tutorial-cats's Introduction

Tutorial cats

Your job is to make all the test green!

Notice that:

Cats implicits for a given type follow this convention import cats.instances.mytype._

Cats syntax for a given type follow this convention import cats.syntax.atype._

TODO `01` - `Eq`

Comparing with Eq, === and =!=

~testOnly example.EqSpec

TODO `02` - `Printable`

Implement the printable for Gato (spanish for Cat).

cats usually split typeclasses in the following way:

A trait defining the general behavior
A companion object that contains the type classes
A syntax object containing implicit class(es)

~testOnly example.PrintableSpec

TODO `03` - `Monoid`

Monoids can combine two elements of the same type (thanks to semigroup) and have an empty element.

|+| is an alias for combine present in semigroup

~testOnly example.MonoidSpec

TODO `04` - `Functor`

Playing with Functors.

Functors can map over something

~testOnly example.FunctorSpec

TODO `05` - `Write and Read`

Handling additional information to the main logic flow.

Write monad add additional information

Read monad expects contextual information

~testOnly example.WriteAndReadSpec

TODO `06` - `State`

Solving equations.

State Monad cumulates values

~testOnly example.StateSpec

TODO `07` - `Validation`

Validation Monad and Either Monad handle errors

~testOnly example.ValidationSpec

Problems?

If you are really, really, really having problems finding the right imports just include the following

import cats._
import cats.implicits._

However one of the goals of this exercise is to be able to identify where the typeclass implementation is located.

Contribution policy

Contributions via GitHub pull requests are gladly accepted from their original author. Along with any pull requests, please state that the contribution is your original work and that you license the work to the project under the project's open source license. Whether or not you state this explicitly, by submitting any copyrighted material via pull request, email, or other means you agree to license the material under the project's open source license and warrant that you have the legal authority to do so.

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tutorial-cats's Issues

Adding an example of `Validated` besides `mapN` could be a good idea.

I'm proposing something along these lines

import cats._
import cats.implicits._
import cats.data.Validated
import cats.data.Validated.{Invalid, Valid}

type ErrorOr[T] = T => Either[List[String], T]
type Result[T]  = T => Validated[List[String], T]

case class Student(country: String, age: Int)

def validateCountry(country: String): ErrorOr[Student] =
  student => Either.cond(student.country == country, student, List(s"Invalid country $country"))

def validateAge(minAge: Int): ErrorOr[Student] =
  student => Either.cond(student.age >= minAge, student, List(s"Invalid age $minAge"))

def run[T](validations: ErrorOr[T]*): Result[T] =
  t => validations.toList
    .map(rule => rule(t).toValidated)
    .foldLeft(t.asRight[List[String]].toValidated) {
      case (i@Valid(_), Valid(_)) => i // any side is ok
      case (i@Invalid(_), Valid(_)) => i
      case (Valid(_), i@Invalid(_)) => i
      case (Invalid(e1), Invalid(e2)) => Invalid(Semigroup[List[String]].combine(e1, e2))
    }

run[Student](
    validateAge(5),
    validateCountry("Frances")
)(Student("France", 3))

cats `Eq`'s `===` with Scalatest

https://stackoverflow.com/questions/47501997/cats-eqs-with-scalatest

Update cats version from master brach

I already migrated the cats version deom the solution branch.

The same changes should be done to the master branch

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fagossa / tutorial-cats Goto Github PK

tutorial-cats's Introduction

Tutorial cats

TODO 01 - Eq

TODO 02 - Printable

TODO 03 - Monoid

TODO 04 - Functor

TODO 05 - Write and Read

TODO 06 - State

TODO 07 - Validation