On 19/06/2009, at 2:40 PM, Erick Tryzelaar wrote:
> But there's no way to say "it's a type error to modify this array"
> like C/C++ does. So, is this something we'd want? I personally think
> something like this would be useful, and I'd bet it'd help with
> optimization. We'd probably want something like this to work beyond
> just arrays and pointers.
"const" is the wrong idea though. It may help, but it isn't the
same as
int immutable x = 1;
int immutable *px = &x;
With "const" a function promises not to modify the pointed
at value. This helps the caller make optimisations.
With "immutable" the caller AND the callee promise
not to modify the value, this helps the callee make
optimisations, in particular it allows
cache = *px
and the function knows cache == *px always.
There is a language (forget the name, something C like) which
specialises in pointer "kinds".
However the original question is syntactic: even if Felix allows
var x = 1;
x += 1;
by simply
incrby : &int * int;
there is no way to make a C primitive return an lvalue. This means
we have to have, for example
Carray::get (a,i)
Carray::set(a,i,v)
and then
x.[i] += v
can be written:
set(a,i,get(x,i)+v)
but that is a long way off the C notation. We could do
a->[i] <- a.[i]
however, where
a->[i] // means &a[i] in C
>
> One way we could limit this a little bit is to make value arrays
> immutable, where these would be type errors:
>
> # val x = 1,2,3;
> # x.[0] = 5; <- type error
> # proc foo[N] (x:array[int,N]) { x.[0] = 5; } <- type error
>
This is already an error, just not a type error.
However this is OK:
var x = 1,2,3;
x.(0) = 99;
as it should be, because x is a variable.
--
john skaller
skal...@users.sourceforge.net
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