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
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