On Tue, 21 Aug 2007, Peter Verswyvelen wrote:
Consider the following example code:
data Vector = V Float Float data Matrix = M Vector Vector
liftV1 f (V x y) = V (f x) (f y) liftV2 f (V x1 y1) (V x2 y2) = V (f x1 x2) (f y1 y2)
liftM1 f (M x y) = M (f x) (f y) liftM2 f (M x1 y1) (M x2 y2) = M (f x1 x2) (f y1 y2)
Both pairs of lift functions have almost identical implementations. Can I merge these somehow? I know data constructors are first class values and are not types, but if I want to merge these lift functions I have to write something like
Maybe you are happy with instances of Control.Applicative (GHC-6.6) http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Applicati... (untested code follows) data Vector a = Vector a a instance Functor a where fmap f (Vector x y) = Vector (f x) (f y) instance Applicative pure x = Vector x x (Vector fx fy) <*> (Vector x y) = Vector (fx x) (fy y) pure f <*> vx pure f2 <*> vx <*> vy However, I'm not convinced that your code becomes more readable or flexible by this change.