Re: [Haskelldb-users] Writing signatures for record-manipulating functions

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

Colin,

HaskellDB is going to give you some complicated type signatures - its
a side-effect of using the type system to enforce things like field
membership. You could just do the record selection in the
topLevelGalleries function:

>  return $ map (\rec -> newGallery (rec DB.! galleryName) (rec DB.! parentGalleryName) (rec DB.! readImageCapabilityName)  (rec DB.! uploadImageCapabilityName)  (rec DB.! administerGalleryCapabilityName)) rs

> newGallery name parent read upload admin = Gallery name parent read upload admin

On Tue, Sep 15, 2009 at 3:20 AM, Colin Paul Adams
<co...@co...> wrote:
> I have the following code:
>
>
> -- | Get list of all top-level galleries from database
> topLevelGalleries :: DB.Database -> IO [Gallery]
> topLevelGalleries db = do
>  let q = do
>        t <- DB.table galleryTable
>        DB.restrict (DB.isNull $ t DB.! parentGalleryName)
>        return t
>  rs <- DB.query db q
>  return $ map newGallery rs
>
> --newGallery :: DB.Record vr -> Gallery
> newGallery rec = Gallery (rec DB.! galleryName) (rec DB.! parentGalleryName) (rec DB.! readImageCapabilityName)  (rec DB.! uploadImageCapabilityName)  (rec DB.! administerGalleryCapabilityName)
>
>
> The reason the signature for newGallery is commented out is that it is
> very hard to write. I fired up ghci and did a :t newGallery. I got:
>
> newGallery
>  :: (Database.HaskellDB.HDBRec.Select
>        (Database.HaskellDB.Query.Attr
>           Database.GalleryTable.GalleryName String)
>        r
>        String,
>      Database.HaskellDB.HDBRec.Select
>        (Database.HaskellDB.Query.Attr
>           Database.GalleryTable.ParentGalleryName (Maybe String))
>        r
>        (Maybe String),
>      Database.HaskellDB.HDBRec.Select
>        (Database.HaskellDB.Query.Attr
>           Database.GalleryTable.ReadImageCapabilityName String)
>        r
>        String,
>      Database.HaskellDB.HDBRec.Select
>        (Database.HaskellDB.Query.Attr
>           Database.GalleryTable.UploadImageCapabilityName String)
>        r
>        String,
>      Database.HaskellDB.HDBRec.Select
>        (Database.HaskellDB.Query.Attr
>           Database.GalleryTable.AdministerGalleryCapabilityName String)
>        r
>        String) =>
>     r -> Dragonfly.ImageGallery.ImageGallery.Gallery
>
> Well, that's a bit of a mouthful. I don't really want to type that in.
>
> I would have hoped that somwhere there was a type definition generated
> from this from the DBInfo, but I can't find it. Any suggestions as to
> where to look?
> --
> Colin Adams
> Preston Lancashire
>
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