Add solutions for problems 19 and 20.

Euler.hs

@@ -7,6 +7,7 @@ module Euler
   , primes
   , zipArraysWith
   , RangeIx(..)
+  , digitsOf
   ) where
 
 import Control.Applicative
@@ -49,3 +50,6 @@ zipArraysWith :: (IArray arrA a, IArray arrB b, IArray arrC c, RangeIx i)
               => (a -> b -> c) -> arrA i a -> arrB i b -> arrC i c
 zipArraysWith f as bs = array newRange $ [ (i, f (as!i) (bs!i)) | i <- range newRange ]
    where newRange = intersectBounds (bounds as) (bounds bs)
+
+digitsOf :: (Read a, Show a, Integral a) => a -> [a]
+digitsOf = map (read . (:[])) . show

Problem19.hs

@@ -0,0 +1,27 @@
+import Data.Monoid
+
+data Month = Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec deriving (Eq,Ord,Enum,Bounded,Show)
+data Day   = Sun | Mon | Tue | Wed | Thu | Fri | Sat deriving (Eq,Ord,Enum,Bounded,Show)
+type Year  = Int
+
+leapYear yr = yr `mod` 400 == 0 || (yr `mod` 4 == 0 && yr `mod` 100 /= 0)
+
+daysInMonth :: (Month, Year) -> Int
+daysInMonth (mn, yr)
+  | mn `elem` [ Sep, Apr, Jun, Nov ] = 30
+  | mn == Feb = if leapYear yr then 29 else 28
+  | otherwise = 31
+
+nextMonth (Dec, yr) = (Jan, yr + 1)
+nextMonth (mn,  yr) = (succ mn, yr)
+
+compareMonths (m1,y1) (m2,y2) = compare y1 y2 <> compare m1 m2
+
+addDays d n = toEnum $ (fromEnum d + n) `mod` 7
+
+startDay = ((Jan, 1900), Mon) : [ (nextMonth m, addDays d n) | (m, d) <- startDay, let n = daysInMonth m ]
+
+main = print $ length $ filter ((== Sun) . snd) $
+       takeWhile (\(m,_) -> m `compareMonths` (Dec, 2000) /= GT) $
+       dropWhile (\(m,_) -> m `compareMonths` (Jan, 1901) == LT) $
+       startDay

Problem20.hs

@@ -0,0 +1,2 @@
+import Euler
+main = print $ sum $ digitsOf $ product [1..100]