Generic programming in GLSL (parametric polymorphism, template metaprogramming) https://www.reddit.com/r/glsl/comments/mmxves/generic_programming_parametric_polymorphism/

generic_example.frag

//I wish there were a better way to do this!
#define func(type,name,param1,param2,body) type name(type param1,type param2) {body}
#define generic(name,param1,param2,body) func(float,name,param1,param2,body) func(vec2,name,param1,param2,body) func(vec3,name,param1,param2,body) func(vec4,name,param1,param2,body)

//define two "generic" functions using this macro
generic(add,a,b,
    return a + b;
)

generic(sub,a,b,
    return a - b;
)

//but this can also be done using a "tagged union":
const int int_ = 1;
const int float_ = int_+1;
const int bool_ = float_+1;

struct Object
{
  bool bool_;
  int int_;
  float float_;
  int type;
};

#define define_new(type1,type1_) Object new(type1 a){Object return1; return1.type = type1_; return1.type1_ = a ; return return1; }
define_new(float,float_)
define_new(bool,bool_)
define_new(int,int_)
Object new(Object a){
    return a;
}

Object And(Object a, Object b){
    return new(a.bool_ && b.bool_);
}
Object Or(Object a, Object b){
    return new(a.bool_ || b.bool_);
}
Object Not(Object a){
    return new(!a.bool_);
}

bool example(bool a,bool b){
    //convert bool to Object and vice-versa
    {
        Object a = new(a),b=new(b);
        return Or(a,b).bool_;
    }   
}