bf3@telenet.be wrote:
I'm learning Haskell. I was surprised that the following example did not compile:
data Vector2 = Vector2 { x :: Float, y :: Float } data Vector3 = Vector3 { x :: Float, y :: Float, z :: Float }
error: "Multiple declarations of `Main.x'"
AFAIK, GHC doesn't implement any fix for this. (I've been wrong before tho...) This is a feature, not a bug. Haskell in general does not let you give two functions the same name (which is what you want to do). This is
Thanks. Yes I read this is syntactic sugar, and I actually like that approach; it automatically "encapsulates" the data fileds by functions, which from an OO programmers point of view, is a good thing. I'm doing my best to get rid of that OO view though, which is not easy after 15 years of OO and 10 years of imperative programming ;-) However, I never understood why Haskell doesn't permit the same name for a function acting on different types, even without using type classes. Must be some deeper reason for it (currying?) Now the type class approach is interesting; it's like saying "any type that has an XXX field"... Lot's of typing, but IMHO it's worth it because it abstracts the concept of a field. I read some papers that some extensions got proposed to treat "fields" as first class values, so one could just do "get X (Vector2 1 2)". Did something like that make it into GHC? So the example becomes: module Main where -- "Vector" is a rather stupid example, because Haskell has tuples data Vector2 = Vector2 Float Float data Vector3 = Vector3 Float Float Float class HasX v where getX :: v -> Float setX :: v -> Float -> v class HasY v where getY :: v -> Float setY :: v -> Float -> v class HasZ v where getZ :: v -> Float setZ :: v -> Float -> v instance HasX Vector2 where getX (Vector2 x y) = x setX (Vector2 x y) value = Vector2 value y instance HasY Vector2 where getY (Vector2 x y) = y setY (Vector2 x y) value = Vector2 x value instance HasX Vector3 where getX (Vector3 x y z) = x setX (Vector3 x y z) value = Vector3 value y z instance HasY Vector3 where getY (Vector3 x y z) = y setY (Vector3 x y z) value = Vector3 x value z instance HasZ Vector3 where getZ (Vector3 x y z) = z setZ (Vector3 x y z) value = Vector3 x y value test v x = getY (setX v x) main = print $ test (Vector2 1 2) 3 -----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Paul Johnson Sent: Saturday, June 16, 2007 12:51 AM To: Andrew Coppin Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Haskell record extension? Andrew Coppin wrote: true of all functions, not just the ones implicitly defined here. Your "Vector2" type is pure syntactic sugar for: data Vector2 = Vector2 Float Float x, y :: Vector2 -> Float x (Vector2 v _) = v y (Vector2 _ v) = v So now you also want x (Vector3 v _ _) = v etc etc. And no, you can't do that because "x" on its own might refer to either version, and its not clear which one you want. Paul. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe