Functional Programming

How I learned to stop worrying and love the monad

WHAT

Composition & Reusable functions

 

passing around class-objects with hidden state in for loops

Not

WHAT

Pure Functions

someObject.property1 = "hej"

const months = ['Jan', 'March'];
months.splice(1, 0, 'Feb');
console.log(months);

Array(3) [ "Jan", "Feb", "March"]
  • Side effects

WHAT

Pure Functions

  • Side effects
  • Deterministic
function add(first, second) {
  first + second
}

WHAT

Pure Functions

  • Side effects
  • Deterministic

WHAT

Immutability

Thou shalt not mutate

Performance losses? Don't worry about it

WHAT

Higher order functions

WHAT

Higher order functions

  • A function that takes another function as argument
  • .map(), reduce(), filter()

WHY (and why not)

WHY

Parallelization

WHY

Parallelization

WHY

Composition

WHY

Composition

Frame
|> Put on wheels

|> Put on windows

|> Put on doors

|> Put on color

WHY

Testability

WHY

Why not

  •  
  •  
  • ⠀
  •  

WHY

Why not

*Cricket Noise*

WHY

  • Parallelization
  • Tests
  • Composability

HOW

HOW

React

import { useState } from 'react';

export default function Counter() {
  const [count, setCount] = useState(0);

  function handleClick() {
    setCount(prevCount => prevCount + 1);
  }

  return (
    <button onClick={handleClick}>
      You pressed me {count} times
    </button>
  );
}

HOW

React

What is useEffect?

HOW

Kubernetes

You set constraints

Kubernetes helps you fullfil them

HOW

More examples

HOW

 function mailPreferences(arr) {
   let result = [];

   for (let index = 0; index < arr.length; index++) {
     const person = arr[index];
     if (person.age >= 18) {
       person.drink = "beer";
       result.push(person);
     }
   }
   if (result) {
     sendMail(result);
   } else {
     console.log("nothing to send");
   }
 }

HOW

function mailPreferencesFunc(arr) {
  res = arr
    .filter(({ age }) => age > 18)
    .map((e) => {
      e.drink = "beer";
      return e;
    });
  if (res) {
    sendMail(res);
  } else {
    console.log("nothing to send");
  }
}

HOW

function mailPreferencesFunc(arr) {
  res = arr
    .filter(({ age }) => age > 18)
    .map((e) =>
      Object.assign({
        drink: "beer",
        e,
      })
    );

  if (res) {
    sendMail(res);
  } else {
    console.log("nothing to send");
  }
}
 function mailPreferences(arr) {
   let result = [];

   for (let index = 0; index < arr.length; index++) {
     const person = arr[index];
     if (person.age >= 18) {
       person.drink = "beer";
       result.push(person);
     }
   }
   if (result) {
     sendMail(result);
   } else {
     console.log("nothing to send");
   }
 }

HOW

function mailPreferencesFunc(arr) {
  res = arr
    .filter(({ age }) => age > 18)
    .map((e) =>
      if (e.age >= 20) {
        return Object.assign({
          drink: "wine",
          e,
        });
      } else {
        return Object.assign({
          drink: "beer",
          e,
        });
      }
    );

  if (res) {
    sendMail(res);
  } else {
    console.log("nothing to send");
  }
}
function mailPreferences(arr) {
  let result = [];

  for (let index = 0; index < arr.length; index++) {
    const person = arr[index];
    if (person.age >= 20) {
      person.drink = "wine";
      result.push(person);
    } else if (person.age >= 18) {
      person.drink = "beer";
      result.push(person);
    }
  }
  if (result) {
    sendMail(result);
  } else {
    console.log("nothing to send");
  }
}

HOW

function mailPreferences(arr) {
  let result = [];
  const drinkers = {
    kombucha: 0,
    wine: 0,
    beer: 0,
  };

  for (let index = 0; index < arr.length; index++) {
    const person = arr[index];
    if (person.age >= 40) {
      const drink = "kombucha";
      person.drink = drink;
      result.push(person);
      drinkers[drink] += 1;
    } else if (person.age >= 20) {
      const drink = "wine";
      // Same thing as above
    } else if (person.age >= 18) {
      const drink = "beer";
      // Same thing
    }
  }
  for (const drink in drinkers) {
    const nrOfDrinkers = drinkers[drink];
    if (nrOfDrinkers > 0) {
      console.log(
        `${drink}Drinkers ${toPercentage(nrOfDrinkers, arr.length)} %`
      );
    }
  }
  console.log(
    `excluded ${toPercentage(arr.length - result.length, arr.length)} %`
  );
  sendMail(result);
}

HOW

function mailPreferencesFunc(arr) {
  const drinkPreferences = getDrinkPreferences(arr)
  const printDrinkerStats = (list, totalNr) =>
    Object.entries(
      list
        .map(({ drink }) => drink)
        .reduce((acc, drink) => {
          if (!acc[drink]) {
            acc[drink] = 1;
          } else {
            acc[drink] = acc[drink] + 1;
          }
          return acc;
        }, {})
    )
      .map(([drink, drinkers]) => [drink, toPercentage(drinkers, totalNr)])
      .map(([drink, percent]) => `${drink}Drinkers ${percent} %`)
      .concat([`excluded ${toPercentage(totalNr - list.length, totalNr)} %`])
      .map(printEntry);
  
  printDrinkerStats(drinkPreferences, arr.length);
  sendMail(drinkPreferences);
}
  const getDrinkPreferences = (arr) =>
    arr
    .filter(({ age }) => age > 18)
    .map((e) => {
      if (e.age >= 40) {
        return Object.assign({
          drink: "kombucha",
          e,
        });
      } else if (e.age >= 20) {
        return Object.assign({
          drink: "wine",
          e,
        });
      } else if (e.age >= 18) {
        return Object.assign({
          drink: "beer",
          e,
        });
      }
    });

Summary

"All told, a monad in X is just a monoid in the category of endofunctors of X, with product × replaced by composition of endofunctors and unit set by the identity endofunctor."

Summary

Its up to you!

Functional Programming

By Mikael Gråborg

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Functional Programming

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