https://wiki.haskell.org/H-99:_Ninety-Nine_Haskell_Problems

1. Find the last element of a list.
2. Find the last but one element of a list.
3. Find the K'th element of a list. The first element in the list is number 1.
4. Find the number of elements of a list.
5. Reverse a list.
6. Find out whether a list is a palindrome. A palindrome can be read forward or backward; e.g. (x a m a x).
7. Flatten a nested list structure.
8. Eliminate consecutive duplicates of list elements.
9. Pack consecutive duplicates of list elements into sublists. If a list contains repeated elements they should be placed in separate sublists.
10. Run-length encoding of a list
11. Run-length encoding of a list modified
12. Decode a run-length encoded list.
13. Run-length encoding of a list (direct solution).
14. Duplicate the elements of a list.
15. Replicate the elements of a list a given number of times.
16. Drop every N'th element from a list.
17. Split a list into two parts; the length of the first part is given.
18. Extract a slice from a list.
19. Rotate a list N places to the left.
20. Remove the K'th element from a list.
21. Insert an element at a given position into a list.
22. Create a list containing all integers within a given range.
23. Extract a given number of randomly selected elements from a list.
24. Lotto: Draw N different random numbers from the set 1..M.
25. Generate a random permutation of the elements of a list.
26. Generate the combinations of K distinct objects chosen from the N elements of a list
27. Group the elements of a set into disjoint subsets.
28. Sorting a list of lists according to length of sublists

31. Determine whether a given integer number is prime.
32. Determine the greatest common divisor of two positive integer numbers. Use Euclid's algorithm.
33. Determine whether two positive integer numbers are coprime. Two numbers are coprime if their greatest common divisor equals 1.
34. Calculate Euler's totient function phi(m).
35. Determine the prime factors of a given positive integer. Construct a flat list containing the prime factors in ascending order.
36. Determine the prime factors of a given positive integer. Construct a list containing the prime factors and their multiplicity.
37. Calculate Euler's totient function phi(m) (improved)
38. A list of prime numbers.
39. Goldbach's conjecture.
40. Given a range of integers by its lower and upper limit, print a list of all even numbers and their Goldbach composition.

46. and 47 Define predicates and/2, or/2, nand/2, nor/2, xor/2, impl/2 and equ/2 (for logical equivalence) which succeed or fail according to the result of their respective operations; e.g. and(A,B) will succeed, if and only if both A and B succeed.
47. Gray codes.
48. Huffman codes.