#+TITLE: 99 Haskell Problems
#+Author: Joseph Ferano

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

*** #1 Find the last element of a list. 

#+begin_src haskell
λ> myLast [1,2,3,4]
4
λ> myLast ['x','y','z']
'z'
#+end_src


**** Solution
#+begin_src haskell
myLast [] = error "Cannot accept an empty list"
myLast [x] = x
myLast (_:xs) = myLast xs
#+end_src

*** #2 Find the last but one element of a list.

#+begin_src haskell
λ> myButLast [1,2,3,4]
3
λ> myButLast ['a'..'z']
'y'
#+end_src


**** Solution
#+begin_src haskell
myButLast x
  | length x < 2 = error "Cannot accept an empty list"
  | otherwise = case x of
                  [x,y] -> x
                  (x:xs) -> myButLast xs
#+end_src


*** #3 Find the K'th element of a list.

The first element in the list is number 1.

#+begin_src haskell
λ> elementAt [1,2,3] 2
2
λ> elementAt "haskell" 5
'e'
#+end_src


**** Solution
#+begin_src haskell
elementAt (x:xs) i = if i == 1 then x else elementAt xs (i - 1)
#+end_src


*** #4 Find the number of elements of a list. 

#+begin_src haskell
λ> myLength [123, 456, 789]
3
λ> myLength "Hello, world!"
13
#+end_src


**** Solution

#+begin_src haskell
myLength [] = 0
myLength xs = foldl (\acc _ -> acc + 1) 0 xs
-- or
myLength [] = 0
myLength (_:xs) = 1 + myLength xs
#+end_src