled or unlabelled). I fact, it should also be possible, to generate all these trees.

Recently, Martin Rubey has done some work to make Aldor-combinat available for Axiom, but for the underlying compilation we still need the Aldor compiler. So, Gaby, I consider Aldor-combinat a testcase for the compiler-improvements branch. ;-)


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