David Menendez writes:

MR K P SCHUPKE writes:

...

I would suggest using the type system as I said earlier so:

toNonEmptyList :: [a] -> Maybe (NonEmpty a) toNonEmptyList (a0:_) = Just (NonEmpty a) toNonEmptyList _ = Nothing

Then redefine head:

head :: NonEmpty a -> a head (NonEmpty (a0:_)) = a0

Oleg described a similar technique a few months ago.

From http://haskell.org/pipermail/haskell/2004-June/014271.html:

|> newtype NonEmpty a = NonEmpty [a] -- the NonEmpty constructor should |> -- be hidden (not exported from its module) |> |> head' (NonEmpty a) = head a -- no error can occur! Can use unsafe version |> tail' (NonEmpty a) = tail a -- no error can occur! Can use unsafe version |> |> -- trusted function: the only one that can use NonEmpty constructor. |> fork_list_len f g x = if null x then f else g (NonEmpty x) |> |> revers x = revers' x [] |> where |> revers' x accum = fork_list_len accum (g accum) x |> g accum x = revers' (tail' x) ((head' x):accum)

We have these equivalences:

toNonEmptyList == fork_list_len Nothing Just fork_list_len d f == maybe d f . toNonEmptyList

I think defining 'toNonEmptyList' in terms of 'fork_list_len' is cleaner, but that's just my personal taste. (As it happens, I ended up defining a very similar function 'cons' in my recursion module[1]).

[1] <http://www.eyrie.org/~zednenem/2004/hsce/Control.Recursion.html#v% 3Acons> -- David Menendez http://www.eyrie.org/~zednenem/