> >         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
  done;
  r

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
  r

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
functions.

> 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
iter.

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.

                                                  Hugo
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