Accompanying material for the paper "Generalizing Generalized Tries"
by Ralf Hinze, in Journal of Functional Programming, 10(4),
pp. 327-351, July 2000.

Perfectly balanced, binary leaf trees.

> module Perfect (
>     Perfect(Null, Succ),
>     perfect,
> ) where
> import Fork

> data Perfect a		=  Null a | Succ (Perfect (Fork a))
>				   deriving (Show)

Constructing a perfect tree (the length of the list must be a perfect
power of 2).

> perfect			:: [a] -> Perfect a
> perfect as
>     | single as		=  Null (unwrap as)
>     | otherwise		=  Succ (perfect (forks as))

> forks				:: [a] -> [Fork a]
> forks []			=  []
> forks (a : b : x)		=  Fork a b : forks x

> single			:: [a] -> Bool
> single [_]			=  True
> single _			=  False

> unwrap			:: [a] -> a
> unwrap [a]			=  a