Haskell: Exercise #9

Haskell.org

@@ -80,11 +80,162 @@ myLength xs = foldl (\acc _ -> acc + 1) 0 xs
 myLength' [] = 0
 myLength' [x] = 1
 myLength' (_:xs) = 1 + myLength xs
-
-myLength [1..5]
-myLength' [1..5]
 #+end_src
 
-#+RESULTS:
+*** #5 Reverse a list.
-: 5
 
+#+begin_src haskell
+λ> myReverse "A man, a plan, a canal, panama!"
+"!amanap ,lanac a ,nalp a ,nam A"
+λ> myReverse [1,2,3,4]
+[4,3,2,1]
+#+end_src
+
+
+**** Solution
+
+#+begin_src haskell
+myReverse = go []
+  where go acc [] = acc
+        go acc (x:xs) = go (x:acc) xs
+#+end_src
+
+*** #6 Find out whether a list is a palindrome
+
+#+begin_src haskell
+λ> isPalindrome [1,2,3]
+False
+λ> isPalindrome "madamimadam"
+True
+λ> isPalindrome [1,2,4,8,16,8,4,2,1]
+True
+#+end_src
+
+**** Solution
+#+begin_src haskell
+isPalindrome xs = xs == reverse xs
+#+end_src
+*** #7 Flatten a nested list structure.
+Transform a list, possibly holding lists as elements into a `flat' list by replacing each list with its elements (recursively).
+
+Example:
+#+begin_src haskell
+,* (my-flatten '(a (b (c d) e)))
+(A B C D E)
+#+end_src
+
+Example in Haskell:
+
+We have to define a new data type, because lists in Haskell are homogeneous.
+
+#+begin_src haskell
+data NestedList a = Elem a | List [NestedList a]
+#+end_src
+
+#+begin_src haskell
+λ> flatten (Elem 5)
+[5]
+λ> flatten (List [Elem 1, List [Elem 2, List [Elem 3, Elem 4], Elem 5]])
+[1,2,3,4,5]
+λ> flatten (List [])
+[]
+#+end_src
+
+**** Solution
+#+begin_src haskell
+flatten = reverse . go []
+  where go acc (List []) = acc
+        go acc (Elem x) = x:acc
+        go acc (List (x:xs)) = go (go acc x) (List xs)
+#+end_src
+*** #8 Eliminate consecutive duplicates of list elements
+
+If a list contains repeated elements they should be replaced with a single copy of the element. The order of the elements should not be changed.
+
+Example:
+
+#+begin_src haskell
+,* (compress '(a a a a b c c a a d e e e e))
+(A B C A D E)
+#+end_src
+
+Example in Haskell:
+
+#+begin_src haskell
+λ> compress "aaaabccaadeeee"
+"abcade"
+#+end_src
+**** Solution
+
+This is not great...
+
+#+begin_src haskell
+compress xs = reverse $ go [] xs
+  where go acc [] = acc
+        go [] (x:xs) = go [x] xs
+        go [a] [x]
+            | a == x = [a]
+            | otherwise = [x,a]
+        go (a:as) [x]
+            | a == x = a:as
+            | otherwise = x:a:as
+        go [a] (x:xs)
+            | a == x = go [a] xs
+            | otherwise = go [x,a] xs
+        go (a:as) (x:xs)
+            | a == x = go (a:as) xs
+            | otherwise = go (x:a:as) xs
+#+end_src
+
+Especially when one of the solutions is the following;
+
+#+begin_src haskell
+import Data.List
+
+compress :: Eq a => [a] -> [a]
+compress = map head . group
+#+end_src
+*** #9 Pack consecutive duplicates of list elements into sublists
+
+If a list contains repeated elements they should be placed in separate sublists.
+
+Example:
+
+#+begin_src haskell
+,* (pack '(a a a a b c c a a d e e e e))
+((A A A A) (B) (C C) (A A) (D) (E E E E))
+#+end_src
+
+Example in Haskell:
+
+#+begin_src haskell
+λ> pack ['a', 'a', 'a', 'a', 'b', 'c', 'c', 'a', 'a', 'd', 'e', 'e', 'e', 'e']
+["aaaa","b","cc","aa","d","eeee"]
+#+end_src
+**** Solution
+In python you can use ~Counter~
+#+begin_src python
+from collections import Counter
+def pack(arr):
+    return [[key] * val for key,val in Counter(arr).items()]
+#+end_src
+
+In Haskell, this is just ~group~
+
+#+begin_src haskell
+pack :: [a] -> [[a]]
+pack = group
+#+end_src
+
+Here's my naive implementation
+
+#+begin_src haskell
+pack :: Eq a => [a] -> [[a]]
+pack xs = reverse $ go [[]] xs
+  where
+    go acc [] = acc
+    go ([]:acc) (x:xs) = go ([x]:acc) xs
+    go ((a:as):acc) (x:xs)
+       | a == x = go ((x:a:as):acc) xs
+       | otherwise = go ([x]:(a:as):acc) xs
+#+end_src

LICENSE

@@ -0,0 +1,20 @@
+Copyright (c) 2023 Joseph Ferano - joseph@ferano.io
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.org

@@ -1,3 +1,21 @@
-** 99 Problems
+#+OPTIONS: toc:nil
 
-Literate documents for OCaml and Haskell 99 problems
+* About
+
+This repository contains my ongoing solutions for both OCaml and Haskell coding
+challenge problems in the style of "99 Problems". See below for references. The
+intention is to practice these languages in the hopes of someday using them in
+anger. The solutions use a style of programming known as [[https://en.wikipedia.org/wiki/Literate_programming][Literate Programming]]
+with the help of Emacs, org-mode, and [[https://orgmode.org/worg/org-contrib/babel/intro.html][org-babel]]. All code is executable from
+within Emacs by invoking the ~C-c C-c~ keybinding while the cursor is in the
+source code block.
+
+* Progress
+- [16/99] OCaml
+- [7/99] Haskell
+- [0/99] Lisp (Soon)
+
+* References
+
+- OCaml Exercises: https://ocaml.org/problems
+- 99 Haskell Problems: https://wiki.haskell.org/H-99:_Ninety-Nine_Haskell_Problems