From 9754a53fdc14f6ee4cf00f851cda68d69889bdcd Mon Sep 17 00:00:00 2001 From: Thomas Letan Date: Tue, 4 Feb 2020 18:13:38 +0100 Subject: Initial commit with previous content and a minimal theme --- site/posts/monad-transformers.org | 71 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 71 insertions(+) create mode 100644 site/posts/monad-transformers.org (limited to 'site/posts/monad-transformers.org') diff --git a/site/posts/monad-transformers.org b/site/posts/monad-transformers.org new file mode 100644 index 0000000..de29053 --- /dev/null +++ b/site/posts/monad-transformers.org @@ -0,0 +1,71 @@ +#+BEGIN_EXPORT html +

Monad Transformers are a Great Abstraction

+ +#+END_EXPORT + +#+OPTIONS: toc:nil + +Monads are hard to get right. I think it took me around a year of Haskelling to +feel like I understood them. The reason is, to my opinion, there is not such +thing as /the/ Monad. It is even the contrary. When someone asks me how I would +define Monads in only a few words, [[https://techn.ical.ist/@lthms/590439][I say Monad is a convenient formalism to +chain specific computations]]. Once I’ve got that, I started noticing “monadic +construction” everywhere, from the Rust ~?~ operator to the [[https://blog.drewolson.org/elixirs-secret-weapon/][Elixir ~with~ +keyword]]. + +Haskell often uses another concept above Monads: Monad Transformers. This allows +you to work not only with /one/ Monad, but rather a stack. Each Monad brings its +own properties and you can mix them into your very own one. That you can’t have +in Rust or Elixir, but it works great in Haskell. Unfortunately, it is not an +easy concept and it can be hard to understand. This article is not an attempt to +do so, but rather a postmortem review of one situation where I found them +extremely useful. If you think you have understood how they work, but don’t see +the point yet, you might find here a beginning of answer. + +Recently, I ran into a very good example of why Monad Transformers worth it. I +have been working on a project called [[https://github.com/ogma-project][ogma]] for a couple years now. In a +nutshell, I want to build “a tool” to visualize in time and space a +storytelling. We are not here just yet, but in the meantime I have wrote a +software called ~celtchar~ to build a novel from a list of files. One of its +newest feature is the choice of language, and by extension, the typographic +rules. This information is read from a configuration file very early in the +program flow. Unfortunately, its use comes much later, after several function +calls. + +In Haskell, you deal with that kind of challenges by relying on the Reader +Monad. It carries an environment in a transparent way. The only thing is, I was +already using the State Monad to carry the computation result. But that’s not an +issue with the Monad Transformers. + +#+BEGIN_SRC patch +-type Builder = StateT Text IO ++type Builder = StateT Text (ReaderT Language IO) +#+END_SRC + +As you may have already understood, I wasn't using the “raw” ~State~ Monad, but +rather the transformer version ~StateT~. The underlying Monad was ~IO~, because +I needed to be able to read some files from the filesystem. By replacing ~IO~ by +~ReaderT Language IO~, I basically fixed my “carry the variable to the correct +function call easily” problem. + +Retrieving the chosen language is as simple as: + +#+BEGIN_SRC patch +getLanguage :: Builder Language +getLanguage = lift ask +#+END_SRC + +And that was basically it. The complete [[https://github.com/ogma-project/celtchar/commit/65fbda8159d21d681e4e711a37fa3f05b49e6cdd][commit]] can be found on Github. + +Now, my point is not that Monad Transformers are the ultimate beast we will have +to tame once and then everything will be shiny and easy. There are a lot of +other way to achieve what I did with my ~Builder~ stack. For instance, in an +OO language, I probably would have to add a new class member to my ~Builder~ +class and I would have done basically the same thing. + +However, there is something I really like about this approach: the ~Builder~ +type definition gives you a lot of useful information already. Both the ~State~ +and ~Reader~ Monads have a well established semantics most Haskellers will +understand in a glance. A bit of documentation won’t hurt, but I suspect it is +not as necessary as one could expect. Moreover, the presence of the ~IO~ Monad +tells everyone using the ~Builder~ Monad might cause I/O. -- cgit v1.2.3