Christian Maeder schrieb:
Ian Lynagh schrieb: [...]
http://hackage.haskell.org/trac/haskell-prime/wiki/NoDatatypeContexts
I'm for this proposal, although I've got an example where I need this context, namely for DrIFT to derive a proper context for instances.
DrIFT doesn't know that the Read instance for Data.Set.Set relies on Ord of the elements. For
data Ord symbol => ExtSign sign symbol = ExtSign { plainSign :: sign , nonImportedSymbols :: Set.Set symbol } deriving Show
DrIFT cannot derive the Read (or our deserialization) instance without the "Ord symbol =>" context.
It would be interesting to know (or if at all) i.e. uniplate (proposed for the haskell platform) or other generic stuff can handle this case (as alternative to DrIFT). A short look at http://www.haskell.org/haskellwiki/Uniplate did not immediate help on this matter. Template Haskell may be possible, too. Can some expert for generic programming elaborate on this? (I once had a good paper comparing the various approaches, but I can no longer find it.) Pointers are welcome. Cheers Christian
However, ghc is able by "deriving (Show, Read)" to see
instance (Ord symbol, Read sign, Read symbol) => Read (ExtSign sign symbol)
without the context.
Cheers Christian