MR K P SCHUPKE wrote:
Hi Oleg, I like the polymorphic list indexed by Ints... there do seem to be a couple of differences between this and the list indexed by natural numbers.
...
Agreed with these differences. Another difference: it is initially a heterogeneous set rather than list! (Because the instance TH a (a, x) is for "found", and the overlapping instance TH a (b,c) is for proceeding with the rest of the set if a and b are not the same.) Of course, you could readily store lists of a's rather than a's, and always use [a] as access type rather than a. This deviation looks as follows: instance (Show a) => TH a ([a],x) where ... alter x (xs,y) = (x:xs,y) So a heteregeneous list would be modelled as a type-indexed set of homogeneous lists. That's pretty close. Still cool and simple. Ralf