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).
