]>
Keith Wansbrough <Keith.Wansbrough@cl.cam.ac.uk> writes:
> Hi.. the OCaml documentation, section 4.2 on polymorphic variants,
> Advanced use, explains that
>
> To make this even more confortable, you may use type definitions as
> abbreviations for or-patterns. That is, if you have defined type
> myvariant = [`Tag1 int | `Tag2 bool], then the pattern #myvariant is
> equivalent to writing (`Tag1(_ : int) | `Tag2(_ : bool)).
>
> But this is not correct! Consider
>
> type de = [`D | `E of de];;
>
> type def = [`D | `E of def | `F of def];;
>
> let rec deproc2 rfun =
> function
> | `D -> print_string "D"; `D
> | `E(x) -> print_string "E"; `E(rfun x)
>
> let rec deproc3 x = deproc2 deproc3 x
>
> let rec defproc2 rfun =
> function
> (* | (`D | `E(_)) as x -> deproc2 rfun x *)
> | #de as x -> deproc2 rfun x
> | `F(x) -> print_string "f"; `F(rfun x)
If I read the documentation, this is rewrote as
let rec defproc2 rfun =
function
| (`D | `E(_:de)) as x -> deproc2 rfun x
| `F(x) -> print_string "f"; `F(rfun x)
that mean that what is in the `E must be a de, not a def. There is
your error.
In fact, your code will do what you want if you don't define your type
as recursive but as polymorphic :
type 'a de = [`D | `E of 'a];;
type 'a def = [`D | `E of 'a | `F of 'a];;
then, your code will work as expected. Well, the last definition became :
let rec defproc3 : ('a def as 'a) -> 'a =
fun x -> defproc2 defproc3 x
There is a very interesting
example about this in the ocaml source : ocaml/testlabl/mixin.ml
[...]
> as expected. It looks to me like #de means (`D | `E(_:de)), rather
> than (`D | `E(_)) as I expected; except that I'm not even sure what
> `E(_:de) means in this case - does it do type-directed matching at
> runtime?
no, it is a constraint on the type of what is the `E done at compile
time. And it is the source of your error because a de is not a def.
>
> Could the documentation please be made more accurate at this point?
>
The documentation is accurate.
--
Rémi Vanicat
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