Re: [htoolkit-users] HSQL, HList

[Frederik E. <fre...@a5...>]

On Tue, Feb 22, 2005 at 09:51:35PM +0100, Shae Matijs Erisson wrote:
> Frederik Eaton <fre...@a5...> writes:
> 
> > Is there anyone on this list?
> 
> Me me! I'm here! I haven't gotten around to reading the HList paper though,
> so I won't be able to give you intelligent feedback on your code till I do.

Oh, don't read the whole paper, in fact you probably don't need to
read any of it, most of it goes far beyond what I use, which is pretty
self-explanatory.

 data a :* l = a :. l
 data HNil = Nil

e.g.

 -- x :: Bool :* String :* Int :* HNil
 x = True :. "Hi" :. 4 :. Nil

It's just a way of giving you something like a tuple type, but with
access to the internals so you can make instances based on the
structure. The important part of my module is:

 instance SqlRow HNil where
     getFieldValues s [] = return Nil
     getFieldValues s _ = fail "Wrong number of fields"

 instance (SqlBind a, SqlRow l) => SqlRow (a :* l) where
     getFieldValues s (f:fs) = do
         a <- getFieldValue s f 
         l <- getFieldValues s fs
         return (a :. l)

The paper does other neat things like defining classes for appending
two HList types and for accessing list elements by index (which has to
be encoded as a type-level natural).

-- 
Frederik Eaton                         http://ofb.net/~frederik/