[Haskell-cafe] Deriving with generics without values

[JP Moresmau]

Hello all,
My problem is the following: I have my own data types, and I'd like to
derive automatically instances of some type class from them. I've started
looking at GHC.Generics, which offer tools to do exactly that. However,
some functions of my typeclass do not take my data type as a parameter, but
as a result. Basically:
 class MyClass where
   fromString :: String -> a

 data MyData=MkMyData {
   myField ::Int
 } deriving (Generic)

and I want to automatically generate the instance instance MyClass MyData,
using default methods, etc.
The GHC Generic class does say that it uses a from function that convert
from the datatype to its representation: from :: a ->
Rep<http://www.haskell.org/ghc/docs/7.4.1/html/libraries/ghc-prim-0.2.0.0/GHC-Generics.html#t:Rep>
a
xfrom :: a -> Rep<http://www.haskell.org/ghc/docs/7.4.1/html/libraries/ghc-prim-0.2.0.0/GHC-Generics.html#t:Rep>
a
x
But I don't have a "a" to start from! I see from the related papers that
the automatically generated code from from actually does pattern matches on
constructors, so I need a value, undefined won't work. However I see the
GHC.Generics also provide :+: (Sums: encode choice between constructors).
If I have to provide an value, then the choice between constructor has been
done! The examples about generics on
http://www.haskell.org/haskellwiki/GHC.Generics do provide an example of
defining the instance for :+: but I don't understand how we can get there.
If I have a class method that takes a value as a parameter, and I pass
undefined to it, the code will crash, since it can't pattern match on
undefined.

Can somebody shed some light on this? Am I using the wrong tool for the
job? How can I achieve what I want? I want the full type representation
with sums, but without a value to start from.

Thanks a million!

JP
-- 
JP Moresmau
http://jpmoresmau.blogspot.com/
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