Substitution Cipher in Haskell

Question

Learn You a Haskell presents the Caesar Cipher:

The Caesar cipher is a primitive method of encoding messages by shifting each character in them by a fixed number of positions in the alphabet

EDIT

Since I'm not limiting this cipher to letters, it's not actually the Caesarian Cipher. Thanks to Anonymous. This implementation is only a Substitution Cipher.

Here's my implementation:

import Data.Char (ord, chr)

encode :: Int -> String -> String
encode n = map $ shift' n
                 where shift' x =  chr . (+ x) . ord

decode :: Int -> String -> String
decode n = map $ shift' n
                 where shift' x =  chr . abs . ((-) x) . ord

Test:

*Main Data.Char> encode 1 "AAA"
"BBB"
*Main Data.Char>
*Main Data.Char> decode 1 "BBB"
"AAA"

I don't like that I'm using abs in my decode. How can I write a function (through currying) that will subtract X from it?

Also, please critique my implementation.

Answer 1

This is a legitimate substitution cipher, but it's not a Caesarean cipher. A Caesarian cipher affects only letters, and it's modulo the alphabet size: caesar 13 "Hello, world!" ⇒ "Urzzb, jbeyq!"

encode and decode can be the same function (caesar?) called with opposite shifts. (Or with the same shift, when it's 13.)

There's no need for a shift' helper.

How can I write a function (through currying) that will subtract X from it?

(x -)