Re: [Caml-list] Efficency of varient types

[Nicolas Cannasse <ncannasse@motion-twin.com>]

Michael D. Adams wrote:
> I have recently learned about OCaml and have been impressed by how
> fast it is in the benchmarks.  However I have discovered that variant
> types can slow down a program quite a bit.  For example, consider the
> Ackermann function implemented for int (see program 1) and the same
> Ackermann function implemented for a "type value = Int of int | String
> of int" (program 2).  The second one is ten times slower!  (Using
> ocamlopt.)

In order to understand what there is such difference, it's useful to 
learn the ocaml memory model at runtime :

- int are 31 bits unboxed value with last bit set to 1 in order to 
differenciate them with GC allocated pointers.
- tagged variants are GC allocated blocks with a discriminating "tag" in 
the header.
- chars and booleans are integers at runtime

The second bit is used to mark an exception but it's only internal and 
temporary when dealing with callbacks.

If you have a tagged variant where all constructors have a parameter, 
you can use Obj module to unbox the Int variant but the code is a lot 
less readable.

Nicolas