On 2/22/11 5:15 AM, Mafi wrote:
Am 22.02.2011 11:29, schrieb %u:
Having learned functional programming in Scheme a couple months ago, I
tried my hand at using map(), reduce(), and filter() in D:
int addend = 5;
map(delegate int(int x) { return x + addend; }, iota(1, 5));
but it didn't work. It turned out that map() actually took the mapper
as its _template_ argument, not as a function argument. Not too much of
a problem, it probably seemed to be...
except that it's a critical problem. It makes map(), reduce(),
filter(), etc. to become 10x less useful, because there's no way to
change
their behavior at runtime, depending on program's state.
I did never try and so I'm unsure but shouldn't you be able to give a
delegate variable as template parameter.
std.algorithms.map's parameter is an alias parameter so it should be
able alias your variable and use it's runtime value.
If it does not work this way it's a bug IMO.
Mafi
Indeed. The solution to OP's problem is std.algorithm.map. Local
instantiation should take care of aliases that refer to local symbols,
so OP's original complaint about std.algorithm.map is invalid (albeit
for a subtle reason). The following code compiles as expected:
import std.algorithm, std.stdio;
auto fun(int[] a)
{
auto y = 3;
auto m = map!((x) { return x < y; })(a);
return m;
}
void main()
{
auto a = [ 1, 2, 3, 4, 5 ];
auto m = fun(a);
writeln(m);
}
Note how map's lambda refers to a symbol local to fun.
Local instantiation, invented by Walter, is a very innovative feature -
perhaps one of D's most innovative. When a template is instantiated with
a local alias (as e.g. map is inside fun) it is in fact instantiated
within the function, so it has access to its locals.
As this is a new feature and a rather subtle one, it has bugs and
limitations. In fact the function above has a bug, it prints:
[false, false, false, false, true]
although it should print
[true, true, false, false, false]
But I'm 100% convinced this is the way to go. I just submitted a bug
report reduced from the code above, see
http://d.puremagic.com/issues/show_bug.cgi?id=5641.
Andrei
