Hi ^_^,
Let's say we have the following data type and functions:
data Tab a = (:↺:)
| a :↓: Tab a
| Tab a :↙↘: (Tab a,Tab a)
deriving (Eq, Show, Read)
map f (:↺:) = (:↺:)
map f (a :↓: t) = f a :↓: map f t
map f (h :↙↘: (l,r)) = map f h
Well, it's certainly not possible for filter, at least, not without
additional hints to the compiler. For example, consider this type:
data Weird a = A | B a (Weird a) (Weird a)
filter p A = A
filter p (B x w1 w2) | p x = B x (filter p w1) (filter p w2)
| otherwise =
Even deriving an instance of Functor seems rather implausable, what
should it do for
data Wierd a b = Nil | A a (Wierd a b) | B b (Wierd a b)
Should fmap's function argument operate on 'a's, 'b's, or both?
Bob
On 5 Jun 2008, at 10:28, Miguel Mitrofanov wrote:
Well, it's certainly not
Thomas Davie wrote:
Even deriving an instance of Functor seems rather implausable, what
should it do for
data Wierd a b = Nil | A a (Wierd a b) | B b (Wierd a b)
Should fmap's function argument operate on 'a's, 'b's, or both?
But for many datatypes it is quite natural. Just google for
It can be even worse:
data X a b = X (X b a - b)
Here (X a) is certainly a functor, but the implementation must also
act on a contravariantly:
mapX :: (a - a') - X a' b - X a b
mapX f (X h) = X $ h . fmap f
instance Functor (X a) where fmap f (X h) = X $ f . h . mapX f
On 5 Jun 2008, at
On Thu, Jun 05, 2008 at 10:39:16AM +0200, Thomas Davie wrote:
Even deriving an instance of Functor seems rather implausable, what
should it do for
data Wierd a b = Nil | A a (Wierd a b) | B b (Wierd a b)
Should fmap's function argument operate on 'a's, 'b's, or both?
Generic Haskell can
On 5 Jun 2008, at 1:39 AM, Thomas Davie wrote:
Even deriving an instance of Functor seems rather implausable, what
should it do for
data Wierd a b = Nil | A a (Wierd a b) | B b (Wierd a b)
Should fmap's function argument operate on 'a's, 'b's, or both?
class Functor (f :: * - *) where ...
Hi
PS Why isn't Functor derivable?
Derive can do it: http://www.cs.york.ac.uk/~ndm/derive
I believe that Twan (the author of Functor deriving in Derive) is
trying to get this suggested for Haskell' as a proper deriving.
As for the original question, Uniplate will certainly do map, and will
Hi
Statutory mathematics warning: lots.
On 5 Jun 2008, at 15:40, Jonathan Cast wrote:
On 5 Jun 2008, at 1:39 AM, Thomas Davie wrote:
Even deriving an instance of Functor seems rather implausable,
what should it do for
data Wierd a b = Nil | A a (Wierd a b) | B b (Wierd a b)
Should
