Do the FP Shake¶

Thinking of recursive data structures

What is functional programming (fp)¶

Method of writing programs
Emphasizes immutability of data
Enforces the idea of function as data

What fp is not¶

fp is not a language

What fp is¶

Immutability - values are not assigned and hence not changed
Functional purity - pure function should not have side effects, for a given set of inputs the output is always the same
Referential transparency - a function can be replaced with its contents wherever it is called

data Maggi = Cheese | Onions | Capsicum | Garlic | Noodles | Masala Maggi Maggi deriving (Eq, Show)

empty :: Maggi
empty = Noodles

append :: Maggi -> Maggi -> Maggi
append a b = Masala a b

empty `append` Cheese `append` Onions

Masala (Masala Noodles Cheese) Onions

append :: Maggi -> Maggi -> Maggi
append = Masala

append Garlic (append Onions (append Cheese empty))

Masala Garlic (Masala Onions (Masala Cheese Noodles))

append Garlic $ append Onions $ append Cheese empty

Layer Olives (Layer Onions (Layer Cheese Pizzabread))

-- Creating a list
data List a = Cons a (List a) | Blank deriving (Eq, Show)

empty :: List a
empty = Blank

append :: a -> List a -> List a
append = Cons

1 `append` (2 `append` Blank)

Cons 1 (Cons 2 Blank)

-- Creating a tree
data Tree a = Branch (Tree a) (Tree a) | Leaf a deriving (Eq, Show)

empty :: a -> Tree a
empty = Leaf

append :: (Ord a) => Tree a -> Tree a -> Tree a
append (Leaf newval) (Leaf val) = Branch (Leaf newval) (Leaf val)
append (Leaf newval) (Branch left right) = Branch (Leaf newval) (Branch left right)
append (Branch a b) (Leaf c) = Branch (Branch a b) (Leaf c)
append a b = Branch a b

append (append (Leaf 1) (Leaf 2)) (Branch (Leaf 2) (Leaf 3))

Branch (Branch (Leaf 1) (Leaf 2)) (Branch (Leaf 2) (Leaf 3))

-- Creating a binary search tree
data Tree a = Branch a (Tree a) (Tree a) | Leaf deriving (Eq, Show)

emptyTree :: Tree a
emptyTree = Leaf

sapling :: a -> Tree a
sapling val = Branch val Leaf Leaf

growTree :: (Ord a) => a -> Tree a -> Tree a
growTree newval (Branch val left right)
    | newval >= val = Branch val left (growTree newval right)
    | newval  < val = Branch val (growTree newval left) right
growTree newval Leaf = sapling newval
    
growTree 10 $ growTree 3 $ sapling 4

Branch 4 (Branch 3 Leaf Leaf) (Branch 10 Leaf Leaf)

Function is data¶

The function decides what goes in and what comes out
There are no variables involved data is only transformed from function to function

Understanding how to understand by understanding the dollar¶

:info $

function defintion
module where it was defined
priority

# Feed me a burger
def giveme_food(count):  # a pure function
    print(f"{count} burgers")
    
giveme_food(2)

2 burgers

# Feed me
# the function can fail due to changes made outside the context of the function, for e.g. deleting the file
def giveme_food(count):  # impure function
    with open("food.txt") as f:
        print(f"{count} {f.readline()}")

giveme_food(4)

4 pizza

# When I said food I meant burger
def pure_func(impure_func):  # emulating a pure function
    cache = {}
    
    def wrapped(arg):
        if arg in cache:
            return cache[arg]
        else:
            cache[arg] = impure_func(arg)
            return cache[arg]
        
    return wrapped

giveme_food_i_mean_burger = pure_func(giveme_food)  # pure func
giveme_food_i_mean_burger(5)

5 "Burger"

-- Immutability
x = 4  -- Ha I made a variable
x = 5  -- Ha I redefined it!!
-- Did you really? It's called shadowing.

x = x + 1  -- check this out

<interactive>:2:1: error:
    Multiple declarations of ‘x’
    Declared at: <interactive>:1:1
                 <interactive>:2:1
<interactive>:3:1: error:
    Multiple declarations of ‘x’
    Declared at: <interactive>:1:1
                 <interactive>:3:1

weirdAddition x y = if x `mod` 2 == 0
                    then x
                    else x + y

weirdAddition 2 undefined
-- Laziness is the single most important quality of a programmer

Do the FP Shake¶

What is functional programming (fp)¶

What fp is not¶

What fp is¶

Function is data¶

Understanding how to understand by understanding the dollar¶

References¶