CODEWITHSUNDEEP
haskellfunctional-programming
Carlos Romero
Carlos Romero

Reputation: 181

Attempting to invert a Binary Tree in Haskell

I've recently started getting into functional programming and have just started to learn Haskell. I am attempting to invert a binary tree and I have the following so far: 

data Tree a = Tip | Bin (Tree a) a (Tree a) deriving (Show,Eq)

mirror :: Tree a -> Tree a
mirror(Bin l v r) = (mirror r) Bin v (mirror l)

However, I don't exactly understand why I get the following error: Couldn't match expected type ‘(Tree a0 -> a0 -> Tree a0 -> Tree a0). Can anyone explain where I am getting confused?

Upvotes: 1

Views: 567

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 477607

You need to put the data constructor first here, so it should be:

mirror :: Tree a -> Tree a
mirror(Bin l v r) = Bin (mirror r) v (mirror l)

By using (mirror r) Bin v mirror l, you call Bin as a parameter on the result of mirror r, so ((mirror r) Bin). But that does not make much sense.

Note that you will need to add a case for Tip as well, since right now, your function does not cover all possible values.

Upvotes: 4

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