[Haskelldb-users] simple question

[Mathijs K. <blu...@gm...>]

Hi all,

I finally got haskelldb working. It took some time, but (after many
tries in the past) I got it.
I'm playing around with querying and I have a simple question:

I have a table with a foreign key that can be NULL.
Now if I try to create an inner-join like restriction, I get a
type-mismatch, which I can understand, but couldn't figure out how to
solve it (it's a little late here :)

 let q = do { u <- table users; g <- table groups; restrict (g!G.xid
.==. u!U.group_id)}

<interactive>:1:4:
    Couldn't match expected type `Int'
           against inferred type `Maybe Int'
      Expected type: Expr Int
      Inferred type: Expr (Maybe Int)
    When using functional dependencies to combine
      Select (Attr f a) (Rel r) (Expr a),
        arising from the dependency `f r -> a'
        in the instance declaration at <no location info>
      Select (Attr U.Group_id (Maybe Int)) (Rel U.Users) (Expr Int),
        arising from a use of `!' at <interactive>:1:97-107
    When generalising the type(s) for `q'

What would be the correct way to fix this?

Thanks,
Mathijs