Re: [Haskell-cafe] represent data sturcture using function
On Mon, Dec 29, 2008 at 4:29 AM,
Would you please give me a complete example of code that I could have more information on the idea?
Sure, I put up an example at http://ryani.freeshell.org/haskell/gmap.hs class MapKey k where data (:->) k :: * -> * newMap :: (k -> v) -> (k :-> v) fetch :: (k :-> v) -> (k -> v) update :: k -> (v -> v) -> (k :-> v) -> (k :-> v) assign :: k -> v -> (k :-> v) -> (k :-> v) assign k v m = update k (const v) m empty :: v -> (k :-> v) empty = newMap . const with instances & associated data types: instance MapKey () where -- A single value newtype () :-> v = UMap v instance MapKey Bool where -- A value for False and True data Bool :-> v = BMap v v instance (MapKey k1, MapKey k2) => MapKey (k1,k2) where -- A "curried" map newtype (k1,k2) :-> v = PMap (k1 :-> k2 :-> v) instance (MapKey k1, MapKey k2) => MapKey (Either k1 k2) where -- sub-maps for Left k1 and Right k2 data (Either k1 k2 :-> v) = EMap (k1 :-> v) (k2 :-> v) instance MapKey k => MapKey (Maybe k) where -- Now we can build up from existing structures! newtype (Maybe k) :-> v = MaybeM (Either () k :-> v) instance MapKey k => MapKey [k] where -- Value for [] and map for (head:tail) -- -- Note that this includes a recursive ([k] :-> v) map -- in the pair map (k,[k]) :-> v data [k] :-> v = ListM v ((k,[k]) :-> v) instance MapKey Positive where -- We just convert a positive number into -- a list of Bools, then make a map of those newtype Positive :-> v = PosMap ([Bool] :-> v) instance MapKey Integer where -- Now an integer is either negative, zero, or positive. -- So we store a map for negative numbers, a zero value, -- and a map for positive numbers. data Integer :-> v = IntMap (Positive :-> v) v (Positive :-> v) The rest of the class functions are reasonably easy to derive from their type and these data types. -- ryan
Hi, how can I make the following code work? (show all the possible values of a type 'a')
showAll :: (Eq a, Bounded a, Enum a) => a -> [a] showAll a = [minBound..maxBound]::[a]
it keeps saying that I could fix the problem by adding (Enum a) and (Bounded a) the the context, but I failed when I try. anything goes wrong? Thanks! Raeck
On Tue, 2008-12-30 at 17:12 +0000, raeck@msn.com wrote:
Hi, how can I make the following code work? (show all the possible values of a type 'a')
showAll :: (Eq a, Bounded a, Enum a) => a -> [a] showAll a = [minBound..maxBound]::[a]
The bottom 'a' has nothing to do with the 'a' in the type signature. Your code is equivalent to
showAll :: (Eq a, Bounded a, Enum a) => a -> [a] showAll a = [minBound..maxBound]::[b]
Which is not type correct as [minBound..maxBound] :: (Bounded a, Enum a) => [a] You don't need that type annotation (or the type signature for that matter), so just omit it.
Raeck said:
Hi, how can I make the following code work? (show all the possible values of a type 'a')
showAll :: (Eq a, Bounded a, Enum a) => a -> [a] showAll a = [minBound..maxBound]::[a]
What you are really looking for, I think, is a polymorphic value of type [a], where a is some enumerable, bounded type. allValues :: (Enum a, Bounded a) => [a] allValues = [minBound .. maxBound] You can omit the type signature, but then you'll run into the monomorphism restriction (which you'll can turn off with, e.g. -XNoMonomorphismRestriction)
participants (4)
-
Derek Elkins -
raeck＠msn.com -
Robert Greayer -
Ryan Ingram