[EMAIL PROTECTED] 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'"
This seems to be a known issue and if I understand it correctly it has to do
with the fact that Haskell does not support function general overloading ala
C++. One either has to use type classes, or put each record in a separate
module and fiddle with the qualified namespaces.
I read multiple papers with proposals to fix this, but does GHC implement
any of these?
AFAIK, GHC doesn't implement any fix for this. (I've been wrong before
tho...)
One easy workaround is to simply do
data Vector2 = Vector2 { x2, y2 :: Float }
data Vector3 = Vector3 { x3, y3, z3 :: Float}
Another (which is usually a bad idea) is
data Vector = Vector2 { x, y :: Float} | Vector3 { x, y, z :: Float}
Usually you'll want to be able to seperate Vector2 functions from
Vector3 functions, which the above won't allow you to do.
And the other possibility is to play with type classes - something like
class V1 v where
x :: v -> Float
instance V1 Vector2 where
x (Vector2 f0 f1) = f0
and so on.
And the final possibility is multiple modules and play with namespaces
that way - but that's probably overkill.
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