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one-time initialization
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Date: 1999-01-28 (12:23)
From: Xavier Leroy <Xavier.Leroy@i...>
Subject: Re: one-time initialization
> 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;

The compiler certainly cannot do it for you, as it involves proving
that f and g are never applied before init is called...

> 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.

Here are a couple of faster, but perhaps less modular approaches:

* Get rid of the ref and the option, and initialize "global" with the
  right value the first time.  That involves reading the file when
  global is initialized, which might involve parsing (partially) the
  command-line to find the right file name.  E.g.:

  let global =
    let filename = <parse command line to find config file name, use default
                    file name if not found> in
    read_config_file global

  let f() = ... use global ...

* Keep the ref but get rid of the option.  You then need a dummy value
  of the type of your global state, and you're also no longer protected
  against forgetting to initialize it.

  let global = ref dummy_global_value
  let init i = global := i
  let f() = ... use !global ...


- Xavier Leroy