Hi
I'm starting to play with GADT's, and I was trying to write a safe
version of head and tail, on a safe version of lists. What I came up
with is:
data ConsT a
data NilT
data List a t where
Cons :: a -> List a b -> List a (ConsT b)
Nil :: List a NilT
instance Show a => Show (List a t) where
show (Cons a b) = show a ++ ":" ++ show b
show Nil = "[]"
safeHead :: List a (ConsT t) -> a
safeHead (Cons a b) = a
safeTail :: List a (ConsT t) -> List a t
safeTail (Cons a b) = b
safeMap :: (a -> b) -> List a t -> List b t
safeMap f Nil = Nil
safeMap f (Cons a b) = Cons (f a) (safeMap f b)
Defining safeMap was trivial, but one thing I couldn't figure out was
how to write something like fromList:
fromList [] = Nil
fromList (a:as) = Cons a (fromList as)
fromList could obviously be anything such as reading from a file etc,
where the input is not known in advance. Are there any techniques for
this?
In addition I was expecting to find some example like this in one of
the papers/examples on GADT's, but I didn't. The Haskell wikibook has
a slight example along these lines, but not with the recursive field
in ConsT. [http://en.wikibooks.org/wiki/Haskell/GADT]
Any help or hints would be appreciated,
Thanks
Neil
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