My first day in Haskell hands-on, encountered the classic Caesar Cipher:
import Data.Char
encode :: Int -> String -> String
encode offset msg = map (chr . (+ offset) . ord) msg
A call for offset 5
on a string delivers:
*Main> encode 5 "abc"
"fgh"
However, a negative offset -3
errs as shown below. Hence how to modify the encode
function to allow for negative values in the offset ?
*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"
Put it in parenthesis:
encode (-3) "abc"