Add a test and some explanation for P39

src/Arithmetic.hs

@@ -126,23 +126,30 @@ Given a range of integers by its lower and upper limit,
 construct a list of all prime numbers in that range.
 
 ANSWER:
-Copied from: https://wiki.haskell.org/Prime_numbers,
-because this prime number $hit is getting boring.
+Code copied from https://wiki.haskell.org/Prime_numbers,
+explanation my own.
 -}
 primesR :: Int -> Int -> [Int]
 primesR a b = if a < 3 then 2 : xs else xs
   where
     xs = [i | i <- [o, o + 2 .. b], arr A.! i]
-    o = max (a + fromEnum (even a)) 3 -- first odd in the segment
+    -- First odd in the segment.
+    o = max (a + fromEnum (even a)) 3
     r = floor . sqrt $ (fromIntegral b :: Float) + 1
     arr =
+      -- Create a boolean array indexed from o to b;
+      -- the indices produced by the association list
+      -- are set to false.
       AU.accumArray
         (\_ _ -> False) -- accumulating fn
-        True -- initial value
+        -- Default value, used when the index
+        -- is not present in the association list.
+        True
         (o, b) -- bounds of the array
         [ (i, ()) -- association list
           | p <- [3, 5 .. r],
-            let q = p * p -- flip every multiple of an odd to False
+            -- Flip every multiple of an odd to False.
+            let q = p * p
                 s = 2 * p
                 -- Difference between divMod and quotRem.
                 -- https://stackoverflow.com/a/339823/839733

test/ArithmeticSpec.hs

@@ -58,6 +58,7 @@ spec = do
   describe "primesR" $ do
     it "constructs a list of primes within a given range" $ do
       primesR 10 20 `shouldBe` [11, 13, 17, 19]
+      primesR 7 31 `shouldBe` [7, 11, 13, 17, 19, 23, 29, 31]
 
   describe "goldbach" $ do
     it "finds two prime numbers that sum to a given even integer" $ do