# types and polymorphic variants

[Manuel Fahndrich <maf@microsoft.com>]

What is the meaning of #foo, when foo is a polymorphic variant set?

type foo = [`A | `B | `C]

I assumed that it was the following:

	#foo = [<`A | `B | `C ]

that is a set containing at most `A | `B | `C. In other words a subtype of
foo.

But consider the following example.

type foo = [`A | `B | `C]

let foo1 (x : #foo as 'a) = x

let y1 = foo1 (`A : [`A])


let foo2 (x : [< `A | `B | `C] as 'a) = x

let y2 = foo2 (`A : [`A])


The complier tells me:

ocamlc -c -i ex1.ml

type foo = [`A|`B|`C]
val foo1 : (#foo as 'a) -> 'a
val y1 : foo
val foo2 : ([<`A|`B|`C] as 'a) -> 'a
val y2 : [`A]

The first function returns me only a foo, the second function returns me
[`A] as expected. Thus my question, what exactly does #foo stand for. I do
understand row variables, but the issue with the lower and upper bounds > <
in variant types seems new. Is there a paper describing these?

I seem to write types of the form [< `A | `B | `C] a lot. Shouldn't there be
an abbreviation for those as well? Maybe #<foo?


Thanks in advance for such a wonderfully expressive type system!

	Manuel