Is there a standard map function for trees in Haskell?

Question

I need to write a non-recursive function that transforms my tree. But I can't use my own map function for trees. Is there a standard map function for trees in Haskell?

data BTree a = Tip a | Bin (BTree a) (BTree a)
exapmeTree = Bin (Bin (Tip [1,2,3,4,5]) (Tip [3,9,7])) (Bin (Tip [6,7,8,9]) (Tip [8,2,1]))
myReverse list = foldl (\acc x -> x : acc) [] list

reverseTree tree = map myReverse tree

Answer 1

You can either write the fmap function yourself, as demonstrated by Rik.

Or you can use the fmap function provided by the automatically generated Functor instance.

Testing under ghci:

$ ghci
GHCi, version 8.6.5: http://www.haskell.org/ghc/  :? for help
 λ> 
 λ> :set -XDeriveFunctor
 λ> 
 λ> data BTree a = Tip a | Bin (BTree a) (BTree a)  deriving (Eq, Show, Functor)
 λ> 
 λ> exampleTree = Bin (Bin (Tip [1,2,3,4,5]) (Tip [3,9,7])) (Bin (Tip [6,7,8,9]) (Tip [8,2,1]))
 λ> 
 λ> reverseTree = fmap reverse exampleTree
 λ> 
 λ> reverseTree 
 Bin (Bin (Tip [5,4,3,2,1]) (Tip [7,9,3])) (Bin (Tip [9,8,7,6]) (Tip [1,2,8]))
 λ> 

Answer 2

Since you've created the BTree type yourself, no, there is no standard function for it. You can quite easily write a Functor instance yourself though.

instance Functor BTree where
  fmap f (Tip a)   = Tip (f a)
  fmap f (Bin l r) = Bin (fmap f l) (fmap f r) 

Now you can write reverseTree tree = fmap myReverse tree.

Note that fmap is the exact same as map in the context of lists, but it is more generic, meaning it works for any Functor instance.