Yes, this looks like a similar task, thanks a million! JP
On Fri, Jul 12, 2013 at 10:57 AM, Roman Cheplyaka <r...@ro-che.info> wrote: > 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 > > -- JP Moresmau http://jpmoresmau.blogspot.com/
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