Re: [Haskell-cafe] Introspection

Thanks. Let me try this. I appreciate all the help. And how do I compute a LR parser graph using haskell? Sent from my iPhone On Jul 22, 2011, at 6:41 AM, Felipe Almeida Lessa wrote:

On Fri, Jul 22, 2011 at 10:31 AM, Stephen Tetley 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.

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