Folks,
I've added Hinze/PJ-style generic class definitions to GHC, based
on summer hacking done by Andrei Serjantov. The design is based
very closely on the Haskell workshop 2000 paper
http://www.informatik.uni-bonn.de/~ralf/Derive.ps.gz
For example:
class Bin a where
toBin :: a -> [Int]
fromBin :: [Int] -> (a, [Int])
toBin {| Unit |} Unit = []
toBin {| a :+: b |} (Inl x) = 0 : toBin x
toBin {| a :+: b |} (Inr y) = 1 : toBin y
toBin {| a :*: b |} (x :*: y) = toBin x ++ toBin y
fromBin {| Unit |} bs = (Unit, bs)
fromBin {| a :+: b |} (0:bs) = (Inl x, bs') where (x,bs') = fromBin
bs
fromBin {| a :+: b |} (1:bs) = (Inr y, bs') where (y,bs') = fromBin
bs
fromBin {| a :*: b |} bs = (x :*: y, bs'') where (x,bs' ) =
fromBin bs
(y,bs'') = fromBin bs'
Now we can say simply
instance Bin a => Bin [a]
and the compiler will derive the appropriate code automatically.
This is now implemented and documented in the CVS version of GHC,
and will be in every future version of GHC. (But I'm afraid we don't plan
a binary release for a while.)
At present we don't (alas) support constructor and field names, so you
can't implement Read and Show generically. But I thought it was worth
getting committed what *is* done.
Simon
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