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Explain purpose of Kleisli Categories from the programmer's perspective #191

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I feel like we need to include such explanation, because at my first read, I was left hanging about what is the purpose of Kleisli Categories.

After all, this book is meant for programmers, so I think it is very important to include such explanation.

@@ -400,6 +400,23 @@ \section{Kleisli Categories}
programs that in imperative languages are traditionally implemented
using side effects.

But, what is the real purpose of this concept from the perspective of a

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I partially agree with this statement because I believe the real purpose of this concept for programmers is a composition for a monadic functions.
e.g.(code is scala)
type Foo = Kleisli[IO, Int, String]
type Bar = Kleisli[IO, String, String]
can be easily composed like Bar compose Foo

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If that is the case it would be good if you can add on upon the content I’ve written.

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If that is the case it would be good if you can add on upon the content I’ve written.

Hi, I'll try to add a description tomorrow

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@a-nigredo a-nigredo May 19, 2019

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Fortunately, I have time to write today:

But, what is the real purpose of this concept from the perspective of a programmer?
Since arrows in Kleisli category is monadic functions and we know from a category definition that arrows have to be composable inside a category.
So the Kleisli category just gives us a composition for monadic function.

Let's say we have to read text from a file and save it to a database.

We have 2 arrows:

type FileReader = String=> IO[String]
type DbWriter = String => IO[Unit]

It is not possible to compose them since FileReader returns IO[String] not String as DbWriter expects. So what can we do?
Just wrapped functions by Kleisli

type FileReaderKleisli = Kleisli[IO, String, String]
type DbWriterKleisli = Kleisli[IO, String, Unit]

and now we can easily compose them as DbWriterKleisli(DbWriter) compose FileReaderKleisli(FileReader)

All code is written in Scala(https://www.scala-lang.org/) and Kleisli type signature is taken from https://typelevel.org/cats/datatypes/kleisli.html

but I don't have time to change pull request directly.
@wongjiahau could you please add and correct if needed my text and add to your pull request?

@hmemcpy hmemcpy force-pushed the master branch 2 times, most recently from 0e79320 to 4986511 Compare February 14, 2020 17:37
@drupol
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drupol commented May 17, 2022

What is the status of this PR ?

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3 participants