Solutions to C9 Haskell (Chapter 6)

1. Figure out if all values in a list are True
   areTrue :: [Bool] -> Bool
   areTrue xs = (length [x|x<-xs, x==True]) == length xs

2. Concatenate a list of lists.
   concat :: [[a]] -> [a]
   concat xss = [x | xs<-xss, x<-xs]
   Main.concat [[1,3], [2,4]]

3. Produce a list with n identical elements
   ident :: (Num n, Enum n) => n -> a -> [a]
   ident n x = [x | _ <- [1..n]]
   ident 3 True

4. Select the nth element of a list
   nSel :: (Num a, Num n, Eq n, Enum n) => [a] -> n -> a
   nSel xs n = head [x | (x, i) <- zip xs [0..], i==n]
   nSel [1,4,0] 1

5. Decide if an element is present in a list
   isElem :: (Eq a) => [a] -> a -> Bool
   isElem xs x = length([e | e <- xs, e==x]) >= 1
   isElem [1, 3, 5, 1] 1
   
6. Other stuff goes here.