Haskell：无法构造无限类型：a ~ Maybe a

Question

I have the following definition of a binary tree and a function that fetches the leftmost element, or return Nothing if the tree is an Empty tree, however it is saying x (with type a ) is an infinite type because it returns Maybe a ?

Thanks any help would be appreciated:)

data Tree a = EmptyTree | Node a (Tree a) (Tree a) deriving (Show, Eq)

leftistElement :: (Ord a) => Tree a -> Maybe a
leftistElement EmptyTree = Nothing
leftistElement (Node x left _) =
  if left == EmptyTree
    then x
    else leftistElement left

types.hs:55:10: error:
    • Occurs check: cannot construct the infinite type: a ~ Maybe a
    • In the expression: x
      In the expression:
        if left == EmptyTree then x else leftistElement left
      In an equation for ‘leftistElement’:
          leftistElement (Node x left _)
            = if left == EmptyTree then x else leftistElement left
    • Relevant bindings include
        left :: Tree a (bound at types.hs:53:24)
        x :: a (bound at types.hs:53:22)
        leftistElement :: Tree a -> Maybe a (bound at types.hs:52:1)
   |
55 |     then x
   |          ^
Failed, no modules loaded.

Answer 1

x has type a , but you try to return it as a value of type Maybe a . Because a is type variable, though, the type checker attempts to see if there is some instance of Maybe that makes a and Maybe a unify, which leads to the infinite-type error.

Use pure (or Just ) to return a value of the correct type.

leftistElement (Node x left _) =
  if left == EmptyTree
    then pure x
    else leftistElement left