Find frequency and cumulative frequency of first letter of last name initials

Initials.hs

import System.IO
import Numeric
import qualified Data.Map as M

-- Given a file with people's first and last initials,
-- one person per line, find the frequency and cumulative
-- frequency for each letter.

freqCount:: M.Map Char Int -> String -> M.Map Char Int
freqCount m s =
  let v = M.lookup (last s) m in
    case v of
      Just a -> M.insert (last s) (a + 1) m
      Nothing -> M.insert (last s) 1 m

-- utility routine to show a float to
-- three decimal places

threeDec :: Float -> String
threeDec x = showFFloat (Just 3) x ""

lastnameDistr:: String -> String
lastnameDistr inputString =
  show outlist
  where
    items = lines inputString
    n = length items
    outmap = foldl freqCount M.empty items
    outlist = M.foldlWithKey (\ acc key val ->
      ((fst acc) ++ [(key, val, threeDec (fromIntegral val / fromIntegral n),
        threeDec (fromIntegral (val + snd acc) / fromIntegral n))],
        (val + snd acc)))  ([], 0) outmap

main = do
  s <- readFile "initials.txt"
  putStrLn (lastnameDistr s)

initials.txt

AR
ES
NH
DK
SG
ED
AA
AB
CD
DG
AN
ET
AB
CC
KO
KH
VJ
JV
RL
ER
KO
JA
MJ
KW
GM
JC
JM
CJ
CF
ZH
BF
AC
EC
TH
AK

This program finds the frequency (and cumulative frequency) of first letters of last names. Here’s why it was written: We were doing registration of students at an event by first letter of their last name and had three registration stations labeled A-H, I-P, and R-Z. THe distribution was very uneven; R-Z was almost empty while A-H had a long line of people waiting. For the next group of people the next day, we opened four stations, but we wanted to know where to split the alphabet to even out the number of people at each registration station. I had registration cards from the previous year, so I entered some 750 data points (first and last name initials -- I was interested in seeing what the distribution for first names was) and wrote this program to get the cumulative frequencies. Having the extra station with equal frequencies made registration go much more smoothly.