haskell - 在 haskell 中的函数中声明字典

Question

我正在编写代码

import Data.Map

main = print $ scores ["APPLE","bbd","cat"]
score :: String -> Int
score = sum . map latterScore

scores :: [String] -> [Int]
scores arrayOfStrings = [ score word | word <- arrayOfStrings]

myLookup :: Char -> Int
myLookup inputChar = x <- Data.Map.fromList([("A",1), ("B",3), ("C", 3), ("E", 1), ("D", 2), ("G", 2), ("F", 4), ("I", 1), ("H", 4), ("K", 5), ("J", 8), ("M", 3), ("L", 1), ("O", 1), ("N", 1), ("Q", 10), ("P", 3), ("S", 1), ("R", 1), ("U", 1), ("T", 1), ("W", 4), ("V", 4), ("Y", 4), ("X", 8), ("Z", 10)])
        case Data.Map.lookup inputChar x of Nothing -> undefined
                                        (Just v) -> v

但是这显示了一个错误，即第 1 行输入“​​<-”的解析错误，即字典我们可以在不使用案例的情况下编写这段代码吗？

Answer 1

<-用于列表推导式和 do符号，而你两者都没有使用。您可能想使用 let :

myLookup inputChar = let x = ... in case Data.Map.lookup inputChar x of ...

其他选项将使用 where :

myLookup inputChar = case Data.Map.lookup inputChar x of ...
  where x = ...

或者只是引入模块级绑定(bind):

x = ...

myLookup inputChar = case Data.Map.lookup inputChar x of ...

至于删除case ，您可以使用 fromJust 导入后Data.Maybe :

fromJust $ Data.Map.lookup inputChar x

哎呀，你甚至可以嵌入 fromList就在那里:

fromJust $ Data.Map.lookup inputChar $ Data.Map.fromList ...