CODEWITHSUNDEEP
haskelltypes
Sasithorn Hannarong
Sasithorn Hannarong

Reputation: 45

Haskell: Couldn't match expected type ‘Tree Char’ with actual type ‘Char -> Tree Char’

I want to construct a binary tree in Haskell. This tree construction is different from normal BST because the input is in Pre-order traversal and I want to reconstruct the binary tree from this input. However, I got an error when compiling this following code:

module Main where

import System.IO
import Text.Printf

data Tree a = Nil | Leaf | Branch a (Tree a) (Tree a) deriving (Eq, Show)

main = do
    -- get input tree
    line <- getLine
    return ()

insert :: Tree Char -> Char -> Tree Char
insert Nil a = Branch a Nil Nil
insert (Branch v t1 t2) a
    | t1 == Nil && t2 == Nil && v == '*' = Branch v (makeB a) t2    --If *     has no child, insert a to leftside of the tree
    | (getElement t1) == '*' && v == '*' = Branch v (insert t1 a) t2
    | (getElement t1) /= '*' && v == '*' = Branch v t1 (makeB a)

getElement :: Tree Char -> Char
getElement (Branch v t1 t2) = v

makeB :: Char -> Tree Char
makeB a =  Branch a Nil Nil

inputTree :: Tree Char -> [Char] -> Tree Char
inputTree tr [] = tr
inputTree tr (x:xs) = inputTree newtree xs
    where newtree = insert (tr x)

The problem is in >> inputTree newtree xs. It said

main.hs:43:33: error:
• Couldn't match expected type ‘Tree Char’
              with actual type ‘Char -> Tree Char’
• Probable cause: ‘newtree’ is applied to too few arguments
  In the first argument of ‘inputTree’, namely ‘newtree’
  In the expression: inputTree newtree xs
  In an equation for ‘inputTree’:
      inputTree tr (x : xs)
        = inputTree newtree xs
        where
            newtree = insert (tr x)
main.hs:44:29: error:
• Couldn't match expected type ‘Char -> Tree Char’
              with actual type ‘Tree Char’
• The function ‘tr’ is applied to one argument,
  but its type ‘Tree Char’ has none
  In the first argument of ‘insert’, namely ‘(tr x)’
  In the expression: insert (tr x)

My newtree should have type Tree Char but it has type Char -> Tree Char instead. I don't know how to fix it. Thanks for any help

Upvotes: 0

Views: 570

Answers (1)

sepp2k
sepp2k

Reputation: 370435

insert (tr x) means "apply the function tr to the argument x and then apply the function insert to the result". Since insert has type Tree Char -> Char -> Tree Char and you're only applying it to one argument, this means that you get back a function of type Char -> Tree Char, which expects the second argument. That's why you get type errors about newTree having type Char -> Tree Char

The next problem is that tr isn't actually a function, so applying it to an argument makes no sense. That's the "The function ‘tr’ is applied to one argument, but its type ‘Tree Char’ has none" error.

You can fix both of those by instead applying insert to the arguments tr and x (insert tr x), as you presumably meant to all along.

Upvotes: 2

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