Short helper presentation on list and folds

All material taken from Haskell book.

list-folds/Makefile

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+all:
+	make slide slideshow
+
+slide:
+	pandoc -t beamer --include-in-header=./style.tex list-folds.md -f markdown-implicit_figures -V colorlinks=true -V linkcolor=blue -V urlcolor=red -o list-folds.pdf
+
+slideshow:
+	pandoc -t beamer --include-in-header=./style.tex list-folds.md -f markdown-implicit_figures -V colorlinks=true -V linkcolor=blue -V urlcolor=red -i -o list-folds-slideshow.pdf
+
+view:
+	zathura --mode=presentation list-folds.pdf &

list-folds/list-folds.md

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+---
+title:
+- List and Folding Lists
+author:
+- Sanchayan Maity
+theme:
+- default
+classoption:
+- aspectratio=169
+---
+
+# Agenda
+
+- Lists
+- Folds
+- **Disclaimer:** No original material in this presentation.
+
+# Lists recap
+
+- Data type
+
+```haskell
+data []  a   =   []  |   a : [a]
+--  [1] [2] [3] [4] [5]   [6]
+```
+
+1. The datatype with the type constructor [],
+2. which takes a single type constructor argument of type a,
+3. at the term level can be constructed via
+4. the nullary list constructor [],
+5. or it can be constructed by
+6. infix data constructor (or cons) :, which is a product of a value of type a from the type constructor and a value of type [a], that is, “more list.”
+
+
+# Pattern matching
+
+```haskell
+ourTail :: [a] -> [a]
+ourTail [] = []
+ourTail (_ : xs) = xs
+```
+
+# Syntactic sugar
+
+```haskell
+ghci> [1, 2, 3] ++ [4]
+[1, 2, 3, 4]
+ghci> (1 : 2 : 3 : []) ++ 4 : []
+[1,2,3,4]
+```
+
+# Construction lists
+
+```haskell
+ghci> [1..10]
+[1,2,3,4,5,6,7,8,9,10]
+ghci> enumFromTo 1 10
+[1,2,3,4,5,6,7,8,9,10]
+ghci> [1,2..10]
+[1,2,3,4,5,6,7,8,9,10]
+ghci> enumFromThenTo 1 2 10
+[1,2,3,4,5,6,7,8,9,10]
+ghci> [1,3..10]
+[1,3,5,7,9]
+ghci> enumFromThenTo 1 3 10
+[1,3,5,7,9]
+ghci> ['t'..'z']
+"tuvwxyz"
+ghci> enumFromTo 't' 'z'
+"tuvwxyz"
+```
+
+# Extracting from lists
+
+```haskell
+take :: Int -> [a] -> [a]
+drop :: Int -> [a] -> [a]
+splitAt :: Int -> [a] -> ([a], [a])
+
+takeWhile :: (a -> Bool) -> [a] -> [a]
+dropWhile :: (a -> Bool) -> [a] -> [a]
+```
+
+# List comprehensions
+
+```haskell
+ghci> [x^y | x <- [1..5], y <- [2, 3]]
+[1,1,4,8,9,27,16,64,25,125]
+```
+
+# Evaluation
+
+```
+1 : (2 : [])
+  :
+  / \
+  1
+    :
+    / \
+    2 []
+```
+
+See `sprint` command.
+
+```haskell
+ghci> blah = enumFromTo 'a' 'z'
+ghci> :sprint blah
+```
+
+**Spines are evaluated independently of values**.
+
+# Miscellaneous
+
+- `map`
+- `filter`
+- `zip`
+
+# Patterns
+
+```haskell
+sum :: [Integer] -> Integer
+sum [] = 0
+sum (x:xs) = x + sum xs
+
+length :: [a] -> Integer
+length [] = 0
+length (_:xs) = 1 + length xs
+
+product :: [Integer] -> Integer
+product [] = 1
+product (x:xs) = x * product xs
+
+concat :: [[a]] -> [a]
+concat [] = []
+concat (x:xs) = x ++ concat xs
+```
+
+# Folds types
+
+```haskell
+foldr :: Foldable t => (a -> b -> b) -> b -> t a -> b
+foldr :: (a -> b -> b) -> b -> [] a -> b
+
+foldl :: (b -> a -> b) -> b -> [a] -> b
+foldl f acc [] = acc
+foldl f acc (x:xs) = foldl f (f acc x) xs
+```
+
+# Right fold transformation[^1]
+
+![*Right Fold*](./Right-fold-transformation.png){width=60%}
+
+[^1]: [Haskell Wiki - Fold](https://wiki.haskell.org/Fold)
+
+
+# Left fold transformation[^2]
+
+![*Left Fold*](./Left-fold-transformation.png){width=60%}
+
+[^2]: [Haskell Wiki - Fold](https://wiki.haskell.org/Fold)
+
+# Folds in-depth
+
+- An aside from Alexis King.
+
+https://github.com/hasura/graphql-engine/pull/2933#discussion_r328821960
+
+# Questions
+
+- Reach out on
+  * Email: sanchayan@sanchayanmaity.net
+  * Mastodon: https://sanchayanmaity.com/@sanchayan
+  * Blog: https://sanchayanmaity.net
+  * Telegram:
+    - `t.me/fpncr`
+    - `t.me/SanchayanMaity`

list-folds/style.tex

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+\logo{\includegraphics[height=0.5cm]{../images/Haskell.jpg}\vspace{220pt}\hspace{8pt}}