On 2/8/12 6:54 AM, Marijn Haverbeke wrote:
That seems a rather heavy-handed restriction.
Perhaps? All I am saying is that assigning to value types that contain
immutable fields is illegal, because it overwrites those immutable
fields. You can declare the record like:
let x = {mutable a: 10};
x = {mutable a: 20};
if you wish to allow mutability. The fact that this notation is painful
is more a by-product of using structural record types than anything else.
An alternative is that declaring a field as immutable does not in fact
mean that it will not be overwritten. We could make that sound (no
covariance, for example), but it means we (and the language's users)
have to ultimately treat all fields as potentially mutable. So I'm not
sure what the point is.
Maybe there are other alternatives?
How about this:
let x = @{mutable y: 10};
let f = fn@(u: int) { x.y = 20; log(error, u); }
f(x.y); // Where f's arg is passed by reference, whichever
notation you'd use for that
Yes, this is precisely the kind of problem I was looking for! Well,
this example itself I think is ok: it prints "20" (the wonders of
aliasing). But an example like this:
let x = @{mutable f: some(10)};
let f = fn@(o: option<int>) {
alt o { some(f) { x.f = none; log(f); }
}
f(x.f);
is certainly bad. We basically have to treat && mode as potentially
mutable for the purposes of matching in alt statements. So the
mutability algorithm from mut.rs needs to be somewhat adjusted to yield
a 3-way answer: immutable, const, or mutable (actually I can't recall if
it does this already or not).
One annoyance is that we don't have a way to express a reference to some
arbitrary immutable field. Moving to something like regions would allow
a type modifier `&` that supported `const` and `mut` just like `@`,
which would make it possible to express that (above, `o` would have type
`&const option<int>`: `&option<int>` would be a type error, as that
would mean a ptr to immutable memory, and `x.f` is not immutable).
Niko
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