Haskellにおける型クラス制約の役割 ref: https://qiita.com/HirotoShioi/items/5474119ba3682448c109

error

    • No instance for (Num a) arising from a use of ‘+’
      Possible fix:
        add (Num a) to the context of
          the type signature for:
            sum' :: forall a. [a] -> a
    • In the expression: x + acc
      In the first argument of ‘foldl1’, namely ‘(\ x acc -> x + acc)’
      In the expression: foldl1 (\ x acc -> x + acc)
  |
5 | sum' = foldl1 (\x acc -> x + acc)
  |    

sum

*Main> sum' [1,2,3,4,5]
15

sum.hs

sum' :: (Num a) => [a] -> a
sum' = foldr1 (\x acc -> x + acc)