102 slides: Section 5 - "do" notation

haskell_102/codelab/Codelab.hs

@@ -56,6 +56,7 @@ may the odds always be in your favor!
 {-# OPTIONS_GHC -fno-warn-unused-imports #-}
 {-# OPTIONS_GHC -fno-warn-unused-matches #-}
 {-# OPTIONS_GHC -fno-warn-unused-binds   #-}
+{-# OPTIONS_GHC -fno-warn-type-defaults  #-}
 
 module Codelab where
 
@@ -341,6 +342,96 @@ countScore c1 c2 = codelab
 
 
 
+{- #####################################################################
+   SECTION 5: "do" notation
+
+   While "do" notation is commonly used for "imperative" code, it can
+   be used with anything that is a monad (or even an applicative).  As
+   we have seen in the slides, lists are monads, meaning we can use
+   "do" notation to write our list generators in a very flexible way!
+
+   For example, a generator like this:
+
+     [x + 1 | x <- [1..10]]
+
+   can be written using "do" notation like this:
+
+     do x <- [1..10]
+        return $ x + 1
+
+   Code inside the "do" block must evaluate to a "wrapped" value,
+   which would be a list in our case.  If you want to return just one
+   value, you can use the "return" function to wrap a single value to
+   produce a value in the context.
+
+-}
+
+-- [5.1]
+-- Can you rewrite allColors to use the "do" notation?
+--
+-- Note that inside "do" blocks Haskell switches to indentation
+-- sensitive mode!  It is like Python but better, as the rules are
+-- more nuanced.  Make sure you follow the indendation proposed in the
+-- examples.
+
+allCodesDo :: Int -> [Code]
+allCodesDo s
+  | s <  0    = error "allCodes: size was lower than 0"
+  | s == 0    = codelab
+  | otherwise = do color <- codelab
+                   code <- codelab
+                   return codelab
+
+-- [5.2]
+-- Unlike generators, a "do" block can return any wrapped value.  For
+-- lists, it means it can return any list, not necessarily a list of
+-- length 1.  Let's build a "generator" using the "do" notation that
+-- would produce a list of duplicates of the specified length.  For
+-- example, for length 5 it should produce
+--
+--   [1, 1, 2, 2, 3, 3, 4, 4, 5, 5]
+
+duplicatesList :: Int -> [Int]
+duplicatesList len =
+  do i <- [1..codelab]
+     codelab
+
+-- [5.3]
+-- How about the case when the length of different "blocks" would be
+-- different?  Let's build a "generator" similar to the previous one,
+-- but that would duplicate only odd values.  For example, for length
+-- 5 it should produce
+--
+--   [1, 1, 2, 3, 3, 4, 5, 5]
+--
+-- I this, this is not something that can be easily expressed with
+-- ordinary generator, is it?
+--
+-- Do not forget about Hoogle, should you need a new function.
+--
+-- Hint: Should you use a conditional expression, make sure to put it
+-- in one line, as we did not discuss correct indentation rules for
+-- that case.
+
+oddlyDuplicateList :: Int -> [Int]
+oddlyDuplicateList len =
+  do codelab
+
+-- When you solve [5.3], think about the fact that when coding in "do"
+-- notation you have the full power of the language, but you are
+-- building something like a generator.  For imperative code that uses
+-- IO, your generator is producing sequences of actions that your
+-- application needs to execute to actually acheive the desired
+-- result.
+--
+-- This approach provides an amazing level of flexibility in terms of
+-- abstraction.  You can apply all the abstraction, code sharing and
+-- other techniques to the code that produces imperatives actions, as
+-- you normally apply to the code that is building lists.
+
+
+
+
 
 {- #####################################################################
    THE END

haskell_102/codelab/Solution.hs

@@ -341,6 +341,93 @@ countScore c1 c2 = Score black white
 
 
 
+{- #####################################################################
+   SECTION 5: "do" notation
+
+   While "do" notation is commonly used for "imperative" code, it can
+   be used with anything that is a monad (or even applicative).  As we
+   have seen in the lecture lists are monads, meaning we can use "do"
+   notation to write our list generators in a very flexible way!
+
+   For example, a generator like this:
+
+     [x + 1 | x <- [1..10]]
+
+   can be written using "do" notation like this:
+
+     do x <- [1..10]
+        x + 1
+
+-}
+
+-- [5.1]
+-- Can you rewrite allColors to use the "do" notation?
+--
+-- Note that inside "do" blocks Haskell switches to indentation
+-- sensitive mode!  It is like Python but better, as the rules are
+-- more nuanced.  Make sure you follow the indendation proposed in the
+-- examples.
+
+allCodesDo :: Int -> [Code]
+allCodesDo s
+  | s <  0    = error "allCodes: size was lower than 0"
+  | s == 0    = [[]]
+  | otherwise = do color <- allColors
+                   code <- allCodesDo (s - 1)
+                   return $ color:code
+
+-- [5.2]
+-- Unlike generators, a "do" block can return any wrapped value.  For
+-- lists, it means it can return any list, not necessarily a list of
+-- length 1.  Let's build a "generator" using the "do" notation that
+-- would produce a list of duplicates of the specified length.  For
+-- example, for length 5 it should produce
+--
+--   [1, 1, 2, 2, 3, 3, 4, 4, 5, 5]
+
+duplicatesList :: Int -> [Int]
+duplicatesList len =
+  do i <- [1..len]
+     [i, i]
+
+
+-- [5.3]
+-- How about the case when the length of different "blocks" would be
+-- different?  Let's build a "generator" similar to the previous one,
+-- but that would duplicate only odd values.  For example, for length
+-- 5 it should produce
+--
+--   [1, 2, 2, 3, 4, 4, 5]
+--
+-- I this, this is not something that can be easily expressed with
+-- ordinary generator, is it?
+--
+-- Hint: Should you use a conditional expression, make sure to put it
+-- in one line, as we did not discuss correct indentation rules for
+-- that case.
+
+oddlyDuplicateList :: Int -> [Int]
+oddlyDuplicateList len =
+  do i <- [1..len]
+     if odd i
+       then [i, i]
+       else [i]
+
+-- When you solve [5.3], think about the fact that when coding in "do"
+-- notation you have the full power of the language, but you are
+-- building something like a generator.  For imperative code that uses
+-- IO, your generator is producing sequences of actions that your
+-- application needs to execute to actually acheive the desired
+-- result.
+--
+-- This approach provides an amazing level of flexibility in terms of
+-- abstraction.  You can apply all the abstraction, code sharing and
+-- other techniques to the code that produces imperatives actions, as
+-- you normally apply to the code that is building lists.
+
+
+
+
 
 {- #####################################################################
    THE END

haskell_102/codelab/Tests.hs

@@ -62,7 +62,7 @@ newtype Section = Section Int
 
 instance Bounded Section where
   minBound = Section 1
-  maxBound = Section 4
+  maxBound = Section 5
 
 instance Ord Section where
   compare (Section s1) (Section s2) = compare s1 s2
@@ -241,6 +241,29 @@ t45_3 = test "[4.5] countScore  [B,B,C,G] [B,B,C,G]" (Score 4 0) $ countScore [B
 
 
 
+{- #####################################################################
+   SECTION 5: "do" notation
+-}
+
+t51_1 = test "[5.1] # codes of size 0: 1"       1  $ length $ allCodesDo 0
+t51_2 = test "[5.1] # codes of size 1: 6"       6  $ length $ allCodesDo 1
+t51_3 = test "[5.1] # codes of size 4: 1296" 1296  $ length $ allCodesDo 4
+t51_4 = test "[5.1] all codes 0 have size 0"  [0]  $ nub $ length <$> allCodesDo 0
+t51_5 = test "[5.1] all codes 1 have size 1"  [1]  $ nub $ length <$> allCodesDo 1
+t51_6 = test "[5.1] all codes 4 have size 4"  [4]  $ nub $ length <$> allCodesDo 4
+t51_7 = test "[5.1] no duplicated codes"      True $ on (==) length (allCodesDo 4) (nub $ allCodesDo 4)
+
+t52_1 = test "[5.2] len: 0: []"                []  $ duplicatesList 0
+t52_2 = test "[5.2] len: 3: [1, 1, 2, 2, 3, 3]" [1, 1, 2, 2, 3, 3] $ duplicatesList 3
+
+t53_1 = test "[5.3] len: 0: []"                       []                       $ oddlyDuplicateList 0
+t53_2 = test "[5.3] len: 3: [1, 1, 2, 3, 3]"          [1, 1, 2, 3, 3]          $ oddlyDuplicateList 3
+t53_3 = test "[5.3] len: 5: [1, 1, 2, 3, 3, 4, 5, 5]" [1, 1, 2, 3, 3, 4, 5, 5] $ oddlyDuplicateList 5
+
+
+
+
+
 {- #####################################################################
    Main
 -}
@@ -275,6 +298,12 @@ tests sections =
                , t44_1, t44_2, t44_3, t44_4, t44_5
                , t45_1, t45_2, t45_3
                ]
+             Section 5 ->
+               [ display "#### Section 5"
+               , t51_1, t51_2, t51_3, t51_4, t51_5, t51_6, t51_7
+               , t52_1, t52_2
+               , t53_1, t53_2, t53_3
+               ]
              Section unexpected ->
                [ display $ "Unexpected section requested: " ++ show unexpected
                ]

haskell_102/slides/haskell_102.tex

@@ -1297,12 +1297,12 @@ \section{Monadic syntax}
 
 %% Codelab
 
-\section{Codelab - Sections 3 and 4}
+\section{Codelab - Sections 3, 4, and 5}
 
 \begin{frame}
-  \frametitle{Codelab - Sections 3 and 4}
+  \frametitle{Codelab - Sections 3, 4, and 5}
   \begin{itemize}
-    \item Codelab - Sections 3 and 4
+    \item Codelab - Sections 3, 4, and 5
   \end{itemize}
 \end{frame}