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diff --git a/site/opinions/MonadTransformers.org b/site/opinions/MonadTransformers.org deleted file mode 100644 index 08f8e1b..0000000 --- a/site/opinions/MonadTransformers.org +++ /dev/null @@ -1,80 +0,0 @@ -#+TITLE: Monad Transformers are a Great Abstraction - -#+SERIES: index.html -#+SERIES_NEXT: StackedGit.html - -[[mn:1][Time has passed since the publication of this article. Whether -or not I remain in sync with its conclusions is an open -question. Monad Transformers are a great abstraction, but given the -choice, I would probably choose another approach, namely extensible -effects.]] This article has originally been published on @@html:<span -id="original-created-at">@@July 15, 2017@@html:</span>@@. - -#+BEGIN_EXPORT html -<div id="history">site/opinions/MonadTransformers.org</div> -#+END_EXPORT - -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, I say monads are 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. |