Well, it's easy to make it so that lists are either finite or bottom,
but it's not so easy to make infinite lists fail to typecheck...
That's what I'm wondering about.
2010/10/13 Miguel Mitrofanov
hdList :: List a n -> Maybe a hdList Nil = Nothing hdList (Cons a _) = Just a
hd :: FiniteList a -> Maybe a hd (FL as) = hdList as
*Finite> hd ones
this hangs, so, my guess is that ones = _|_
13.10.2010 12:13, Eugene Kirpichov пишет:
{-# LANGUAGE ExistentialQuantification, GADTs, EmptyDataDecls #-} module Finite where
data Zero data Succ a
class Finite a where
instance Finite Zero instance (Finite a) => Finite (Succ a)
data List a n where Nil :: List a Zero Cons :: (Finite n) => a -> List a n -> List a (Succ n)
data FiniteList a where FL :: (Finite n) => List a n -> FiniteList a
nil :: FiniteList a nil = FL Nil
cons :: a -> FiniteList a -> FiniteList a cons a (FL x) = FL (Cons a x)
list123 = cons 1 (cons 2 (cons 3 nil))
ones = cons 1 ones -- typechecks ok
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-- Eugene Kirpichov Senior Software Engineer, Grid Dynamics http://www.griddynamics.com/