What are row types? Are they algebraic data types?

Question

I often hear that F# lacks support for OCaml row types, that makes the language more powerful than F#.

What are they? Are they algebraic data types, such as sum type

Answer 1

I'll complete PatJ's excellent answer with his example, written using classes.

Given the classes below:

class t1 = object
  method a = 42
  method b = "Hello world"
end

class t2 = object
  method a = 1337
  method b = false
end

And the objects below:

let o1 = new t1
let o2 = new t2

You can write the following:

let print_a t = print_int t#a;;
val print_a : < a : int; .. > -> unit = 

print_a o1;;

42
- : unit = ()

print_a o2;;

1337
- : unit = ()

You can see the row type in print_a's signature. The < a : int; .. > is a type that literally means "any object that has at least a method a with signature int".