Re: [Haskelldb-users] Improving 'project'

[<bri...@cs...>]

On Jan 30, 2008, at 19:01 , Justin Bailey wrote:

> All,
>
> project doesn't easily let me add a few columns to an existing query
> (or take a few columns away). Instead, each use of project requires me
> to build the entire list of columns I'd like to pass on by hand.
> Before I go further, if there is a way to do that, please let me know.

No, there is no existing way to achieve this. Indeed, it's one of the =20=

most common complaints about HaskellDB :-)

> An example of what I'd like to do is below. Imagine I want to add a
> "selected" boolean column to my query.
>
>   select_customers =3D do
>      cust <- table customers
>      project ( (selectedField <<- False) # cust)
>
> So I get all the columns in my customers table, plus the "selected"
> column. Unfortunately the code above doesn't work.
>
> cust has type "Rel r", and project wants a "Record r", so I originally
> tried something like:
>
>   select_customers =3D do
>     let toRec :: Rel r -> Record r
>          toRec _ =3D undefined
>     ...
>     project (unRel cust)
>
> But project needs to recover the "Expr" values associated with 'r' and
> this fails. I then came up with this solution, which lets me add
> columns at the head of a query:
>
>   class RecoverExpr a b where
>     toExpr :: a -> b
>
>   instance RecoverExpr RecNil RecNil where
>     toExpr rec =3D RecNil
>
>   instance (FieldTag f, RecoverExpr r r) =3D> RecoverExpr (RecCons f
> (Expr b) r) (RecCons f (Expr b) r) where
>     toExpr ~rec@(RecCons _ r) =3D RecCons (attribute . fieldName $
> recField $ rec) (toExpr r)
>
>   recField :: RecCons f a b -> f
>   recField _ =3D undefined
>
>   rest :: (RecoverExpr r r) =3D> Rel r -> r
>   rest qry =3D
>     let unRel :: Rel r -> r
>         unRel _ =3D undefined
>     in toExpr . unRel $ qry
>
> Then I can write:
>
>   select_customers  =3D do
>     cust <- table customers_tbl
>     ...
>     project (selectField <<- False # (rest cust))
>
> And it actually works. However, it seems over-engineered. My =20
> questions are:
>
>  1) Is there a way to make project behave like I want without these =20=

> tricks?

I doubt that there is. In general, manipulating record types with =20
Haskell's roundabout type level programming is messy, there's no =20
avoiding that.

>  2) If not 1, would it make more sense for the Query monad to pack
> around the original Expr values?

Hmm, wouldn't it need to carry those through arbitrary query =20
computations?

>  3) If not 2, how can the solution above be better? I'd like to extend
> it to adding/removing arbitrary columns and  more control over column
> order.

The best way to do it would be to add powerful record types to =20
Haskell...

You may want to look at HList, http://homepages.cwi.nl/~ralf/HList/ =20
I'm not sure how much HList can do now, perhaps they have added =20
things since they wrote the paper. There might be some nice record =20
manipulation stuff there that you could use. Porting HaskellDB to use =20=

HList instead of its own records would be nice, if the HList code is =20
in good shape.

> Thanks for any and all thoughts!
>
> Justin
>
> p.s. Sorry for the spam on cafe - I accidently sent this there =20
> first ...


/Bj=F6rn