José Pedro Magalhães wrote:
Complex has a derived instance. In Data.Complex:
data (RealFloat a) => Complex a = !a :+ !a # if __GLASGOW_HASKELL__ deriving (Eq, Show, Read, Data) # else deriving (Eq, Show, Read) # endif
Was there a problem with this?
Yes, isn't it exactly the same problem as Ratio? The derived instance 'leaks' the implementation detail of a complex number as a real and imaginary component. As part of a large structure I might want to apply some operation to all the 'Doubles' therein, but certainly not to the components of any Complex Doubles. You can imagine particular situations where I want to apply only to the real component, or something more subtle. You can also imagine that an alternative implementation of Complexes in polar coordinates (with a suitable solution to the principle value problem) is supposed to be abstractly the same, but would gain a totally different Data instance. Jules