Set differences

[monarch_dodra]

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));