# Hardcore Functional Programming in JavaScript, v2

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Table of Contents

## Introduction

### Introduction

Brian introduces the course, and explains what the intended audience is.### Mathematical, Pure Functions

Brian explores three small tests to determine if one is writing a function: the object needs to be total, have an input for every output, be deterministic, and have no observable side effects.### Pure Functions Checklist

Brian reviews the checklist of what makes a mathematical, pure function, and asks the audience to determine if the examples presented are pure functions or not.### Pure Functions Advantages

Brian explains how the mathematics used in functions can be useful when programming. Pure functions are reliable, portable, reusable, testable, and composable.

## Currying

### Properties, Arguments & Currying

Brian explores the properties of a few calculation functions, and demonstrates how to write a curried function.### Currying Example & Argument Order

Brian gives the example of the filter function, curries it, explains the argument order, and imports the currying function from the Ramda library to be able to curry with more than two arguments.### Ramda Generalized Currying

Brian explains how to use the Ramba library while currying a function, and demonstrates how to write point free functions.### Partial Application vs Currying

Brian explains that partial application and curried functions are dual functions. Partial application only uses one of its arguments, a curried function uses one argument at a time. The two functions have the same effect, but are written differently.### Currying Exercises

The students are instructed to code the solutions to various currying exercises.### Currying Solutions

Brian live codes the solution to the currying exercises.

## Composition

### Composition

Brian defines composition and gives examples of composed functions. A function is composed when it contains one function nested in the other, and returns a new function.### Creating Programs with Curry & Compose

Brian demonstrates how to build a simple program using currying and compose, and argues that this way of writing code leads to consistent state mutations, but not efficient programming because of the absence of a clean data pipeline.### Composition is Dot Chaining

Brian explains that composition and dot chaining are the same, and says that dot chaining is more ergonomic in JavaScript.### Logging in Composition

Brian demonstrates how to use logging in function composition to debug code.### Compose Practice

Brian demonstrates how to use compose in various examples, and rewrites functions into pipelines, making code easier to read.### Refactoring with Compose

Brian demonstrates how refactoring with function composition can make code easier to understand.### Refactoring to Point Free Style

Brian demonstrates how to refactor functions using the point free style, explains that it allows parallelization and a better data processing workflow thanks to the high level declarative syntax.

## Functors

### Creating the Identity Functor

Brian explains that a functor is a container that holds an object that is mapped over, demonstrates how to dot chain a function into a linear control flow, and explains that a functor is an object that has a map method. A functor is useful in functional programming because it abstracts function application.### Refactoring to Dot Chaining

Brian demonstrates how to use dot chaining with a functor and refactors the code written in the previous lesson.### Functor Practices

Brian demonstrates how to use composition and sub-composition to nest functions. Nested functions are useful to have a clear idea of the data flow.### Adding Chain for Nested Functors

Brian demonstrates how to write a function that allows for chaining methods and argues that one could use promise.then instead, but in an effort to stay deterministic and mathematical, the use of chaining is better.

## Either Monad

### Either Monad

Brian explains that the type Either is a functor and a monad, it has both a map, a chain method, and a fold method. The Either type respects function purity and is effectively an if else statement, but inverted.### fromNullable

Brian explains that fromNullable is useful because it is a definitive null check for every function, and avoids repetition.### Refactoring Using the Either Monad

Brian demonstrates through a server example how to use the Either monad to refactor code.### Flattening Either Monads with Chain

Brian demonstrates how to flatten the Either monad using both the chain and map methods.### Adding Syntax vs Generalized Solutions

Brian recommends using the Either type instead of exploring specific syntax that only covers one unique way of using the type.### Either Practices

Brian goes over examples of using the Either type using the dot syntax, and explains that, when working with Either, it is not possible to flatten an array of another type.### Debugging with Logging

Brian demonstrates how to use logs to debug the code, and answers questions about the Writer monad, a monad that, along with values, returns an error message.

## Task

### Task Monad

Brian explains that the Task monad is the functional equivalent of promise. Similarly to promise, Task takes resolve and reject functions, but in reversed order. A Task monad only starts running once it reaches the fork method, and this way avoids race conditions.### Refactoring Node IO with Task

Brian explains that Node IO, although commonly used, does not bring in any asynchronicity, and demonstrates how to use Task instead which gives a clean control flow.### Task Practices

Brian goes over different Task monad examples and familiarizes the audience with the Task syntax, and characteristics.### Transforms & Monad Patterns

Brian explores various transformations, such as type transformations, free monads, and monoids with both the either and task monad.### Inspecting the Weather API

Brian demonstrates how to fetch weather data from the weather API, and explains the next data parsing steps.### Separate Pure & Impure Code

Brian builds a task monad to fetch data from an API and refactors the code using the Task monad.### Creating a Weather Module

Brian demonstrates how to use the Task monad when creating a weather module, and how to use the monad in different places within the same application.### Parsing Weather API Data into HTML

Brian uses HTML to show some of the results gathered through the weather API on the screen, live codes a weather data type, and refactors the code into new types, making the application cleaner, and the functions within it more accessible.