• Josh (10/10) Apr 05 2014 Can anyone please explain why this works:
• monarch_dodra (14/25) Apr 06 2014 setDifference returns a lazy range, whose type depends on the
• bearophile (23/32) Apr 06 2014 This compiles:

"Josh" <moonburntm gmail.com> writes:

Can anyone please explain why this works:

	auto numbers = sequence!("n")();
	auto trimmed = setDifference(setDifference(numbers, sequence!("n 
* a[0]")(2)), sequence!("n * a[0]")(3));

but this doesn't?

	auto numbers = sequence!("n")();
	auto trimmed = setDifference(numbers, sequence!("n * a[0]")(2));
	trimmed = setDifference(trimmed, sequence!("n * a[0]")(3));

Thanks,
Josh

"monarch_dodra" <monarchdodra gmail.com> writes:

On Sunday, 6 April 2014 at 05:27:44 UTC, Josh wrote:
 Can anyone please explain why this works:

 	auto numbers = sequence!("n")();
 	auto trimmed = setDifference(setDifference(numbers, 
 sequence!("n * a[0]")(2)), sequence!("n * a[0]")(3));

 but this doesn't?

 	auto numbers = sequence!("n")();
 	auto trimmed = setDifference(numbers, sequence!("n * 
 a[0]")(2));
 	trimmed = setDifference(trimmed, sequence!("n * a[0]")(3));

 Thanks,
 Josh

setDifference returns a lazy range, whose type depends on the 
inputs. It's not just a generic array. Because of this, while 
these *look* like they are just ranges of numbers:
setDifference(numbers, sequence!("n * a[0]")(2));
setDifference(trimmed, sequence!("n * a[0]")(3));

Their *types* and what goes on under the hood is *complelty* 
different. Because of this, you can't use the result of the 
first, to store the result of the second, the types don't match.

Try this:
auto tmpTrimmed = setDifference(numbers, sequence!("n * 
a[0]")(2));
auto trimmed = setDifference(tmpTrimmed, sequence!("n * 
a[0]")(3));

"bearophile" <bearophileHUGS lycos.com> writes:

Josh:

 Can anyone please explain why this works:

 	auto numbers = sequence!("n")();
 	auto trimmed = setDifference(setDifference(numbers, 
 sequence!("n * a[0]")(2)), sequence!("n * a[0]")(3));

 but this doesn't?

 	auto numbers = sequence!("n")();
 	auto trimmed = setDifference(numbers, sequence!("n * 
 a[0]")(2));
 	trimmed = setDifference(trimmed, sequence!("n * a[0]")(3));

This compiles:

void main() {
     import std.stdio, std.algorithm, std.range;

     auto numbers = sequence!("n")();
     auto trimmed = setDifference(numbers, sequence!("n * 
a[0]")(2));
     auto trimmed2 = setDifference(trimmed, sequence!("n * 
a[0]")(3));
     pragma(msg, typeof(trimmed));
     pragma(msg, typeof(trimmed2));
}

Output:

SetDifference!("a < b", Sequence!("n", Tuple!()), Sequence!("n * 
a[0]", Tuple!int))
SetDifference!("a < b", SetDifference!("a < b", Sequence!("n", 
Tuple!()), Sequence!("n * a[0]", Tuple!int)), Sequence!("n * 
a[0]", Tuple!int))


The two trimmed have different type, because in D ranges are 
template-based, so often things have different type, unless they 
are identical.

Bye,
bearophile