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-<h1>Monad Transformers are a Great Abstraction</h1>
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-<p>This article has originally been published on <span
-id="original-created-at">July 15, 2017</span>.</p>
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-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 diff
--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 haskell
-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.