凯撒密码用于负偏移量

Question

我在Haskell动手的第一天，遇到了经典的凯撒密码:

import Data.Char
encode :: Int -> String -> String
encode offset msg = map (chr . (+ offset) . ord) msg

在字符串上调用offset 5传递:

*Main> encode 5 "abc"
"fgh"

然而，负的offset -3出错如下所示。因此如何修改encode函数以允许偏移量中的负值?

*Main> encode -3 "abc"
<interactive>:7:8:
    No instance for (Num (Int -> String -> String))
      arising from a use of `-'
    Possible fix:
      add an instance declaration for (Num (Int -> String -> String))
    In the expression: encode - 3 "abc"
    In an equation for `it': it = encode - 3 "abc"
<interactive>:7:9:
    No instance for (Num ([Char] -> Int -> String -> String))
      arising from the literal `3'
    Possible fix:
      add an instance declaration for
      (Num ([Char] -> Int -> String -> String))
    In the expression: 3
    In the second argument of `(-)', namely `3 "abc"'
    In the expression: encode - 3 "abc"

Answer 1

放入括号内:

encode (-3) "abc"