Not Lisp again (2009)

Re:Not Lisp again (2009) by smitherfield on Tuesday May 02, 2017 @03:04PM (#14249532)

He is right, you could do something roughly equivalent in 1983-era C, albeit a fair amount clunkier due to the lack of partial application or (any kind of) type-checking:

  #define DX (double)0.0001

  double f_prime(f, x)
          register double (*f)();
          double x;
  {
          return ((*f)(x + DX) - (*f)(x)) / DX;
  }
  #define DERIV(F, X) f_prime((F), (double)(X))

  double cube(x)
          double x;
  {
          return x * x * x;
  }
  #define DERIV_CUBE(X) DERIV(cube,(X))

  main()
  {
          printf("%f\n%f\n%f\n", DERIV_CUBE(2), DERIV_CUBE(3), DERIV_CUBE(4));
  }

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Re:Not Lisp again (2009) by smitherfield on Tuesday May 02, 2017 @04:03PM (#14250054)

And in >=C89:

  #include <stdio.h>

  double deriv(double (*f)(double), double x)
  {
          static const double dx = 1e-8;
          return (f(x + dx) - f(x)) / dx;
  }

  static double cube(double x)
  {
          return x * x * x;
  }

  double deriv_cube(double x)
  {
          return deriv(cube, x);
  }

  int main(void)
  {
          printf("%f\n%f\n%f\n", deriv_cube(2), deriv_cube(3), deriv_cube(4));
          return 0;
  }

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