[Template-haskell] Can I reify types?

[Simon Peyton-Jones]

See http://hackage.haskell.org/trac/ghc/ticket/2339

Simon


| -----Original Message-----
| From: template-haskell-bounces at haskell.org [mailto:template-haskell-bounces at haskell.org] On Behalf Of
| Marc Weber
| Sent: 25 May 2008 16:19
| To: template-haskell at haskell.org
| Subject: [Template-haskell] Can I reify types?
|
| 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
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