On Mon, 30 Nov 2009 15:46:45 +0300, KennyTM~ <kenn...@gmail.com> wrote:
On Nov 30, 09 19:15, Denis Koroskin wrote:
On Mon, 30 Nov 2009 13:52:48 +0300, Lars T. Kyllingstad
<pub...@kyllingen.nospamnet> wrote:
The following is a quote from a comment in the Reddit post about
asserts that Walter just linked to:
"Asserts are not without pitfalls, though. If you aren't careful,
what to put in the asserts, the program could behave differently"
A trivial example of this is:
int i;
write(i);
assert (++i == 1);
write(i);
Normally, this program prints "01", but in release mode it prints
"00". Asserts should never change the behaviour of a program, but
currently it is up to the programmer to verify that this is in fact
the case.
Would it be possible (and desirable) for the D compiler to statically
check that asserts have no side effects?
-Lars
Compiler must do full code flow analysis to do that:
bool foo(); // no body or body is very complicated
void main()
{
assert(foo()); // okay to call or not?
}
I believe this is doable with introduction of a new attribute -
@hasNoSideEffects (shorter name would be better) with the following
rules:
- @hasNoSideEffects functions cannot modify variables outside of a
function scope. It means that they can read globals, but can not get a
non-const reference (or pointer) to them.
- @hasNoSideEffects can only call other @hasNoSideEffects functions
- Body of an assert check has a @hasNoSideEffects attribute, and follows
the same rules.
@pure is a subclass of @hasNoSideEffects. @hasNoSideEffects functions
are also very useful for various performance optimizations.
I think it is as valid attribute as @pure (even though it might affect
code generation outside of a function body due to open optimization
opportunities).
Perhaps @pure semantics should be changed to @hasNoSideEffects (i.e.
allow non-invariant input arguments and allow accessing globals).
It will lose a memoization ability, but to be honest I don't think
memoization needs to be implemented based on @pure attribute alone. For
example, std.math.pow() is @pure, but is it worth memoizing? I think
it's programmer who should decide what functions are expensive enough to
be memoized, and these should be marked as such explicitly (@memoizable,
or something).
Can you give an example which @hasNoSideEffects but not @pure?
Sure:
// can't be pure because pure functions
// accept only immutable args
@hasNoSideEffects int deref(int* ptr)
{
return *ptr;
}
class Array
{
@hasNoSideEffects int length() const
{
return _array.length;
}
@hasNoSideEffects ref T opIndex(size_t index)
{
return _array[index];
}
private T[] _array;
}
Array!(int) array = ...;
for (int i = 0; i < array.length(); ++i) {
int v1 = array[i]; //
int v2 = array[i];
int v3 = array[i];
}
It the example above, array.length and i-th value are only evaluated once,
because length and opIndex both have no side effects.
BTW, any class invariant should also be marked as @hasNoSideEffects
implicitly and checked accordingly.
I think @pure attribute is too restrictive to be useful. I believe there
are an order of magnitude more functions that could be marked as
@hasNoSideEffects than those that could be marked as @pure.
