Well, in your case, you need not 'from', but 'to', in order to convert from a generic representation to yours.
Take a look at how a similar task is done in SmallCheck: https://github.com/feuerbach/smallcheck/blob/master/Test/SmallCheck/Series.hs#L180 https://github.com/feuerbach/smallcheck/blob/master/Test/SmallCheck/Series.hs#L352 Roman * JP Moresmau <jpmores...@gmail.com> [2013-07-12 10:45:39+0200] > 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/ > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
