Re: [Haskelldb-users] simple question

[Justin B. <jgb...@gm...>]

The error is telling you that you can't compare an "Int" to a "Maybe
Int", which makes sense becuse one column can hold NULL or an int,
while the other can only hold ints. Use the fromNull operator.
Assuming group_id is the column which can be null, I think you want:

  let q = do { u <- table users; g <- table groups; restrict (g !
G.xid .==. fromNull (constant 0) (u ! U.group_id))}

Justin

On Fri, May 22, 2009 at 6:36 PM, Mathijs Kwik <blu...@gm...> wrote:
> 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
>
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