Flolac week of code 4: caesar cipher

Caesar.hs

import Data.List (group, sort, sortOn)
import qualified Data.Map as M

encode :: Int -> String -> String
encode n = fmap (fetchChar . shift . fromEnum) where
  shift     = (+ n) . subtract 65
  fetchChar = (cycle ['A'..'Z'] !!)

decode :: String -> (String, Int)
decode str = fst . head $ sortOn snd matrix where
  matrix = [ row offset str | offset <- [0..25]]

row :: Int -> String -> ((String, Int), Double)
row offset str = ((encoded, offset), likelihood) where
  encoded = encode offset str
  grouped = group . sort $ encoded
  likelihood = foldr sumDistance 0.0 grouped
  sumDistance xs acc = acc + distance xs
  distance xs = abs $ M.findWithDefault 0.0 (head xs) distribution - freq xs
  freq xs = fromIntegral (length xs) / len
  len = fromIntegral $ length str

distribution :: M.Map Char Double
distribution = M.fromList [ ('A', 0.08167) , ('B', 0.01492) , ('C', 0.02782) , ('D', 0.04253)
                          , ('E', 0.02702) , ('F', 0.02228) , ('G', 0.02015) , ('H', 0.06094)
                          , ('I', 0.06966) , ('J', 0.00153) , ('K', 0.00772) , ('L', 0.04025)
                          , ('M', 0.02406) , ('N', 0.06749) , ('O', 0.07507) , ('P', 0.01929)
                          , ('Q', 0.00095) , ('R', 0.05987) , ('S', 0.06327) , ('T', 0.09056)
                          , ('U', 0.02758) , ('V', 0.00978) , ('W', 0.02360) , ('X', 0.00150)
                          , ('Y', 0.01974) , ('Z', 0.00074)]