On 21:00 Sun 10 Jul , Christopher Howard wrote:
I'm trying to understand parametric polymorphism and polymorphic data types. I especially want to go beyond simply using the defined polymorphic data types (lists and so forth) and see how I can make my own useful ones.
As my first stab at it, it seemed like I should be able to create my own heterogeneous "list" data type -- i.e., a "list" data type that can contain elements of different types. (like, [3,'a',True], for example)
But I'm a little stuck. My first try was like so:
data HeteroList a b = Null | Element a (HeteroList a b) deriving (Show)
...but this of course did not work, because all elements end up having to be the same type.
Then I tried
data HeteroList a b = Null | Element a (HeteroList b a) deriving (Show)
...but this doesn't work because every other other element has to be the same type:
Element 'a' (Element 1 (Element 'a' (Element 2 Null)))
...I could go on and embarrass myself some more, but since I'm likely widely off-base I'll just ask if somebody can point me in the right direction.
I'm not an expert on the subject, but existential quantification allows it. {-# LANGUAGE ExistentialQuantification #-} data HeteroElement = forall a. Element a list = [Element 1, Element 'a', Element True] http://en.wikibooks.org/wiki/Haskell/Existentially_quantified_types http://haskell.org/haskellwiki/Heterogenous_collections -- Mats Rauhala MasseR