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# Functional Programming

Last updated Feb 21, 2022 Edit Source

## # Syntax Quirks

• $:: (a -> b) -> a -> b is function application (adds implicit parentheses and makes it right associative instead of left associative) 1. Normally, sort "abc" ++ "def" would be interpreted as (sort "abc") ++ "def" 2. If we use the $ operator, we can do sort $"abc" ++ "def" which is interpreted as sort ("abc" ++ "def") as intended. • . is function composition. Read the dot as the little dot in$f \circ g$• <> is a synonym for mappend :: Monoid m => m -> m -> m or the monoidal append • <$> is a synonym for fmap :: (a -> b) -> f a -> f b
• Intuitively like applying a function to a container
• <*> is like <$> but for wrapped functions (<*>) :: Applicative f => f (a -> b) -> f a -> f b • Intuitively like applying a function in a container to another container • Remember that (<$) and ($>) point towards the value that will be kept • void :: Functor f => f a -> f () is implemented as void x = () <$ x. Read as: whatever you give me, I will return the unit value

## # Terminology

### # Category Theory

In essence, a simple collection which can be thought of as a graph. Three components

1. A collection of objects (nodes)
2. A collection of morphisms (edges).
• If $f$ is a morphism with source C and target B, we write $f: C \rightarrow B$
3. A notion of composition of morphisms.
• If we have $g: A \rightarrow B$ and $f: B \rightarrow C$, they can be composed resulting in a morphism $f \circ g: A \rightarrow C$
• Composition of morphisms needs to be associative. Typically applied right to left

Category theory to Haskell

• Objects are types
• Morphisms are functions
• Things that take a type and return another type are type constructors
• Things that take a function and return another function are higher-order functions
• Typeclasses capture the fact that things are often defined for a ‘set’ of objects at once

### # Functor

A ‘container’ of some sort, along with the ability to apply a function uniformly to every element in it

Essentially a transformation between categories. Given categories $C$ and $D$ and a functor $F: C \rightarrow D$

1. $F$ maps any object $A \in C$ to $F(A) \in D$ (the type constructor)
2. $F$ maps morphisms $f: A \rightarrow B \in C$ to $F(f): F(A) \rightarrow F(B) \in D$ (fmap). Importantly, this means that all functors must be generic over at least one parameter (e.g. Maybe and not Integer)
• applying fmap is sometimes called ’lifting’ as it lifts a function from the normal context into the ‘f’ world
 1 2 3 4 5 6 7 8  class Functor f where -- fmap maps morphisms fmap :: (a -> b) -> f a -> f b -- applies a 'constant' function to replace the values in a container (<$) :: a -> f b -> f a -- default implementation (<$) = fmap . const 

fmap takes a function which maps a value from a to b and applies it to a Functor f. Think of f as the container, (a -> b) as the function that operates on the ‘inner’ values.

Monads are functors from a category $A$ to that same category. A container for values that can be mapped over.

Think of it like a context-specific environment. You need a function to transform things outside of it to things in it. You also need a function to manipulate stuff inside of that environment.

A monad is a functor $M: C \rightarrow C$ along with two morphisms $\forall X \in C$

1. $\textrm{unit}_X : X \rightarrow M(X)$ (return)
2. $\textrm{join}_X: M(M(X)) \rightarrow M(X)$ (can be recovered from bind)
  1 2 3 4 5 6 7 8 9 10 11 12 13 14  class Monad m where -- join operation (optional, only one of bind or join need to be defined) join :: m (m a) -> m a -- bind operation -- takes an f :: (a -> m b) and applies it to -- the inner value a of m (>>=) :: m a -> (a -> m b) -> m b -- replaces m a with m b (>>) :: m a -> m b -> m b -- constructs the simplest monad m using a return :: a -> m a 

 1 2 3  return a >>= k = k a m >>= return = m m >>= (\x -> k x >>= h) = (m >>= k) >>= h 

## # Left and Right Associativity

Associativity of an operator determines how operators are grouped in the absence of parentheses.

For the following examples, we consider a fictional operator ~

1. Associative: operations can be grouped arbitrarily (e.g. addition, order doesn’t matter)
2. Left-associative: operations are grouped left to right
1. a ~ b ~ c is interpreted as (a ~ b) ~ c
2. Examples include
1. Function application operator
3. Right-associative: operations are grouped right to left
1. a ~ b ~ c is interpreted as a ~ (b ~ c)
2. Examples include
1. Variable assignment (=)
2. Exponentiation (^)
4. Non-associative: operations cannot be chained

## # Parser Combinators

Parser combinators are a technique for implementing parsers by defining them in terms of other parsers

Notes on Chumsky

Where a and b are both parsers.

Parser Methods

1. just(a) accepts a single string a
2. a.or(b) parse a, if a fails, try parsing b
3. a.choice(b,c,d...) try parsing b, c, d, return first one that succeeds
4. a.or_not() optionally parse a
5. a.ignore_then(b) ignore pattern a then parse b
6. a.then_ignore(b) parse a then ignore b
7. a.then(b) parse both a and b and return a tuple of (a,b)
8. a.padded() ignore whitespace around a
9. a.repeated().at_least(n) parse a at least n times
10. a.filter(fn) only accept a if fn(a) evaluates to true

Result Methods

1. a.collect() turn results of a into an iterator
2. a.map(b) map results of a into type b
3. a.chain(b) concatenate results of parsers a and b into collection
4. a.copy(b) duplicate parser definition
5. a.flatten() flatten nested collection
6. a.to(b) marks result of a as type b
7. a.labelled(b) label result of a with b
8. a.end() indicate end of parser