Günther Schmidt
I need a data structure as in my example without the [] being possible to be empty.
Well, a list can by definition be empty, so this is clearly impossible. The best you can do is to hide the constructors and have "smart" constructor functions that guarantee not to construct your data structure with an empty list component. I'm puzzled why you need to use a list here though.
The idea behind this is that an "a" can "pocket" siblings, but only one level deep and that an "a's" list of "pocketed/swallowed" siblings must not be empty, because otherwise it would automatically be an "Single a".
data Container a = Container a [a]
If you cannot use this directly, then perhaps you can use it as a replacement for normal lists that cannot be non-empty? chead (Container x _) = x ctail (Container _ []) = error "Can't take tail of singleton list" ctail (Container _ (x:xs) = x:xs -k -- If I haven't seen further, it is by standing in the footprints of giants