[Aldor-l] [Axiom-developer] spad: language and compiler

[Ralf Hemmecke]

> Probably what we need is some kind of recursive type constructor.
> See for example:
> 
> http://www.cs.kent.ac.uk/people/staff/sjt/Atypical/AldorExs/vector.as

If you look closer to that, then you see that vector.as is implementing 
"List" in a terribly complicated way.

Look at

-- Add up the entries of a Vector(n).
addVec(n:Integer,v:Vector(n)):Integer
     == {
     import from Record(fst:Integer,rst:Vector(n-1));
     if n<=1
           then 0
	  else vec.fst + addVec(n-1,vec.rst);
	  where { vec == (v pretend				-- A
	  		 Record(fst:Integer,rst:Vector(n-1))); }};

in particular look at the argument. If it were a list you could forget 
about the n, but here you have to give it. I am sure, nobody would ever 
use such code.

If you must have a union over Vectors of arbitrary lenght then a cleaner 
design would be to define vectors of lenght n and in a second step 
define operations on vectors of different length.

In aldor-combinat you would define that thing like in the BinaryTree 
example I gave before but just use the equation V = 1 + X * V.
And, by the way, I did not use "pretend" in my code.

Ralf