Propsal: NoDatatypeContexts

John Meacham john at
Mon Jul 19 17:31:01 EDT 2010

On Mon, Jul 19, 2010 at 11:29:37AM +0200, Christian Maeder wrote:
> 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.

Hmm... if you can provide a fix for this I'll integrate it, perhaps a
manual annotation will be the only way, I don't think there is any easy
way for DrIFT to derive that information otherwise.


John Meacham - ⑆⑆john⑈ -

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