Fix Free Monad example

[youta32449999]

Type Mismatch Error occurred impureCompiler and pureCompiler in scalaVersion 3.1.3

kvs.get(key).map(_.asInstanceOf[A])
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Found:    Option[A]
Required: cats.Id[A]

State.inspect(_.get(key).map(_.asInstanceOf[A]))
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Found:    Option[A]
Required: A

[armanbilge]

Thanks!

[armanbilge]

Hmm, I may have merged this a bit too hastily 😓

.get returns an Option, so we can't cast to it to A. Seems like what we should be doing here is directly calling .apply on the key.

[armanbilge]

Also, it's annoying that mdoc in CI passes for this even if it's not working.

[youta32449999]

Thank you for review my pull request.

The definition of Get is case class Get[T](key: String) extends KVStoreA[Option[T]].
In this case, I think type A is Option[T]🤔.

[armanbilge]

@youta32449999 ah, indeed your are right.,I am confusing A with T. Hasty reviewing again! 😅 Sorry about that, and thanks for clarifying. I guess the reason it worked before is because of type erasure at runtime, so that the mistake did not matter.

It looks to be correct on the website. Thank you again for fixing this!
https://typelevel.org/cats/datatypes/freemonad.html#_5-run-your-program

[armanbilge]

I think I am also confused by the comment.

cats/docs/datatypes/freemonad.md

Line 172 in 038705e

// the program will crash if a key is not found,

Since Get returns an Option, in what situation will it crash?

[youta32449999]

Hmm, I think it will crash if a type is incorrectly specified.
For example:

def program: KVStore[Option[Int]] = for {
  _ <- put("wild-cats", "two")  // expect Int, but value is String.
  _ <- update[Int]("wild-cats", (_ + 12))
  _ <- put("tame-cats", 5)
  n <- get[Int]("wild-cats")
  _ <- delete("tame-cats")
} yield n

So, I'm going to fix the comment as below.
the program will crash if a type is incorrectly specified.