Propsal: NoDatatypeContexts

Christian Maeder Christian.Maeder at
Tue Jul 20 04:46:17 EDT 2010

Christian Maeder schrieb:
> Ian Lynagh schrieb:
> 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 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

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