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

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2015/05/11 06:58, 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;
>       }

There is a misunderstanding here: the declaration “type t = Input.t” does not
re-export Input.t, it just defines a type abbreviation pointing to Input.t.
A type abbreviation does not contain field information.
You should qualify the field name with the original module name; unfortunately
there is no such module here since you defined the module inline.
Here is the type you get for ClonedModuleWithRecordType:
   module ClonedModuleWithRecordType : sig type t end
Since there is no module to refer to, all information about the type is lost.

The solution is to properly define the module argument:

   module MyRecord = struct type t = {innerRecordField: int} end
   module ClonedModuleWithRecordType = EchoModule (MyRecord)
   … {MyRecord.innerRecordField = 10 } …

which gives you:

   module ClonedModuleWithRecordType : sig type t = MyRecord.t end

Note that since 4.02 you can also omit the module name when the type is known.
But again, this will only work if there is a module name.

Jacques Garrigue