Minor code improvements for Haskell

Haskell/p004.hs

@@ -7,7 +7,7 @@ isPalindrome n = show n == reverse (show n)
 
 maxPalindrome :: Int
 maxPalindrome =
-    maximum $ filter isPalindrome [ x * y | x <- [100..999], y <- [100..999] ]
+    maximum . filter isPalindrome $ (*) <$> [100..999] <*> [100..999]
 
 main = do
     let result = maxPalindrome

Haskell/p013.hs

@@ -101,9 +101,8 @@
 -- 20849603980134001723930671666823555245252804609722
 -- 53503534226472524250874054075591789781264330331690
 
---firstDigitsSum :: (Show a, Read a, Integral a) => Int -> [a] -> a
-firstDigitsSum :: Int -> [Integer] -> Integer
-firstDigitsSum n xs = read $ take n $ show $ sum xs
+firstDigitsSum :: Int -> [Integer] -> Int
+firstDigitsSum n xs = read . take n . show $ sum xs
 
 main = do
     let result = firstDigitsSum 10 [37107287533902102798797998220837590246510135740250

Haskell/p014.hs

@@ -14,7 +14,7 @@
 --
 -- NOTE: Once the chain starts the terms are allowed to go above one million.
 
-collatz :: (Integral a) => a -> [a]
+collatz :: Int -> [Int]
 collatz 1 = [1]
 collatz n
     | even n = n:collatz (n `div` 2)