Ian Lynagh schrieb:
Hi all,
H98 and H2010 allow a context to be given for datatypes, e.g. the "Eq a" in
data Eq a => Foo a = Constr a
I have made a proposal to remove support for that context (ticket #139). More details are on the proposal wiki page:
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. 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