[Haskell-cafe] Introspection

[Felipe Almeida Lessa]

On Fri, Jul 22, 2011 at 10:31 AM, Stephen Tetley
<stephen.tetley at gmail.com> wrote:
> As Haskell is statically typed, if y.length < 100, y is still of type Big...

You could achieve this kind of thing with smart constructors and
GADTs.  I don't know how useful it is, though =).

data Sized t a where
  Small :: a -> Sized Small a
  Big :: a -> Sized Big a

data Small -- empty
data Big -- empty

retrieve :: Sized t a -> a
retrieve (Small a) = a
retrieve (Big a) = a

toSmall :: [a] -> Maybe (Sized Small [a])
toSmall xs = if length xs < 100 then Just (Small xs) else Nothing

toBig :: ... -- almost the same as toSmall

toUnknown :: [a] -> (forall t. Sized t [a] -> r) -> r
toUnknown xs f = if length xs < 100 then f (Small xs) else f (Big xs)

-- 
Felipe.