Steven Schveighoffer wrote:
On Mon, 20 Sep 2010 15:45:10 -0400, Don <nos...@nospam.com> wrote:
bearophile wrote:
Jonathan M Davis:
I assume that if you declare a member function as pure, then all of
its parameters - including the invisible this - are included in
that. That is, if all of them - including the invisible this - have
the same value, then the result will be the same.
This D2 program runs with no errors, and here there isn't a D
language/compiler bug:
struct Foo {
int x;
this (int xx) { this.x = xx; }
pure int bar() { return x; }
}
void main() {
Foo f = Foo(1);
assert(f.bar() == 1);
f.x *= 2;
assert(f.bar() == 2);
}
Bye,
bearophile
You do need to be careful about concluding how 'pure' works based on
the current behaviour of the compiler.
There's a trap here. What if you use a hypothetical startTimer()
function which executes a delegate every few clock ticks?
void main() {
Foo f = Foo(1);
startTimer( () { f.x++; });
scope(exit)
killTimer();
assert(f.bar() == 1); // may fail!
f.x *= 2;
assert(f.bar() == 2);
}
Wouldn't f have to be shared for this to be asynchronous?
That's an excellent point. 'pure' was put into the language long before
'shared' and '__gshared'. It could now just mean, "doesn't use static,
globals, shared, or __gshared".
And then cachable pure is just: pure, + all reference parameters are
immutable.
If this becomes the rule, it seems likely that pure functions would
become far more common than impure ones.
I actually think that 'pure' on a member function can only mean, it's
cacheably pure if and only if 'this' can be cast to immutable. Which
includes the important case where the call is made from a pure
function (this implies that the 'this' pointer is either a local
variable of a pure function, or an immutable object).
Since pure functions cannot call impure functions, they can't do any
of this nasty asynchronous stuff.
I think it's ok for a function to be pure if all the arguments are
unshared, regardless of immutability. However, in order to cache the
return value, the reference itself must not be used as the key, but the
entire data of the reference. Even if it's immutable, wouldn't you not
want to cache the return values between two identical immutable objects?
Possibly, but my guess is that it would take too long to check.
