[Haskell-cafe] Generic data constructor in pattern?

Henning Thielemann lemming at henning-thielemann.de
Tue Aug 21 11:53:46 EDT 2007

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)

(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.

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