Haskell列出列表中的变量

Question

假设我有数据

data Expense = MkExpense Double String deriving (Eq,Ord)

和一个功能

balanced :: [Expense] -> Double -> Bool
balanced expenses epsilon =
  let amounts = ...
  in maximum amounts - minimum amounts < epsilon;

计算支出清单是否为余额（即每个人都支付相同的费用，由于浮点误差导致的epsilon的最大差额），epsilon的值为0.01。

alogrithm是正确的，但是三个点在哪里，我想提取一个双打列表。 即我参数中所有费用的两倍。 如何在不更改数据类型的情况下做到这一点？

Answer 1

您可以按以下步骤进行：

balanced :: [Expense] -> Double -> Bool
balanced expenses epsilon =
  let amounts = map extractExpense expenses
  in maximum amounts - minimum amounts < epsilon

而extractExpense将成为数据的Double值的展开器：

extractExpense :: Expense -> Double
extractExpense (MkExpense expense _) = expense

或更简洁：

balanced :: [Expense] -> Double -> Bool
balanced expenses epsilon =
  let amounts = map extractExpense expenses
  in maximum amounts - minimum amounts < epsilon
    where extractExpense (MkExpense e _) = e