とりあえず、ここの3.までを完了。下記のエラーの意味が理解できるようになる。
Prelude> foldr1 (++) ['a','b','c','d','e']
ERROR - Type error in application
Expression : foldr1 (++) ['a','b','c','d','e']
Term : (++)
Type : [a] -> [a] -> [a]
Does not match : Char -> Char -> Char
Prelude> foldr1 (++) ["a","b","c","d","e"]
"abcde"
Prelude>