# 99 questions/Solutions/54A

### From HaskellWiki

(*) Check whether a given term represents a binary tree

In Haskell, we characterize binary trees with a datatype definition:

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

The above tree is represented as:

tree1 = Branch 'a' (Branch 'b' (Branch 'd' Empty Empty) (Branch 'e' Empty Empty)) (Branch 'c' Empty (Branch 'f' (Branch 'g' Empty Empty) Empty)))

Other examples of binary trees:

tree2 = Branch 'a' Empty Empty -- a binary tree consisting of a root node only tree3 = nil -- an empty binary tree tree4 = Branch 1 (Branch 2 Empty (Branch 4 Empty Empty)) (Branch 2 Empty Empty)

Tree a

are binary trees: it is just not possible to construct an invalid tree with this type. Hence, it is redundant to introduce a predicate to

check this property: it would always returnTrue