[Template-haskell] Can I reify types?

25 May 2008

      This doesn't work:

-- packages: template-haskell 
{-# OPTIONS_GHC -XTemplateHaskell #-}
module Main where
import Language.Haskell.TH

data A a = A a

type C = A Int

data ABC = ABC Int

$(do
  a <- reify $ mkName "C"
  report False $ show a
  return []
  )
main = print ""

|| [1 of 1] Compiling Main             ( test.hs, test.o )
|| 
test.hs|1| `C' is not in scope at a reify

Using mkName "A" results in
test.hs|1 error| 
||     DataConI Main.A (ForallT [a_1627391370] [] (AppT (AppT ArrowT (VarT a_1627391370)) (AppT (ConT Main.A) (VarT a_1627391370)))) Main.A (Fixity 9 InfixL)
as expected

Why do I need it?
I'd like to implement kind of very basic relational data representation
the way IxSet is doing it but without dynamics..

It will look like this:

type CDs = Table (Autoinc, Artist, Title, Year) -- col types
                 (Artist, Title, Year) -- keys 
                 () -- is detail of 
type Tracks = Table (Autoinc, Title, RecordingDate)
                    (Title, RecordingDate)
                    (CDs)
$(mkDB ["CDs","Tracks"])

To be able to automatically derive
insert{CDs,Tracks}
delete{CDs,Tracks}
update{CDs,Tracks}
functions I need to get information about those types.. Is this
possible?

Thanks
Marc Weber