Functional programming in D trouble

Question

I'm having trouble with creating a template in D that works:

pure T BSpline(int k:1, T)(in T x, in T[] t)
{
    if (t[0] <= x && x < t[k])
        return 1;
    else
        return 0;
}

pure T BSpline(int k, T)(in T x, in T[] t)
{
    if (t[0] <= x && x < t[k])
    {
        T a = (x - t[0]) / (t[k-1] - t[0]);
        T b = (t[k] - x) / (t[k] - t[1]);

        return a * BSpline!(k-1,T)(x, t[0..k-1]) + b * BSpline!(k-1,T)(x, t[1..k]);
    }
    else
        return 0;
}

And then my unit-tests:

unittest {
    real a = .5;
    real[] b = [0,0,1,2,3];
    assert(BSpline!(1,real)(a,b[0..2]) == 0);
    assert(BSpline!(2,real)(a,b[0..3]) == .5);
    assert(BSpline!(3,real)(a,b[0..4]) == .625);
    assert(BSpline!(4,real)(a,b[0..5]) == 0.260417);
}

which are failing with the following error:

bspline.d(18): Error: template BSplineBasis.BSpline(int k : 1,T) does not match any function template declaration
bspline.d(18): Error: template BSplineBasis.BSpline(int k : 1,T) cannot deduce template function from argument types !(1,real)(const(real),const(real[]))
bspline.d(18): Error: template instance errors instantiating template

I wouldn't be asking, however, I don't understand why D is having trouble deducing the template function from the rather explicit argument types...

What am I doing wrong.

If this should be in code review stack exchange rather than here, let me know, but I expect this to be a misunderstanding on my part about how templates work, rather than a bug.

Answer 1

I can't test it right now, but I think it's because in T[] is scope const T[] which is scope const(T[]), which is a lot more annoying for everyone to deal with than scope const(T)[], and at the same time, has pretty much no benefits.

Try changing

in T x, in T[] t

to

scope T x, scope const(T)[] t

in the parameter list, to see if that solves the problem.