Passing Functions in D

So, the problem is to create a function (foo) that:

• takes two values x₁ and x₂.
• returns a a function that 
• takes as argument a single number
• return the  interpolated value between the two intial values

For example:
x₁ = 10
x₂ = 15
f(x₁, x₂) = (x₂ - x₁)*t + x₁ = F(t)
F(0.5) = 12.5

---

import std.stdio;

auto interp_proc_maker(float x1, float x2){
    float interp(float s) {
        return (x2-x1)*s+x1;}
    return  &interp;}

void main()
{
  auto f1 = interp_proc_maker(10, 15);
  writefln("f1(0.5) = %f", f1(0.5));
}

---

$ gdc example.d
$ ./a.out
f1(0.5) = 12.500000