On 1/19/11 7:19 PM, Jens Mueller wrote:
Andrei Alexandrescu wrote:
On 1/19/11 5:53 PM, Jens Mueller wrote:
Hi,
I cannot make the following compile.
import std.functional;
import std.array;
import std.algorithm;
import std.stdio;
void main() {
auto numbers = [0, 1, 2, 3, 4, 5];
bool alwaysTrue(uint a) { return true; }
alias not!(alwaysTrue) alwaysFalse;
numbers = array(filter!(alwaysTrue)(numbers));
writeln(numbers);
numbers = array(filter!(alwaysFalse)(numbers)); // does not compile
writeln(numbers);
}
The line with alwaysFalse fails with:
/path/to/../src/phobos/std/algorithm.d(854): Error: constructor
std.algorithm.Filter!(not,int[]).Filter.this cannot get frame pointer to
not
Any ideas what I'm doing wrong or workarounds?
Jens
Place the call to not!alwaysTrue in a local function inside main:
bool alwaysFalse(uint a) { return not!alwaysTrue(a); }
Thanks. Can you elaborate a bit please? I wonder why the alias won't
work.
I thought of it for a bit. It's a limitation of the compiler that's
worth a bug report. The explanation is a bit involved. Let me start by
remarking that if you prepend "static" to alwaysTrue, the alias works as
expected:
static bool alwaysTrue(uint a) { return true; }
alias not!(alwaysTrue) alwaysFalse;
Without "static", alwaysTrue has access to a hidden pointer to the stack
frame of the function is in.
In theory a template should be unable to manipulate alwaysTrue because
of that frame pointer. But Walter has had this great idea of
instantiating templates in the context of the function they're in, so
they gain access to the frame pointer too. However, that instantiation
mechanism still has a few limitations. I think the code above runs into
one of them.
Andrei
