Re: [Caml-list] Record fields not reexported from functors.

[Leo White <leo@lpw25.net>]

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To expand slightly on Jacques' answer. Record fields (and variant
constructors) must be explicitly exported, using the:

type t = Foo.t = { bar : int }

syntax.

For example, the following version of your example works:

module type HasType = sig type t = { innerRecordField: int } end

module EchoModule (Input : HasType) : HasType with type t = Input.t =
struct type t = Input.t = {innerRecordField: int} end

module ClonedModuleWithRecordType = EchoModule(struct type t =
{innerRecordField: int} end)

let myRecord: ClonedModuleWithRecordType.t = {
ClonedModuleWithRecordType.innerRecordField=10; }

Of course, the `EchoModule` functor can now only operate on modules
where the type t is defined as:

type t = { innerRecordField : int }

Jacques' solution of giving the functor argument a name, and using that
name to access the field, is a much more flexible approach.

Regards,

Leo

On Sun, 10 May 2015, at 05:58 PM, Jordan W wrote:
> I believe there is an issue with record fields and functors. It
> appears that while types are correctly exported from functors, the
> corresponding record field scopes are not. Here is an example.
>
> module type HasType = sig type t end
>
> module EchoModule (Input:HasType): (HasType with type t = Input.t) =
> (struct type t = Input.t end)
>
> module ClonedModuleWithRecordType = EchoModule (struct type t =
> {innerRecordField: int} end)
>
> (* This doesn't type check, but I can't see why it shouldn't. Record
> fields' scopes should be reexported just like the types themselves.
>
> let myRecord: ClonedModuleWithRecordType.t = {
> ClonedModuleWithRecordType.innerRecordField=10; }
>
> In this case, the original "scope" of the record fields is "lost"
> because the record type was declared in an anonymous module
> expression. I believe that it *should* be "lost" as it is, but then
> reexported as an identical type in the module returned from the
> functor (identically because of the "with constraint").
>
> *)
>
> module ClonedModuleWithNonRecordType = EchoModule (struct type t =
> string list end)
>
> (* This typechecks perfectly fine! Evidence that types are reexported,
> but not the "scope" of record labels *) let myRecord:
> ClonedModuleWithNonRecordType.t = ["asdf"; "fff"]
>
>
> Is anyone aware of whether or not this issue is already being tracked?
>


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