[Haskell-cafe] Lazy Lists and IO

Jonathan Cast jcast at ou.edu
Wed Jul 11 17:52:24 EDT 2007

On Wednesday 11 July 2007, Chaddaï Fouché wrote:
> 2007/7/11, Stefan O'Rear <stefanor at cox.net>:
> > Interestingly, the function is already there; it's called genericLength.
> >
> > However, the lazy natural type isn't.
> I'm not sure what you mean there : genericLength is just a length that
> can return any Num (eg. an Integer) and not just Int, it has nothing
> to do with what Andrew wanted, which was a function that checked if a
> list was longer than n without swallowing more of the list than
> necessary.
> Is there something I misunderstood in the exchange ?

Yeah.  The reference to the "lazy natural type", which is:

data Nat
  = Zero
  | Succ Nat
  deriving (Eq, Ord, Read, Show)

instance Num Nat where
  fromInteger 0 = Zero
  fromInteger (n + 1) = Succ (fromInteger n)

then genericLength xn > n does exactly what Andrew wants, when n :: Nat.

Jonathan Cast

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