[Caml-list] Even at compile time 2*2=4!

[Caml-list] Even at compile time 2*2=4!

[Jakob Lichtenberg]

Fellow O'Caml hackers,

I have this complicated module 'fourLib.ml' that binds the value 'four' to
the *computed* value of four:
---
let four = (Printf.printf "Now we do the complicated calculation of
2*2...\n";
            let res = 2*2
            in res);
---

A sample application 'sixteen.ml':
---
let _ = Printf.printf "4 * 4  is %d" (FourLib.four * FourLib.four);
---


I wish to package/compile my 'fourLib' so that I can avoid doing the
complicated calculation '2*2' each and every time I load 'fourLib'.  (I am
sure that you can imagine an even more requiring computational task than
evaluating '2*2'.)


Currently I build as follows:
  > ocamlc -c fourLib.ml
  > ocamlc  -c sixteen.ml
  > ocamlc -o sixteen fourLib.cmo sixteen.cmo

Unfortunately 2*2 is first calculated when I run the application:
  > ./sixteen
  Now we do the complicated calculation of 2*2...
  4 * 4  is 16
---

1. I am willing to rewrite the library, the way stuff is build, etc....  I
basically just want ocaml to evaluate 2*2 before the final linking.  Is my
only hope to serialize Four.four and then load it?


2. Can I build a new top-level interactive environment 'ocaml_with_fourLib'
where this module is 'pre-loaded'.  Again, the actual calculation of 2*2
should be done when I build ocaml_with_fourLib - not when I start it.

Thanks,

- Jakob Lichtenberg

--
Jakob Lichtenberg - jl@itu.dk - http://www.itu.dk/people/jl/
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Re: [Caml-list] Even at compile time 2*2=4!

[Alessandro Baretta]

Jakob Lichtenberg wrote:

> 
> I wish to package/compile my 'fourLib' so that I can avoid doing the
> complicated calculation '2*2' each and every time I load 'fourLib'.  (I am
> sure that you can imagine an even more requiring computational task than
> evaluating '2*2'.)

I wish to suggest one possibility.

1) You write a module defining the type of the datastructure 
you wish to precompute and load
2) You write a program importing the above module, which 
computes the datastructure and marshals it to a file. You 
run this program.
3) You write a library which, upon being loaded, searches 
the filesystem for the appropriate file, unmarshals it and 
binds the precomputed data to an identifier.
4) You write your programs referring to the the identifier 
exported by #3.

eg.

module: type_of_four.ml
*********************
type four = int


module: compute_four.ml
**********************
open Type_of_four

let four:four = 2 * 2

let _ = Marshal.to_channel (open_out "foo") four []


module: four.ml
***************
open Type_of_four

let (four:four) = Marshal.from_channel (open_in "foo")


module: sixteen.ml
******************
open Four

let sixteen = four * four


Cheers,
Alex

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Re: [Caml-list] Even at compile time 2*2=4!

[Pierre Weis]

> Jakob Lichtenberg wrote:
> 
> > 
> > I wish to package/compile my 'fourLib' so that I can avoid doing the
> > complicated calculation '2*2' each and every time I load 'fourLib'.  (I am
> > sure that you can imagine an even more requiring computational task than
> > evaluating '2*2'.)
> 
> I wish to suggest one possibility.
> 
> 1) You write a module defining the type of the datastructure 
> you wish to precompute and load
> 2) You write a program importing the above module, which 
> computes the datastructure and marshals it to a file. You 
> run this program.
> 3) You write a library which, upon being loaded, searches 
> the filesystem for the appropriate file, unmarshals it and 
> binds the precomputed data to an identifier.
> 4) You write your programs referring to the the identifier 
> exported by #3.
[...]
> Cheers,
> Alex

I clearly see what you want to do, this is as old as functional
programming (remember the old LISP eval-when-compile form).

As far as I know people want that feature because they think (well in
fact, they say they KNOW for sure) that this will speed up their
programs. This is unclear to me.

I remember to have read the source code of a CAS, entirely written in
Lisp, that started by computing a lot of constants with a huge number
of decimals (including $\pi$ and $e$). So, I said "he guys, you could
easily speed up your stuff by just defining the pre-calculated
constants instead of computing them from scratch again and again at
each start of your program!"

They politely thanked me for the suggestion, but they had to let me
know that they knew from time measurements that this ``presumably
enhanced'' method was much slower than re-computation: building the
constants directly in memory via computation was much faster than any
way they had found to get them stored into the memory at the beginning
of the program. Effectively, at launch time the constants have to be
read from files and this is really slow !

(The constants written at compile time in the executable are stored to
disk anyway and have to be read back when launching the program: in
the case of the CAS program, loading those huge constants with the
executable was slower than loading a small executable that re-computed
them from scratch!)

Life is not that simple...

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://pauillac.inria.fr/~weis/


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Re: [Caml-list] Even at compile time 2*2=4!

[Alessandro Baretta]

Pierre Weis wrote:

> (The constants written at compile time in the executable are stored to
> disk anyway and have to be read back when launching the program: in
> the case of the CAS program, loading those huge constants with the
> executable was slower than loading a small executable that re-computed
> them from scratch!)
> 
> Life is not that simple...
> 
> Pierre Weis

I agree. This method is only useful if computational 
complexity is much greater than space complexity. Besides, 
even complexity is actually a fairly volatile concept in 
this case, because we are talking about computing a constant 
function (no input).

To be honest, I can't think of a situtation where I'd use my 
own hack. I wonder what was Jakob's reason for posting 
originally.

Alex

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Re: [Caml-list] Even at compile time 2*2=4!

[Daniel de Rauglaudre]

Hi,

On Mon, Nov 18, 2002 at 07:18:34PM -0800, Jakob Lichtenberg wrote:

> 2. Can I build a new top-level interactive environment 'ocaml_with_fourLib'
> where this module is 'pre-loaded'.  Again, the actual calculation of 2*2
> should be done when I build ocaml_with_fourLib - not when I start it.

There is no difference between "load" a module and "start" it: the fact
of loading it is starting it.

In some cases, you could resolve your problem with Camlp4, if your
computation can be done at parse time. We can imagine a syntax
extension, e.g. "compute" followed by an expression, ask the
preprocessor to evalutate it and build the syntax tree of the
result. For example:
    compute (2*2)
would generate at parse time:
    4

But this could not work:
    let x = 2 in
    compute(x*x)
because the preprocessor does not know what is "x".

On the other hand, if "x" is defined as a directive to the preprocessor,
for example with "DEFINE x = 2" (new syntax extension pa_macro.cmo), it
could work.

-- 
Daniel de RAUGLAUDRE
http://cristal.inria.fr/~ddr/
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Re: [Caml-list] Even at compile time 2*2=4!

[David Brown]

On Tue, Nov 19, 2002 at 12:12:01PM +0100, Daniel de Rauglaudre wrote:

> In some cases, you could resolve your problem with Camlp4, if your
> computation can be done at parse time. We can imagine a syntax
> extension, e.g. "compute" followed by an expression, ask the
> preprocessor to evalutate it and build the syntax tree of the
> result. For example:
>     compute (2*2)
> would generate at parse time:
>     4
> 
> But this could not work:
>     let x = 2 in
>     compute(x*x)
> because the preprocessor does not know what is "x".

To be useful, you would probably also have to be able to define
functions for the preprocessor that could be used to compute the value.

This also makes the interesting possibility of compiles that don't
terminate...

      compute (forever ())

Dave Brown
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