sirlensalot

λ> import Data.Tree
λ> import Control.Comonad
λ> let t = Node (1::Int) [Node 2 [Node 4 [],Node 5 []],Node 3 [Node 6 [],Node 7 []]]
λ> let draw = putStrLn . drawTree . fmap show
λ> draw t
1
|
+- 2
|  |
|  +- 4

sirlensalot

-- Transforming between classy stuff.

-- | Foo is present in input and output.
data Foo = Foo
-- | Classy.
class HasFoo a where foo :: Lens' a Foo

-- | Maybe Bar is added to output type.
data Bar = Bar