Haskell范围错误

Question

formatBill :: BillType -> String
formatBill bill = merge' [pname ++ dots ++ show pprice | (pname, pprice) <- bill]
    where
    dots = ['.' | x<-[1..(lineLength - length pname - length (show pprice))]]

这是我的代码 - formatBill是一个必须返回的函数String.

它应该返回的一个例子:

Product name.................. PRICE
Other product................. 4555

merge' 只是 [String] -> String

type BillType = [(String, Int)] -- BillType definition
lineLength = 30 -- length of row

这些是我得到的错误:

code.hs:69:51:不在范围内:`pname'

code.hs:69:72:不在范围内:`pprice'

Answer 1

一个where条款作用域在整个函数定义,所以你不能使用的东西,只有在列表中的理解范围.

要么dots将它们作为参数制作成函数:

formatBill :: BillType -> String
formatBill bill =
  merge' [pname ++ dots pname pprice ++ show pprice | (pname, pprice) <- bill]
    where
    dots pname pprice =
          ['.' | x<-[1..(lineLength - length pname - length (show pprice))]]

或者使用let列表内部理解:

formatBill :: BillType -> String
formatBill bill =
  merge' [pname ++ dots ++ show pprice
               | (pname, pprice) <- bill
               , let dots = ['.' | x <- [1..(lineLength
                                                - length pname
                                                - length (show pprice))]]]