99 questions/11 to 20

These are Haskell translations of Ninety Nine Lisp Problems.

If you want to work on one of these, put your name in the block so we know someone's working on it. Then, change n in your block to the appropriate problem number, and fill in the <Problem description>,<example in lisp>,<example in Haskell>,<solution in haskell> and <description of implementation> fields.

Problem 11

<Problem description>

```Example:
<example in lisp>

```

Solution:

```<solution in haskell>
```

<description of implementation>

Problem 12

<Problem description>

```Example:
<example in lisp>

```

Solution:

```<solution in haskell>
```

<description of implementation>

Problem 13

<Problem description>

```Example:
<example in lisp>

```

Solution:

```<solution in haskell>
```

<description of implementation>

Problem 14

(*) Duplicate the elements of a list.

```Example:
* (dupli '(a b c c d))
(A A B B C C C C D D)

> dupli [1, 2, 3]
[1,1,2,2,3,3]
```

Solution:

```dupli [] = []
dupli (x:xs) = [x,x] ++ dupli(xs)
```

Problem 15

<Problem description>

```Example:
<example in lisp>

```

Solution:

```<solution in haskell>
```

<description of implementation>

Problem 16

<Problem description>

```Example:
<example in lisp>

```

Solution:

```<solution in haskell>
```

<description of implementation>

Problem 17

<Problem description>

```Example:
<example in lisp>

```

Solution:

```<solution in haskell>
```

<description of implementation>

Problem 18

(**) Extract a slice from a list.

Given two indices, i and k, the slice is the list containing the elements between the i'th and i'th element of the original list (both limits included). Start counting the elements with 1.

```Example:
* (slice '(a b c d e f g h i k) 3 7)
(C D E F G)

*Main> slice ['a','b','c','d','e','f','g','h','i','k'] 3 7
```

Solution:

```slice xs (i+1) k = take (k-i) \$ drop i xs
```

Problem 19

(**) Rotate a list N places to the left.

Hint: Use the predefined functions length and (++).

```Examples:
* (rotate '(a b c d e f g h) 3)
(D E F G H A B C)

* (rotate '(a b c d e f g h) -2)
(G H A B C D E F)

*Main> rotate ['a','b','c','d','e','f','g','h'] 3

*Main> rotate ['a','b','c','d','e','f','g','h'] (-2)
```

Solution:

```rotate [] _ = []
rotate l 0 = l
rotate (x:xs) (n+1) = rotate (xs ++ [x]) n
rotate l n = rotate l (length l + n)
```

There are two separate cases:
- If n > 0, move the first element to the end of the list n times.
- If n < 0, convert the problem to the equivalent problem for n > 0 by adding the list's length to n.

Problem 20

<Problem description>

```Example:
<example in lisp>

```<solution in haskell>