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[Caml-list] OCaml Speed for Block Convolutions
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Date: 2001-06-06 (02:04)
From: Hugo Herbelin <Hugo.Herbelin@i...>
Subject: Re: [Caml-list] OCaml Speed for Block Convolutions
> >         let a = ref 0. in
> >         for i = 0 to n-1 do
> >           a := !a +. Array.unsafe_get xs i
> >         done
> ...
> Why not have a construct that expresses what
> the programmer really wants:
>   let mutable a = 0. in
>   for i = 0 to n-1 do
>     a <- a +. Array.unsafe_get xs i
>   done
> Tom

I was precisely asking myself the same question, initially motivated
by explaining the imperative constructs of ML to programmers coming
from the imperative side. I mean how to explain that you find in ML a
regular for-loop, a regular while-loop but you need to use pointers to
translate assignments as in the code at top above which really only
corresponds to the following C code:

int i;
double *a = (double *) malloc(sizeof(double)); *a = 0;
for (i=0;i<=n-1;i++) *a = *a + xs[i];

Assume more generally that you can modify any local variable as in the
(standard) following example:

let fact (mutable n) =
  let mutable r = 1 in
  while n > 0 do
     r <- r * n;
     n <- n - 1

At first, this is in contradiction with the substitution property of
purely functional languages, like "let x = 1 in ... x ..." should
behave the same as "... 1 ...", but it then should be enough to
explain that a while-loop binds over all the variables modified in its
body, as the functional interpretation of the while-loop shows it

let fact n =
  let r = 1 in
  let rec loop (r,n as effects) =
     if n > 0 then
       let r = r * n in
       let n = n - 1 in
       loop (r,n)
     else effects in
  let (r,n) = loop (r,n) in

and the substitution property is preserved.

The compilation model of ocaml is already able to handle this (at
least the bytecode, in fact to realize the incrementation of the
for-loops counter) and the typing would just be monomorphic as for 
any mutable. May there be any problem I don't see? Is there a risk
to allow more imperative-style algorithms? Could it really lead to
a sensible benefit in efficiency for some typical algorithms?

At least, it seems there is some limitations with higher-order

> let a = ref 0 in
> Hashtbl.iter (fun k d -> a := !a + d) my_hash

Following the same principe as before, an alternative would be

let a = mutable 0 in
Hashtbl.iter (fun k d -> a <- a + d) my_hash

explaining that the substitution property is preserved by the fact
that a function too binds over the variables it modifies, even if in
this case, the functional translation needs to add an extra arg to

But since only "immediate" variables of a function are not copied (all
the variables declared by and since the closer surrounding "fun"), the
last option will not work: when building the closure, a new copy of
the value of a is put in the closure environment and further updatings
will not affect the initial cell.

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