Re: one-time initialization

[Gerd Stolpmann <Gerd.Stolpmann@darmstadt.netsurf.de>]

On Thu, 28 Jan 1999, Michael Hicks wrote:
>I wonder if anyone knows how to optimize the following (simplified for the
>sake of dicussion) situation:
>
>let global = ref None
>let init i =
>  global := Some i
>let f () =
>  match (!global) with
>    Some x -> x
>  | None -> failwith "not initialized";;
>let g() =
>  match (!global) with
>    ...
>
>Essentially, there is some global state that is initialized once, and is
>used by all functions in the module.  In a more realistic situation, this
>state might be initialized by reading in a file.  Given that following
>initialization the global state never changes, it should be conceivable to
>eliminate the match and dereference; on my machine (pentium 166), the match
>and dereference result in about a 30% slowdown.  I've fooled around with
>some things, but haven't found anything that performs better than this
>straightforward approach or is any more elegant.

If you encapsulate your module, there is a solution that may eliminate
the need of an 'option' type
(I assume that 'global' has type t ref, and that v is a known value of t):

module M :
  sig
    type initialized
    val init : t -> initialized
    val f : initialized -> t
    val g : initialized -> ...
  end =
  struct
    type initialized = unit
    let global = ref v
    let init i = global := i
    let f () = !global
    let g () = ...
  end
    

You can only call f and g if you have a value of type 'initialized', and your
only chance to get it is to call 'init'. This is a good example how to use
types as assertions about state. This simply means that the type checker prooves
for you that f and g are never called before init.

To get rid of dereferencing is impossible. Perhaps a mutable
record is faster in some cases.

--
----------------------------------------------------------------------------
Gerd Stolpmann      Telefon: +49 6151 997705 (privat)
Viktoriastr. 100             
64293 Darmstadt     EMail:   Gerd.Stolpmann@darmstadt.netsurf.de (privat)
Germany                     
----------------------------------------------------------------------------