kripke

    Error: This expression has type t/1 but an expression was expected of type
             t/2
           Hint: The type t has been defined multiple times in this toplevel
             session. Some toplevel values still refer to old versions of this
             type. Did you try to redefine them?

    #include<stdio.h>
    #define DX 0.0001
    typedef double (*function)(double);
    typedef double (*closure_body)(void*, double);
    struct deriv_env { function f; };
    struct closure { void* env; closure_body body; };
    #define CALL(c, x) ((c)->body((c)->env, (x)))
    double deriv_body(void *env, double x) {
        function f = (deriv_env *)env->f;
        return (f(x + DX) - f(x)) / DX;
    }
    void deriv(function f, struct closure *out) {
        out->env = (void *)f;
        out->body = deriv_body;
    }
    double cube(double x) { return x * x * x; }

    int main(void) {
        struct closure cube_deriv;
        deriv(&cube, &cube_deriv);

        printf("%f\n", CALL(&cube_deriv, 2.));
        printf("%f\n", CALL(&cube_deriv, 3.));
        printf("%f\n", CALL(&cube_deriv, 4.));

        return 0;
    }