On Thu, 2006-08-24 at 09:49 -0700, Erick Tryzelaar wrote:
> skaller wrote:
>
> I really think that something like this would be the simplest solution.
> And technically, wouldn't it decompose from this:
>
> proc f(x:int): int {
> return x + 1;
> }
> Into this:
>
> proc f(x:int) (px:int) {
> *px = x + 1;
> }
You should note this is only one possibility. Another is:
side_effect fun f(x:int): int { .. }
and there are several more :)
In general, a 'function' can:
* depend on constants in its environment or not
* depend on variables in its environment of not
* depend on a variable in the callers environment
via a pointer argument
* modify an environment variable
* modify a caller variable via a pointer
The first case is already optimised: if no part
of the environment is used, the function can be
modelled as a "C" function without any thread frame
pointer being passed.
However the dependence on environmental variables can
be annotated in the type .. variable by variable!
This is very precise:
fun: A -> B depending on x,y modifying z
but interestingly, whilst this determines how far
we can move the call, without changing the semantics,
it doesn't determine the _correct_ place to do the call!
Given:
var x = 1;
print x;
x = 2;
we expect to print 1: the call must be done fairly
eagerly. But here:
var i = 0;
while { i < 10 } { ..++i; }
we had better evaluate i < 10 lazily. In this case the
notation { .. } ensures lazy evaluation. But more
generally, what would you define as the meaning
of an expression with interacting side effects?
It isn't enough to say "eager evaluation" because as I
pointed out before that defeats:
cond? then_expr : else_expr
where you have to evaluate lazily.
--
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net
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