std.range should support recursion (Was: One-line FFT, nice!)

[jerro]

> import std.algorithm, std.math, std.range;
> typeof(R.init.stride(0)) dft(R)(R v)
> {
> 	return v.length <= 1
> 	? v.stride(1)
> 	: (p =>
> 	   chain(map!(q => q[0] + q[1])(p),
> 	         map!(q => q[0] - q[1])(p)))
> 	  (zip(dft(v.stride(2)),
> 		map!(p => p[1] * expi(p[0] * -2 * PI / v.length))
> 		(zip(iota(v.length / 2),
> 		 dft(v.drop(1).stride(2))))));
> }
> void main() { dft([1.0, 2, 3]); }
>
>
> Which gives the following error:
>
>
> Test.d(5): Error: incompatible types for ((stride(v,1u)) ? 
> ((*delegate @system 
> Result(Zip!(Result,MapResult!(__lambda8,Zip!(Result,Result))) p)
> {
> 	return chain(map(p),map(p));
> }
> )(zip(dft(stride(v,2u)),map(zip(iota(v.length() / 
> 2u),dft(stride(drop(v,1u),2u)))))))): 'Result' and 'Result'
> Test.d(10): Error: template instance Test.dft!(Result) error 
> instantiating
> Test.d:19: instantiated from here: dft!(double[])
> Test.d(5): Error: incompatible types for ((stride(v,1u)) ? 
> ((*delegate @system 
> Result(Zip!(Result,MapResult!(__lambda8,Zip!(Result,Result))) p)
> {
> 	return chain(map(p),map(p));
> }
> )(zip(dft(stride(v,2u)),map(zip(iota(v.length / 
> 2u),dft(stride(drop(v,1u),2u)))))))): 'Result' and 'Result'
> Test.d(19): Error: template instance Test.dft!(double[]) error 
> instantiating
>
>
>
> How should I interpret it?
>
> Thanks!

One possible reason for this error could be that you are 
returning a result of chain() if length is larger than 1 and a 
result of stride() otherwise.