Added solutions for lesson 15

Author: rober-m

Homework15/CHANGELOG.md

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+# Revision history for Homework15
+
+## 0.1.0.0 -- YYYY-mm-dd
+
+* First version. Released on an unsuspecting world.

Homework15/Homework15.cabal

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+cabal-version:   3.0
+name:            Homework15
+version:         0.1.0.0
+license:         MIT
+license-file:    LICENSE
+author:          Robertino Martinez
+maintainer:      robertino.martinez@iohk.io
+build-type:      Simple
+extra-doc-files: CHANGELOG.md
+
+common warnings
+    ghc-options: -Wall
+
+executable Homework15A
+    import:           warnings
+    main-is:          Homework15A.hs
+    build-depends:    base ^>=4.16.4.0
+    hs-source-dirs:   app
+    default-language: Haskell2010
+
+
+library
+    import:           warnings
+    build-depends:    base ^>=4.16.4.0
+    hs-source-dirs:   src
+    exposed-modules:  Homework15B
+    default-language: Haskell2010

Homework15/LICENSE

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+Copyright (c) 2023 Robertino Martinez
+
+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.

Homework15/README.md

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+
+## Homework
+
+1. Read the [Data.Maybe](https://hackage.haskell.org/package/base-4.18.0.0/docs/Data-Maybe.html) and [Data.Either](https://hackage.haskell.org/package/base-4.18.0.0/docs/Data-Either.html) modules.
+2. Read the [Control.Exception](https://hackage.haskell.org/package/base-4.18.0.0/docs/Control-Exception.html) module skipping the "Asynchronous Exceptions" section.
+3. Solve Homework15A (inside `app` directory) and Homework15B (inside `src`).

Homework15/app/Homework15A.hs

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+module Main where
+
+import Control.Exception (catch, try)
+import Text.Read (readMaybe)
+
+--------------------------------------------------------------------------------
+--------------------------------------------------------------------------------
+-- IMPORTANT: Read the README.md file before completing the homework.
+--------------------------------------------------------------------------------
+--------------------------------------------------------------------------------
+
+-- This is a CLI application that allows the user to manage a TODO list.
+-- The user can add and remove items from the list and save and load the list
+-- from a file.
+-- It's a working prototype, but it has some bugs. Specifically, it doesn't
+-- handle errors very well. Your mission, should you choose to accept it, is to
+-- fix those bugs and make the application more robust. Hint: Try to interact
+-- with the application in unexpected ways and see what happens! You should be
+-- able to find and fix 3 bugs.
+
+printTodoItem :: (Int, String) -> IO ()
+printTodoItem (n, todo) = putStrLn (show n ++ ": " ++ todo)
+
+prompt :: [String] -> IO ()
+prompt todos = do
+  putStrLn ""
+  putStrLn "Current TODO list:"
+  foldr (\x k -> printTodoItem x >> k) (return ()) (zip [0 ..] todos)
+  command <- getLine
+  interpretCommand command todos
+
+delete :: Maybe Int -> [a] -> [a]
+delete Nothing as = as
+delete _ [] = []
+delete (Just 0) (_ : as) = as
+delete (Just n) (a : as) = a : delete (Just (n - 1)) as
+
+interpretCommand :: String -> [String] -> IO ()
+interpretCommand cmd todos = case cmd of
+  "q" -> return ()
+  ('+' : ' ' : todo) -> prompt (todo : todos)
+  ('-' : ' ' : num) -> prompt $ delete (readMaybe num) todos
+  ('s' : ' ' : fn) ->
+    writeFile fn (show todos) `catch` \e -> do
+      putStrLn $ "Could not write to file because: " ++ show (e :: IOError)
+      prompt todos
+  -- There's no reason as to why this and the previous case should handle
+  -- errors differently. I did it this way so you can see how to handle errors
+  -- in different ways.
+  ('l' : ' ' : fn) -> do
+    contents <- try $ readFile fn >>= return . read
+    case contents of
+      Left e -> do
+        putStrLn $ "Could not read from file because: " ++ show (e :: IOError)
+        prompt todos
+      Right todos' -> prompt todos'
+  _ -> do
+    putStrLn ("Invalid command: `" ++ cmd ++ "`")
+    prompt todos
+
+printCommands :: IO ()
+printCommands = do
+  putStrLn "Commands:"
+  putStrLn "+ <Item Name>   - Add a TODO entry"
+  putStrLn "- <Item Number> - Delete the numbered entry"
+  putStrLn "s <File Name>   - Save the current list of TODOs"
+  putStrLn "l <File Name>   - Load the saved list of TODOs"
+  putStrLn "q               - Quit without saving"
+
+main :: IO ()
+main = do
+  printCommands
+  prompt []

Homework15/hye.yml

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+cradle:
+  cabal: 
+

Homework15/src/Homework15B.hs

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+{-# LANGUAGE ScopedTypeVariables #-}
+
+module Homework15B where
+
+import Control.Exception (IOException, try)
+
+--------------------------------------------------------------------------------
+--------------------------------------------------------------------------------
+-- IMPORTANT: Read the README.md file before completing the homework.
+--------------------------------------------------------------------------------
+--------------------------------------------------------------------------------
+
+-- 1. Write a function that takes a list and returns the head if the list is not empty.
+-- If the list is empty, return Nothing.
+
+headMaybe :: [a] -> Maybe a
+headMaybe [] = Nothing
+headMaybe (x : _) = Just x
+
+-- 2. Write a function that takes a list of Maybe values and returns a list of all the Just values.
+-- If there are no Just values, return an empty list.
+
+catMaybes :: [Maybe a] -> [a]
+catMaybes [] = []
+catMaybes (Nothing : xs) = catMaybes xs
+catMaybes (Just x : xs) = x : catMaybes xs
+
+-- 3. Write a function that tries to read from a file and returns the contents of the file.
+-- If the file does not exist, return Nothing.
+
+readFileMaybe :: FilePath -> IO (Maybe String)
+readFileMaybe path = do
+  contents <- try $ readFile path
+  case contents of
+    Left (_ :: IOException) -> return Nothing
+    Right c -> return (Just c)
+
+-- 4. Write a function that checks all the requirements for a password using the
+-- Either type with a custom data type for errors.
+-- The requirements are:
+-- - The password must be at least 10 characters long.
+-- - The password must contain at least one digit.
+-- - The password must contain at least one uppercase letter.
+-- - The password must contain at least one lowercase letter.
+
+data PasswordError
+  = NotLongEnough
+  | NoDigit
+  | NoUppercase
+  | NoLowercase
+  deriving (Eq, Show)
+
+passwordLongEnough :: String -> Either PasswordError String
+passwordLongEnough password =
+  if length password >= 10
+    then Right password
+    else Left NotLongEnough
+
+passwordHasDigit :: String -> Either PasswordError String
+passwordHasDigit password =
+  if any (`elem` ['0' .. '9']) password
+    then Right password
+    else Left NoDigit
+
+passwordHasUppercase :: String -> Either PasswordError String
+passwordHasUppercase password =
+  if any (`elem` ['A' .. 'Z']) password
+    then Right password
+    else Left NoUppercase
+
+passwordHasLowercase :: String -> Either PasswordError String
+passwordHasLowercase password =
+  if any (`elem` ['a' .. 'z']) password
+    then Right password
+    else Left NoLowercase
+
+--------------------------------------------------------------------------------
+--------------------------------------------------------------------------------
+-- As you can see, the passwordRequirements function is very repetitive.
+-- This is one of the downsides of using nested optional values.
+-- We're going to solve this problem in the "Basic Abstractions" section of the coruse.
+--------------------------------------------------------------------------------
+--------------------------------------------------------------------------------
+
+passwordRequirements :: String -> Either PasswordError String
+passwordRequirements password =
+  case passwordLongEnough password of
+    Left err -> Left err
+    Right _ -> case passwordHasDigit password of
+      Left err -> Left err
+      Right _ -> case passwordHasUppercase password of
+        Left err -> Left err
+        Right _ -> case passwordHasLowercase password of
+          Left err -> Left err
+          Right _ -> Right password