[TYS] O'Caml TYS for the day

[wtd]

Consider the following snippet from the toplevel interpreter.

code:

# class foo = object method baz = 42 end   class bar = object method baz = 27 end     let qux (wooble : foo) =      print_int wooble#baz;      print_newline ();; class foo : object method baz : int end class bar : object method baz : int end val qux : foo -> unit = <fun>

Just reasoning about this with your brain, what is the smallest change necessary such that an object of class bar can be passed to the function qux?

[Null]

I'm pretty sure this:

code:

let qux (wooble : foo) =      print_int wooble#baz;      print_newline ();;

becomes this:

code:

let qux wooble =      print_int wooble#baz;      print_newline ();;

Because O'Caml will realize that the method will work on any object with a baz method.

[wtd]

Not quite.

[richcash]

I know this is old, but I'm bored.


No change is necessary for objects of class bar to be passed to qux.

Ocaml:

class foo = object method baz = 42 end class bar = object method baz = 27 end   let qux (wooble : foo) =         print_int wooble#baz;         print_newline ();; let f = new foo;; let b = new bar;; qux f;; qux b;;

The above code compiles and runs as expected.

The type for the qux function is :

code:

foo -> unit

But the compiler simplifies this by replacing the type of foo in automatically :

code:

(object baz -> int end) -> unit

It just so happens that objects of class bar have the exact same type signature as objects of foo, so you can use bar objects as well.

If bar still has the same method baz but we change its type by redifining it, then qux will no longer be able to take bar objects :

Ocaml:

class foo = object method baz = 42 end class bar = object method baz = 27; method fizz = 35 end let qux (wooble : foo) =         print_int wooble#baz;         print_newline ();; let f = new foo;; let b = new bar;; qux f;; qux b;;

The above will cause an error on the last line.

[wtd]

Good. Now, with your second bit of code, what would one have to change about the method signature to make it work with both classes?

Or, put another way, what should the type of wooble be?

[richcash]

Well, we can cheat and let o'caml's type inferencing figure out the type of wooble for us (not give wooble a type at all). Or, we can do it the safe way :

Ocaml:

class foo = object method baz = 42 end class bar = object method baz = 27; method fizz = 35 end let qux (wooble : < baz : int; .. >) =         print_int wooble#baz;         print_newline ();; let f = new foo;; let b = new bar;; qux f;; qux b;;

We give wooble the special type signature

code:

< baz : int; .. >

which means allow any object that has a method (baz : int), regardless of what other methods the object might have.

[wtd]

Alternatively:

code:

let qux wooble = Printf.printf "%d\n" wooble#baz

[richcash]

Yep, that would be a good alternative. But I think you broke your own rules there by completely redefining qux.

[wtd]

I made it better!