haskelldb-users

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

[Colin P. A. <co...@co...>]

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

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

[Colin P. A. <co...@co...>]

>>>>> "Justin" == Justin Bailey <jgb...@gm...> writes:

    Justin> Colin, HaskellDB is going to give you some complicated
    Justin> type signatures - its a side-effect of using the type
    Justin> system to enforce things like field membership.
    
Would it be possible to exploit context synonyms (see
http://martijn.van.steenbergen.nl/journal/2009/10/11/context-synonyms/)
to automatically generate more readable (and therefore writeable) signatures?
-- 
Colin Adams
Preston Lancashire

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

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

That is really slick and it sounds like it would work.
UndecideableInstances are already allowed anyways.

On Sun, Oct 11, 2009 at 10:47 PM, Colin Paul Adams
<co...@co...> wrote:
>>>>>> "Justin" == Justin Bailey <jgb...@gm...> writes:
>
>    Justin> Colin, HaskellDB is going to give you some complicated
>    Justin> type signatures - its a side-effect of using the type
>    Justin> system to enforce things like field membership.
>
> Would it be possible to exploit context synonyms (see
> http://martijn.van.steenbergen.nl/journal/2009/10/11/context-synonyms/)
> to automatically generate more readable (and therefore writeable) signatures?
> --
> Colin Adams
> Preston Lancashire
>