shouting versus dotting

[Andrei Alexandrescu]

Benji Smith wrote:
> Hmmmmmm... I still hate opCall, though :)

One context in which I found opCall very valuable is parameterized 
functions: functions that take, say, 1 argument but are parameterized by 
another. An example is a Gaussian kernel of parameterized bandwidth:

double gaussianKernel(double d)
{
     return exp(-d * d / alpha * alpha);
}

We'd like to be able to configure alpha properly (not via a global, 
because there could be several Gaussian kernels around). A solution 
would be to use delegates, but that's inefficient and a kernel is 
evaluated so often it's not even funny. So a good solution is to make 
the function a struct with state:

struct GaussianKernel
{
     private double alpha2InvNeg = -1;

     double opCall(double d) const
     {
         return exp(d * d * alpha2InvNeg);
     }

     void alpha(double a)
     {
         enforce(a > 0);
         alpha2InvNeg = -1 / (a * a);
     }

     double alpha() const
     {
         return sqrt(-1 / alpha2InvNeg);
     }
}


Andrei