[Haskell-cafe] Deriving with generics without values

[JP Moresmau]

Yes, this looks like a similar task, thanks a million!

JP


On Fri, Jul 12, 2013 at 10:57 AM, Roman Cheplyaka <roma at 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 <jpmoresmau at 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 at haskell.org
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>
>


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