How do I store Either (Key a) (Key b)?

I have the following model:

User
  ...
Group
  ...
Sharing
  objectId (Either UserId GroupId)

In Sharing entity I want to store either UserId or GroupId and differentiate between them. Simply using Either doesn't work:

Not in scope: type constructor or class `UserId'

Not in scope: type constructor or class `GroupId'

Adding a new sum-type also doesn't work:

data SharingIdType = SharingUserId UserId | SharingGroupId GroupId

Not in scope: type constructor or class `SharingIdType'

Moving SharingIdType into another module isn't possible, because it uses UserId and GroupId types. The only way I see is to create an entity for each sharing type, like UserSharing/GroupSharing.

Other than that, how to approach this problem?

Solution

After searching for some time and thinking about it I concluded there are two possible solutions:

1.

If number of SharingIdTypes is static or rarely changes (means, it is OK to recompile the source to change it or alter the DB schema), the proper way to handle the problem is to have to entities for each sharing type:

User
  ...
Group
  ...
UserSharing
  userId UserId
GroupSharing
  groupId GroupId

Here the "sumness" of the problem is moved to DB queries. Whenever I need to find out with what something shared, I make two selectLists and query two tables instead of one.

2.

If number of SharingIdTypes needs to be altered dynamically, the SharingType entity is needed:

User
  ...
Group
  ...
SharingType
  description String
Sharing
  objectId SharingTypeId

This table is filled up with values corresponding to SharingIdTypes constructors:

do
  insert $ SharingType "user"
  insert $ SharingType "group"

Now whenever we share something, we refer SharingTypeId.