Re: [Haskell-cafe] Finite but not fixed length...

Then viewl can be defined anywhere,

viewl :: Finite s a -> Either () (a, Finite s a) viewl = finite (Left ()) (Right . (,))

... or more like: viewl :: Finite s a -> Either () (a, Finite s a) viewl = finite (Left ()) (\a b -> Right (a,b)) /J 2010/10/13 Jonas Almström Duregård

Hi Stephen,

I'm not sure I see the problem. You can do what you require with the function i supplied (minus the typo). This is in the module (where the Finite constructor is exposed)

finite :: b -> (a -> Finite s a -> b) -> Finite s a -> b finite b _ (Finite []) = b finite _ f (Finite (x:xs)) = f x (Finite xs)

Then viewl can be defined anywhere,

viewl :: Finite s a -> Either () (a, Finite s a) viewl = finite (Left ()) (Right . (,))

Why would you ever decrease the Peano numbers? It's just an upper bound, not an exact size.

/J

2010/10/13 Stephen Tetley :

...

Hi Jonas

Thanks - I was meaning an equivalent to viewl on Data.Sequence, on plain lists:

viewl :: [a] -> Either () (a,[a]) viewl [] = Left () viewl (x:xs) = Right (x,xs)

It was a trick question because I can't see how you can do it without decrement on the Peano numbers.

Best wishes

Stephen