Combining Free Monads with Free Applicatives workshop

Running

Command Line:

sbt "run monad"
sbt "run applicative"

IDE: Launch app.Main

Problem

Given two Twitter accounts, determine which one has the most followers who have tweeted in the last week. Then return a url in the form http://<app-url>/details/<twitter-handle>. Note that this endpoint does not exist, but assume later we might develop one.

Initial solution

A solution has been provided (loosely based on the interpreter pattern at REA). However the solution has some limitations.

Intellij IDE notes

Note that some of the valid code in this solution will be highlighted by intellij as having structural issues. Especially around the case statement in the interpreter. https://youtrack.jetbrains.com/issue/SCL-3170

Intellij also gets confused by the |@| operator when combining applicatives. Any alternative is to use Apply[M].map2(a, b)(handleBoth)

Monix Notes

The problem makes use of Monix tasks as an abstraction over Scala Future. Monix has been used as it cleanly separates the intention of asynchronous execution from the actual execution. A Scala Future does not provide this level of flexibility.

See https://monix.io/docs/2x/eval/task.html.

1. Single ADT

We have defined a single top level ADT which becomes bloated with all possible operations. A single top level interpreters needs to concern itself with all operations.

2. Monadic composition

All side effects are monadic. A monad defines flatMap (aka Bind) as:

trait Monad[A] {
   def flatMap[B](f: A => F[B]): F[B]   

So when doing

for {
   lukeFollowers <- getFollowers("lukestephenson")
   composeFollowers <- getFollowers("composemelbourne")
} yield ()

Even though there is no logical dependency between the two lookups for followers, the monadic chaining introduces one.

Given there is no logical dependency between the two lookups, our logic should ideally reflect that.

Exercises

Tackle either of the above.

For problem 1 refer to http://typelevel.org/cats/tut/freemonad.html under the heading Composing Free monads ADTs.

For problem 2, see http://typelevel.org/cats/tut/freeapplicative.html.
Note that you will still need monadic composition as well (to fetch the most recent tweet for each follower).

For this problem, there is a branch free-applicative to use as a starting point.