M05-burritos-everywhere

title	author	patat
M05 - Burritos Everywhere	Walker Leite	eval purescript purescript' command purs-eval | node --experimental-network-imports --input-type module command purs-eval

Introduction

Getting Started

In this module we will see more fundamental type classes of PureScript and Haskell.

To run this presentation type (you will need nix):

../../slide README.md

Community Support

• LovelaceAcademy Discord
• StackExchange (:bulb: use the tag lovelace-academy)
• Plutonomicon Discord

What you should know

1. PureScript (modules 2-4)

Apply and Applicatives

Recap Functor

Recap Functor Type Class

Functor is a type class which enables map operations over some type

class Functor :: (Type -> Type) -> Constraint
class Functor f where
  map :: forall a b. (a -> b) -> f a -> f b

infixl 4 map <$>

So we can:

(+) 3 <$> Just 2
-- Just 5

Functor problem

What happens when you map a function that receives more than one argument?

-- + :: Int -> Int -> Int
x = (+) <$> Just 3
-- ok: x = Just ((+) 3) :: Maybe (Int -> Int)
y = x ?? Just 2
-- ?? can't be <$> :: (a -> b) -> f a -> f b
-- what ?? would be to have: y = Just 5 :: Maybe Int

Applicative Just

Apply Type Class

class Apply :: (Type -> Type) -> Constraint
class (Functor f) <= Apply f where
  apply :: forall a b. f (a -> b) -> f a -> f b

infixl 4 apply <*>

So we can:

x = (+) <$> Just 2
-- x = Just ((+) 3) :: Maybe (Int -> Int)

y =  x <*> Just 3
-- y = Just 5 :: Maybe Int

Or even:

(+) <$> Just 2 <*> Just 3
-- Just 5 :: Maybe Int

You can have any number of arguments:

foo = functionTakingNArguments <$> computationProducingArg1
                               <*> computationProducingArg2
                               <*> ...
                               <*> computationProducingArgN

💡 nothing says that computations must happen sequentially, actually Apply allows parallel computation and we'll see it in the future

Apply problem

What if you don't know the box type, eg:

x :: forall f. Apply f => f Int
x = (+) <$> ?? 2 <*> ?? 3
-- ?? would be :: Int -> f Int

Applicative Type Class

class Applicative :: (Type -> Type) -> Constraint
class (Apply f) <= Applicative f where
  pure :: forall a. a -> f a

So we can:

x :: forall f. Applicative f => f Int
x = (+) <$> pure 2 <*> pure 3

y :: Maybe Int
y = x
-- like magic: y = Just 5

Applicative problem

data Maybe a = Nothing | Just a

half x = case even x of
    true -> Just (x `div` 2)
    false -> Nothing

Half

Half Ouch

Plunger

Bind Type Class

data Maybe a = Nothing | Just a

half :: Int -> Maybe Int
half x = case even x of
    true -> Just (x `div` 2)
    false -> Nothing

> Just 3 >>= half
Nothing
> Just 4 >>= half
Just 2
> Nothing >>= half
Nothing
-- (>>=) :: f a -> (a -> f b) -> f b

class Bind :: (Type -> Type) -> Constraint
class (Apply m) <= Bind m where
    bind :: forall a b. m a -> (a -> m b) -> m b

infixl 1 bind as >>=

instance Bind Maybe where
    bind (Just x) fn = fn x
    bind Nothing  _ = Nothing

Bind Just

Bind Nothing

Bind Chain

> Just 20 >>= half >>= half >>= half
Nothing

Bind Chain 2

Monad

Put simply Monad is Bind (>>=) + Applicative (pure):

class (Applicative m, Bind m) <= Monad m

So, as in Applicative, we can:

xs :: forall m. Monad m => m Int
xs = pure 20 >>= half

ys :: Maybe Int
ys = xs

ys :: Tuple String Int
ys = xs
-- Tuple bind works on the snd value

Do notation

The do notation is a syntax suggar for >>=:

module Main where

import Prelude (($), (>>=), (==), div, bind, show)
import Data.Maybe (Maybe(Nothing, Just))
import Data.Int (even)
import Effect.Console (log)

half x = case even x of
  true -> Just (x `div` 2)
  false -> Nothing

x = do
  v <- Just 20
  v' <- half v
  half v'
  
-- translates to
y = Just 20 >>= \v -> half v >>= \v' -> half v'

-- which is the same of
z = Just 20 >>= half >>= half

main = log $ show [x, y, z]

Effect (Haskell IO)

The Effect is a type without constructors that represent effects in the runtime.

data Effect a

The underlying ais the return type, it can be Unit to represent a non-returning Effect

module Main where

import Prelude (Unit, unit, pure)
import Effect (Effect)
import Effect.Console (log)

main :: Effect Unit
main = log "hello"
-- the side-effect in this case is logging in the console

module Main where

import Prelude (Unit, ($), (>>=), (<>), show)
import Effect (Effect)
import Effect.Console (log)
import Effect.Now (nowDate)

main :: Effect Unit
main = nowDate >>= \d -> log $ "the current date is " <> show d

Combining Effects

module Main where

import Prelude (Unit, ($), (>>=), (<>), (>>>), (<$>), (<*>), bind, pure, discard, show)
import Effect (Effect)
import Effect.Console (log)
import Effect.Now (nowDate, nowTime)
import Effect.Random (random)

showNowDate :: Effect String
showNowDate = do
    d <- nowDate
    pure $ "the current date is: " <> show d 

showNowTime :: Effect String
showNowTime = (\d -> "the current date is: " <> show d) <$> nowTime

logRandom :: Effect Unit
logRandom = show >>> (<>) "here is a random number: " <$> random >>= log

main :: Effect Unit
main = do
  showNowDate >>= log
  showNowTime >>= log
  logRandom

Credits

Pictures and examples from adit.io

Breakthrough

Exercise

In module 04 we've used buildTx to simulate a transaction build, in this module we'll simulate a Contract behavior using what we've learned so far.

type Validator = Redeemer -> ScriptContext -> Datum -> Boolean

data Contract a = Contract a

buildTx :: Inputs -> Outputs -> Redeemer -> Validator -> Contract Boolean

runContract :: forall a. Contract a -> Effect a

Change previous created buildTx to return the value using the Contract monad and implement the missing definitions. runContract must log in the console the contract execution.