subtyping

Given these types type a = [ `A ] type b = [ a | `B | `C ] and this function let pp: [< b] -> string = function | `A -> "A" | `B -> "B" | `C -> "C" applying a value of type a works without issue, as expected: let a: a = `A let _ = pp a However, if the function is modified to include a wildcard pattern let pp: [< b] -> string = function | `A -> "A" | `B -> "B" | _ -> "?" even though everything else remains the same, it now yields the following error (on let _ = pp a ): This expression has type b -> string but an expression was expected of type a -> 'a Type b = [ `A | `B ] is not compatible with

The database I'm designing has an employees table; there can be multiple types of employees, one of which are medical employees. The database needs to also describe a many-to-many relation between medical employees and what competences they have. Is it okay to create a table medical_employees with only an id column, whose only purpose is to specify which employees are medics? The id column has a foreign key constraint that references the employees table. The code below should make my question clearer: /* Defines a generic employee */ CREATE TABLE employees ( id INT PRIMARY KEY AUTO_INCREMENT,

Regarding the following C++ program: class Base { }; class Child : public Base { }; int main() { // Normal: using child as base is allowed Child *c = new Child(); Base *b = c; // Double pointers: apparently can't use Child** as Base** Child **cc = &c; Base **bb = cc; return 0; } GCC produces the following error on the last assignment statement: error: invalid conversion from ‘Child**’ to ‘Base**’ My question is in two parts: Why is there no implicit conversion from Child** to Base**? I can make this example work with a C-style cast or a reinterpret_cast . Using these casts means throwing away