1202 lines
37 KiB
Org Mode
1202 lines
37 KiB
Org Mode
# Created 2019-04-01 Mon 13:31
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#+TITLE: Haskell for the working programmer
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#+AUTHOR: Yann Esposito
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#+TODO: TODO TO-CLEAN TO-REVIEW | DONE
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#+LANGUAGE: en
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#+KEYWORDS: haskell
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#+PROPERTY: header-args :output-dir HWP :mkdirp yes :tangle-mode (identity #o755)
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* TO-CLEAN Introduction
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This is somehow a follow-up from Learn Haskell Fast and Hard.
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Which was more about being able to /play/ with Haskell than to /work/ with it.
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This is also an experiment.
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I'm not sure if it will be as positive as I hope.
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This book try to be a good resource to learn Haskell but to speed up the
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learning in the first part I'll skip the explanation about why Haskell does
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things the way it does.
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As a consequence if you don't keep in mind that there is *very good* reasons to
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make some things way more difficult in Haskell than in other languages you
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might miss the real reason.
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Also don't forget in the beginning you might only see what is more difficult or
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harder to achieve in Haskell.
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But for each thing harder, keep in mind that there are very difficult things in
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other languages that are solved extremely easily in Haskell.
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And I personnally believe the things Haskell make easier are essential to reach
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the best balance between speed, elegance, safety and pragmatism with regards to
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any programming language I ever used before.
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So this book might be a bit raw.
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And in fact not really "fun" unfortunately.
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But it should be efficient.
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This book is aimed to be one of the fastest way to learn how to be productive
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with Haskell.
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Know that there still will be a very long road ahead once this book will be
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finished to master Haskell. That should be ok. Even with those basic knowledge,
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you should already be more productive in Haskell than in most other programming.
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Modern computing has unfortunately less to do with algorithmic than to create a
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mashup of libs and external APIs.
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So while learning all the details of Haskell can seems like an impossible challenge.
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Learning the necessary skills to be productive shouldn't be that hard.
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What does this book will talk about.
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1. Having a clean and stable dev environment
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2. Basic Introduction to the language
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3. Professional Project developement workflow
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4. Make command line program
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5. Use external libraries
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6. Handle the filesystem
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7. Handle a few DBs
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8. Make a basic REST API
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What Haskell can do few other programming language can.
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Ability to put strong constraingt on part of the code. For exemple you can have
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confidence in 3rd party functions. You can be certain that there will be NO side
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effect. Or you can also ensure that part of you code can only write logs and not
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access the DB. We'll technique that will ensure that subpart of the code will
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only access the User table in your DB etc...
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Writting parallel and concurrent code because /very/ easy to write.
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While this is generally a nightmare in most programming language.
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** TO-CLEAN What does "working programmer" stand for?
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Being able to:
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- create a new working program from scratch,
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- work with the filesystem (read/write files/directories),
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- work with BDD (SQLite, PostgresSQL, MongoDB, etc...),
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- work with network (send/receive HTTP request),
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- make a REST API,
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- use most libraries (OpenGL, ncurses, etc...)
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- write test for your application,
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- to deploy your application
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This is more about being an user, consumer from the Haskell community than being
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an active contributor. Hopefully the gap won't be hard to pass from user to
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contributor. So I'll write a minimal chapter about how to write your own library
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and publish it for other developpers.
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** TO-CLEAN Prerequiste
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The target audience I'm writting this book for is software developpers.
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You should:
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- be familiar with some programming language,
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- be familiar with command line in a shell,
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- know how to editing text files (I try to focus on generic editors like emacs,
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vim, etc...),
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- know the basic usage of =git=
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If you don't know that, your journey with this book might be a bit difficult but
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I'll do my best to not make it impossible.
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** TO-CLEAN Opinionated
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Keep in mind that Haskell has a very active and open ecosystem.
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And the language itself let you make very different choices to the fundamentals.
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This book is very opinionated, because I wanted to be efficient in learning
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fast for some specific kind of personalities.
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It might not be for you.
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One of my goal is to shortcircuit some classic learning detour.
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For a lot of decisions I generally make only one choice. I'll try to talk about
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the other choices and it will be your duty to explore other choices after you
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completed this book to decide which is the one that has your preference.
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Also note that this book was written in the past. And as I said Haskell
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ecosystem evolve very fast. And some choices which are an evidence today might
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be deprecated in a few months from now.
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Typically there are many different and concurrent web frameworks, db libs, etc..
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** TO-CLEAN A Word about Haskell philosophy
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One Haskell main characteristic is that it tends to make the right/most secure
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choice by default.
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A very simple example is that it is generally harder to write unsafe code than
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to write safe and pure code.
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Also one of the reason I think Haskell is percieved as hard to learn by many
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people is that you generally need to ingest a lot of concepts before being able
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to be productive.
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#+TODO: TODO TO-CLEAN TO-REVIEW | DONE
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#+LANGUAGE: en
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#+KEYWORDS: haskell
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#+PROPERTY: header-args :output-dir HWP :mkdirp yes :tangle-mode (identity #o755)
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** TO-CLEAN Install a dev environment (about 30 minutes)
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Installing a dev environment should hopefully be the most boring part of this
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book. But this is a necessary price to pay to really get why Haskell is
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considered so great by people using it.
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**** TO-CLEAN Working environment
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A thing to note is the distinction between learning a language for personal
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interrest for some personal project and learning with the goal to achieve a
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"product" with some hard deadline.
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So for example, it can be nice to understand the language by playing inside a
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REPL. We won't use that much in this book as the goal is not to really gain a
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deeper knowledge but perhaps to be able to /use/ the language.
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The problem I try to solve in this book is to make you a professional /user/ of
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Haskell more than a /contributor/ to Haskell. While I encourage everybody to
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gain deeper understanding on the internals of Haskell this is not the primary
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goal of this book.
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What I mean by professional /user/ is that you should have the following
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features at your disposal:
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- DCVS
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- Generated documentation
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- Tests (Unit tests, Generative tests, Integration tests, etc...)
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- Benchmark
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- Continuous Integration
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- Continuous Deployment
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Choices:
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- Raw: get GHC and cabal exectuable and work with that. Too long and manual
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- Nix: this is really great because it's like a super make that can deal with
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external dependencies. Certainly the best tool in the long term.
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- Stack: fast to install focused on being user friendly. Has a lot of easy to
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use features like:
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- integration with docker that will make it easy to cross-compile, deploy.
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- integration with nix
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- easy to deal with many private repositories
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- good professional starting templates
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**** TO-CLEAN Stack
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I recommend [[https://haskellstack.org][stack]]. But there are many different method to install Haskell.
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Stack should be simple and straight to the point.
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If thing haven't changed since the book was written it could be installed with:
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#+BEGIN_SRC shell
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curl -sSL https://get.haskellstack.org/ | sh
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#+END_SRC
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**** TO-CLEAN git
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You should have =[[https://git-scm.com][git]]= installed.
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**** TO-CLEAN Editor
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You should check any of the supported editor here:
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https://github.com/rainbyte/haskell-ide-chart#the-chart-with-a-link-to-each-plug-in
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I personnaly use [[http://spacemacs.org][spacemacs]] with the haskell layer because it comes with battery
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included. If you're not used to vim keybindings I believe it is easy to switch
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to more classical editor keybindings easily.
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Even if I don't have a strong opinion on the editor you should choose. It should
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at least be easy to support the Haskell tooling, like intero or ghc-mod. Because
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it's one of the best part of Haskell.
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For example without any configuration I have the following features:
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- I see errors, warn and even code hints while I'm typing my code.
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- very good code completion
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- auto styling of my source code and be able to change the style of my entire
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buffer
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- be able to get the type of the expression under my cursor
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- be able to add the type of a top level declaration
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- be able to launch a repl easily loading the current code of the file I'm
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currently editing
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And many other nice features.
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Note that in the past I had some problem with ghc-mod during upgrades while
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intero was mostly a no problem story.
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It is also useful to have hoogle and hayoo, which are search engine focused on
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Haskell.
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***** TO-CLEAN Spacemacs
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So if you want to choose spacemacs:
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1. Install a recent emacs
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2. =git clone https://github.com/syl20bnr/spacemacs ~/.emacs.d=
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3. Launch emacs
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4. Edit your =~/.spacemacs= file to add to the layer list:
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#+BEGIN_SRC elisp
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haskell
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(auto-completion :variables
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auto-completion-enable-help-tooltip t
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auto-completion-enable-short-usage t)
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#+END_SRC
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If you're not used to vim keybinding and it is too much to handle for you.
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I think you can change the value of =dotspacemacs-editing-style= from ='vim=
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to ='hybrid= or ='emacs= in the =.spacemacs= file.
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It should be good now.
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**** TO-CLEAN Conclusion
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First you can congratulate yourself to have installed all the prerequiste to
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have a great working development environment.
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I know it was already a lot of boring tasks to perform before being able to
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write any line of code. But I promise it will be worth it.
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By starting with this template, you won't use the classic prelude. It's quite a
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strong opinionated move. Because many classic function will be overwritten by
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safer/more generic one.
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So be prepared that the actual learning route is jumping other classical
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learning steps you can find in other learning resources. Don't worry I'll do my
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best to make the jump as natural as possible.
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#+TODO: TODO TO-CLEAN TO-REVIEW | DONE
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#+LANGUAGE: en
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#+KEYWORDS: haskell
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#+PROPERTY: header-args :output-dir HWP :mkdirp yes :tangle-mode (identity #o755)
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* TODO Data Management
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You can think of Haskell and its Prelude as a Linux installation.
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You have a kernel and a bunch of executable.
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So in order to manage your Unix env you need to know a few command line first.
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Then, slowly with the need and experience, you'll learn more and more commands.
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So we'll do the same thing here.
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We'll start with only what we need to start a project.
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And You'll be introduced with new notion as they are needed.
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** Prelude/Protolude
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So Haskell prelude is old and it is very easy to use more modern prelude.
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So I'll tend to do just that.
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*** Data Manipulation Functions
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* TODO Effect Management
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In that part of the book, we'll use simple examples. Thus instead of going
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directly to a full project structure we'll focus on the language. That file can
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be treated as a single executable strict.
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For example:
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#+BEGIN_SRC haskell
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#!/usr/bin/env stack
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{- stack script
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--resolver lts-13.13
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--install-ghc
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--package protolude
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-}
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{-# LANGUAGE NoImplicitPrelude #-}
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{-# LANGUAGE OverloadedStrings #-}
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import Protolude
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main = putText "Hello World!"
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#+END_SRC
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The firsts line are simply here to set the correct execution environment.
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The real program starts after them.
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Once =stack= will be installed (see the /Install a dev environment/ section)
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if you put that content in a file named =hello.hs= then you can launch it with:
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#+BEGIN_SRC
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> chmod +x hello.hs
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> ./hello.hs
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#+END_SRC
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The first time it is launched can take a little bit of time because it will
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download all dependencies. The advantage of this form of distribution is that it
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is a quasi self-contained exectuable. That's a good one for minimal examples.
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But after a short introduction we'll use full projects.
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We'll start by example first and all notion will be introduced as they appear.
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If you find confident you could feel free to skip some descriptions and
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explanations.
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** TODO Short Examples / Scripts
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*** TO-CLEAN Guess a number
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**** TO-CLEAN Print and read things
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Now let's modify the code of =main= to print things.
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First comment the import line for =Lib=.
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Haskell comment are =--= till the end of the line or ={- .... -}=
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for multiline comments.
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Without this comment you'll get a warning that this import is unused.
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And by default we compile using =-Werror= flag to GHC which tell that the
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compilation should fail also on warnings as well as on errors.
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The default template tend to be a professional environment and has more
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restrictions in order to maximize confidence in quality.
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#+BEGIN_SRC haskell
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#!/usr/bin/env stack
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{- stack script
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--resolver lts-11.6
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--install-ghc
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--package protolude
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-}
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{-# LANGUAGE NoImplicitPrelude #-}
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{-# LANGUAGE OverloadedStrings #-}
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import Protolude
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main = putText "Hello, world!"
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#+END_SRC
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Simple and natural.
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Now let's ask your name.
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#+BEGIN_SRC haskell
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#!/usr/bin/env stack
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{- stack script
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--resolver lts-11.6
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--install-ghc
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--package protolude
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-}
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{-# LANGUAGE NoImplicitPrelude #-}
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{-# LANGUAGE OverloadedStrings #-}
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import Protolude
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main = do
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putText "What is your name?"
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name <- getLine
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putText ("Hello " <> name <> "!")
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#+END_SRC
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We can try that in the REPL (GHCI). You should be able to start it from your
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editor. For example in spacemacs I can load the current buffer (open file) in
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the REPL with =SPC m s b=.
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You could also start the repl in a terminal with =stack ghci=
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And then load the module with =:l hello_name.hs=.
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The =:l= is a shortcut for =:load=.
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#+BEGIN_SRC
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> stack ghci
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Warning: No local targets specified, so ghci will not use any options from your package.yaml / *.cabal files.
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Potential ways to resolve this:
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,* If you want to use the package.yaml / *.cabal package in the current directory, use stack init to create a new stack.yaml.
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,* Add to the 'packages' field of ~/.stack/global-project/stack.yaml
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Configuring GHCi with the following packages:
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GHCi, version 8.2.2: http://www.haskell.org/ghc/ :? for help
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Loaded GHCi configuration from /private/var/folders/bp/_8thkcjd4k3g81mpxtkq44h80000gn/T/ghci70782/ghci-script
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Prelude> :l hello_name.hs
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[1 of 1] Compiling Main ( hello_name.hs, interpreted ) [flags changed]
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Ok, one module loaded.
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,*Main> main
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What is your name?
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Yann
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Hello Yann!
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#+END_SRC
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But you should also simply run it from command line:
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#+BEGIN_SRC
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> ./hello_name.sh
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What is your name?
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Yann
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Hello Yann!
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#+END_SRC
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OK simple enough.
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|
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But let's take a moment to understand a bit more what's going on.
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We started with the =do= keyword.
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It's a syntactical sugar that helps in combining multiple lines easily.
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Let's take a look at the type of each part.
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#+BEGIN_SRC haskell
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putText :: Text -> IO ()
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#+END_SRC
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It means that =putText= is a function that take a =Text= as parameter and return
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an =IO ()=.
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Mainly =IO ()= simply means, it will return =()= (nothing) while doing some IO
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||
or border effect.
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||
The border effect here being, writing the text to the standard output.
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#+BEGIN_SRC haskell
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putText "What is your name?" :: IO ()
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#+END_SRC
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So yes this line make an IO but returns nothing significant.
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#+BEGIN_SRC haskell
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name <- getLine
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#+END_SRC
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|
||
The function =getLine= will read from standard input and provide the line read
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and send the value as a =Text=. If you look at the type of =getLine= you have:
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|
||
#+BEGIN_SRC haskell
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getLine :: IO Text
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#+END_SRC
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|
||
And that means that to be able to retrieve and manipulate the Text returned by
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in an "IO context" you can use the =<-= notation.
|
||
So in the code the type of =name= is =Text=
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|
||
More generally if =foo :: IO a= then when you write
|
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#+BEGIN_SRC haskell
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do
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x <- foo :: IO a
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#+END_SRC
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Then the type of =x= is =a=.
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Finally the last line:
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||
|
||
#+BEGIN_SRC haskell
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putText ("Hello " <> name <> "!")
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#+END_SRC
|
||
|
||
=putText= take a =Text= as argument so: =("Hello " <> name <> "!") :: Text=.
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||
|
||
So =(<>)= is the infix operator equivalent to the function =mappend=.
|
||
Here are equivalent way to write the same thing:
|
||
|
||
#+BEGIN_SRC haskell
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"Hello" <> name <> "!"
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"Hello" `mappend` name `mappend` "!"
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mappend "Hello" (mappend name "!")
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(<>) "Hello" ((<>) name "!")
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#+END_SRC
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|
||
So in Haskell if your function contains chars it will be a prefix function.
|
||
If your function contains special chars then it is considered to be an infix
|
||
operator.
|
||
|
||
You can use your function as infix if you put "`" around it name.
|
||
And you can make your operator prefix if you put it inside parentheses.
|
||
|
||
So you should have remarqued a pattern here.
|
||
Which is really important. Each line of a =do= bloc has a type of =IO a=.
|
||
|
||
#+BEGIN_SRC haskell
|
||
main = do
|
||
putText "What is your name?" :: IO ()
|
||
name <- getLine :: IO Text
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||
putText ("Hello " <> name <> "!") :: IO ()
|
||
#+END_SRC
|
||
|
||
So whenever you have an error message try to think about the type of your
|
||
expression.
|
||
|
||
Another very important aspect to notice.
|
||
The type of ="Hello " <> name <> "!"= is =Text= not =IO Text=.
|
||
This is because this expression can be evaluated purely.
|
||
Without any side effect.
|
||
|
||
Here we see a clear distinction between a pure part of our code and the impure
|
||
part.
|
||
|
||
#+BEGIN_QUOTE
|
||
|
||
|
||
*☞ Pure vs Impure* (function vs procedure)
|
||
|
||
That is one of the major difference between Haskell and other languages.
|
||
Haskell provide a list of function that are considered to have border effects.
|
||
Those functions are given a type of the form =IO a=.
|
||
|
||
And the type system will restrict the way you can manipulate function with type
|
||
=IO a=.
|
||
|
||
So, first thing that might be counter intuitive.
|
||
If an expression has a type of =IO a= it means that we potentially perform a
|
||
side effect and we "return" something of type =a=.
|
||
|
||
And we don't want to ever perform a side effect while doing any pure evaluation.
|
||
This is why you can't write something like:
|
||
|
||
#+BEGIN_SRC haskell
|
||
-- DOESN'T COMPILE
|
||
main = do
|
||
putText ("Hello " <> getLine <> "!")
|
||
#+END_SRC
|
||
|
||
Because you need to "traverse" the =IO= barrier to get back the value after the
|
||
evaluation.
|
||
This is why you NEED to use the =<-= notation.
|
||
Now knowing if a code is potentially making any side effect is /explicit/.
|
||
#+END_QUOTE
|
||
|
||
***** TO-CLEAN Strings in Haskell digression
|
||
|
||
Generally working with string is something you do at the beginning of learning a
|
||
programming language.
|
||
It is straightforward.
|
||
In Haskell you have many different choices when dealing with Strings depending
|
||
on the context.
|
||
But let just say that 95% of the time, you'll want to use Strict =Text=.
|
||
|
||
Here are all the possible choices:
|
||
|
||
- =String=: Just a list of =Char= very inefficient representation,
|
||
- =Text=: UTF-16 strings can be Lazy or Strict,
|
||
- =Bytestring=: Raw stream of =Char= and also =Lazy.Bytestring=.
|
||
|
||
That is already 5 different choices. But there is another package that provide other string choices.
|
||
In =Foundation= the strings are =UTF-8=.
|
||
|
||
Hmmm... so much choices.
|
||
|
||
A rule of thumbs is to never use =String= for anything serious.
|
||
Use =Text= most of the time because they support encoding.
|
||
Use =Bytestring= if you need efficient bytes arrays.
|
||
|
||
By using Protolude, we naturally don't use =String=.
|
||
|
||
**** TO-CLEAN Guess my age program
|
||
|
||
So far so good.
|
||
But the logic part of the code should be in a library in =src/= directory.
|
||
Because this part is easier to test.
|
||
|
||
The =src-exe/Main.hs= should be very minimalist, so now let's change its content
|
||
by:
|
||
|
||
#+BEGIN_SRC haskell
|
||
#!/usr/bin/env stack
|
||
{- stack script
|
||
--resolver lts-11.6
|
||
--install-ghc
|
||
--package protolude
|
||
-}
|
||
{-# LANGUAGE NoImplicitPrelude #-}
|
||
{-# LANGUAGE OverloadedStrings #-}
|
||
import Protolude
|
||
|
||
guess :: IO ()
|
||
guess = undefined
|
||
|
||
main :: IO ()
|
||
main = do
|
||
guess
|
||
putText "Thanks for playing!"
|
||
#+END_SRC
|
||
|
||
We know that the type of guess must be =IO ()=.
|
||
We don't know yet what the code will be so I just used =undefined=.
|
||
This way the program will be able to typecheck.
|
||
|
||
The next step is to define the ~guess~ function.
|
||
|
||
#+BEGIN_SRC haskell
|
||
#!/usr/bin/env stack
|
||
{- stack script
|
||
--resolver lts-11.6
|
||
--install-ghc
|
||
--package protolude
|
||
-}
|
||
{-# LANGUAGE NoImplicitPrelude #-}
|
||
{-# LANGUAGE OverloadedStrings #-}
|
||
import Protolude
|
||
|
||
guess :: IO ()
|
||
guess = guessBetween 0 120
|
||
|
||
guessBetween :: Integer -> Integer -> IO ()
|
||
guessBetween minAge maxAge = do
|
||
let age = (maxAge + minAge) `div` 2
|
||
if minAge == maxAge
|
||
then putText ("You are " <> show minAge)
|
||
else do
|
||
putText ("Are you younger than " <> show age <> "?")
|
||
answer <- getLine
|
||
case answer of
|
||
"y" -> guessBetween minAge (age - 1)
|
||
_ -> guessBetween (if age == minAge then age + 1 else age) maxAge
|
||
|
||
main :: IO ()
|
||
main = do
|
||
guess
|
||
putText "Thanks for playing!"
|
||
#+END_SRC
|
||
|
||
So going from there we declared the =guess= function to call the =guessBetween=
|
||
function with the two paramters 0 and 120 to guess an age between 0 and 120.
|
||
|
||
And the main function is a classic recursive function.
|
||
We ask for each age if the user is younger than some age.
|
||
|
||
the =let= keyword permit to introduce pure values in between =IO= ones.
|
||
so =age = (maxAge + minAge) `div` 2= is mostly straightforward.
|
||
Note that we manipulate =Integer= and so that mean =`div`= is the integer division.
|
||
so =3 `div` 2 == 1=.
|
||
|
||
We see that working in IO you can put print statements in the middle of your
|
||
code. First remark we used a recursive function. In most imperative programming
|
||
languages explicit loops are preferred to recursive functions for efficiency reasons.
|
||
That shouldn't be the case in Haskell.
|
||
|
||
In Haskell recursive functions are the natural way to program things.
|
||
|
||
Important Remarks to note:
|
||
- to test equality we use the =(==)= operator.
|
||
- Haskell is lazy, so the =age= value is only computed if needed. So if you are
|
||
in the case where =minAge == maxAge=, =age= value is not evaluated.
|
||
- In Haskell =if .. then .. else ..= form always have an else body. There is no
|
||
Implicit "no result" value in Haskell. Each expression need to return
|
||
something explicitely. Even if it is the empty tuple =()=.
|
||
|
||
So now here we go:
|
||
|
||
#+BEGIN_SRC
|
||
> stack build
|
||
> stack exec -- guess-exe
|
||
Are you younger than 60?
|
||
y
|
||
Are you younger than 29?
|
||
n
|
||
Are you younger than 44?
|
||
y
|
||
Are you younger than 36?
|
||
n
|
||
Are you younger than 39?
|
||
n
|
||
Are you younger than 41?
|
||
y
|
||
Are you younger than 39?
|
||
n
|
||
You are 40
|
||
Bye!
|
||
#+END_SRC
|
||
|
||
We see we can still make the program better.
|
||
For example, the same question is asked twice in that example.
|
||
Still, it works.
|
||
|
||
*** TO-CLEAN Guess a random number
|
||
|
||
Let's write another slightly more complex example.
|
||
Instead of guessing the age of somebody.
|
||
This will be the role of the user to guess a random number choosen by the
|
||
program.
|
||
|
||
First we'll need to generate random numbers.
|
||
To that end we'll use a the =random= package as a new dependency.
|
||
|
||
You can get more information either on hackage or on stackage:
|
||
|
||
- https://hackage.haskell.org/package/random
|
||
- https://www.stackage.org/lts-11.7/package/random
|
||
|
||
Hackage is the official place where to put Haskell public libraries.
|
||
Stackage works in conjunction with =stack= and mainly it takes care of having a
|
||
list of packages version working together.
|
||
So that means that all packages in an LTS (Long Term Support) release can work
|
||
together without any build conflict.
|
||
|
||
Now let's use that package.
|
||
Notice the added =--package random= argument.
|
||
|
||
We'll start by writing a =guessNumber= function:
|
||
|
||
#+BEGIN_SRC haskell
|
||
#!/usr/bin/env stack
|
||
{- stack script
|
||
--resolver lts-11.6
|
||
--install-ghc
|
||
--package protolude
|
||
--package random
|
||
-}
|
||
{-# LANGUAGE NoImplicitPrelude #-}
|
||
{-# LANGUAGE OverloadedStrings #-}
|
||
|
||
import Protolude
|
||
|
||
import System.Random (randomRIO)
|
||
|
||
...
|
||
|
||
-- | Choose a random number and ask the user to find it.
|
||
guessNumber :: IO ()
|
||
guessNumber = do
|
||
n <- randomRIO (0,100)
|
||
putText "I've choosen a number bettween 0 and 100"
|
||
putText "Can you guess which number it was?"
|
||
guessNum 0 n
|
||
|
||
-- | Given a number of try the user already made and the number to find
|
||
-- ask the user to find it.
|
||
guessNum :: Int -> Int -> IO ()
|
||
guessNum nbTry nbToFound = undefined
|
||
#+END_SRC
|
||
|
||
So for now we just focus on how to get a random number:
|
||
|
||
#+BEGIN_SRC haskell
|
||
do
|
||
n <- randomRIO (0::Int,100)
|
||
-- do stuff with n
|
||
#+END_SRC
|
||
|
||
You NEED to use the =<-= notation inside a =do= bloc.
|
||
If you try to use =let n = randomRIO (0,100)= it will fail because the
|
||
types won't match.
|
||
|
||
And that's it!
|
||
|
||
Now to write the =guessNum= function, we'll write a classical recursive function:
|
||
|
||
#+BEGIN_SRC haskell
|
||
-- | Given a number of try the user already made and the number to find
|
||
-- ask the user to find it.
|
||
guessNum :: Int -> Int -> IO ()
|
||
guessNum nbTry nbToFound = do
|
||
putText "What is your guess?"
|
||
answer <- getLine
|
||
let guessedNumber = readMaybe (toS answer)
|
||
case guessedNumber of
|
||
Nothing -> putText "Please enter a number"
|
||
Just n ->
|
||
if n == nbToFound
|
||
then putText ("You found it in " <> show (nbTry + 1) <> " tries.")
|
||
else do
|
||
if n < nbToFound
|
||
then putText "Your answer is too low, try a higher number"
|
||
else putText "Your answer is too high, try a lower number"
|
||
guessNum (nbTry + 1) nbToFound
|
||
#+END_SRC
|
||
|
||
Let's read the program line by line:
|
||
|
||
- ~putText "What is your guess?"~ should be straightforward.
|
||
- ~answer <- getLine~ So the ~getLine~ read from standard input and returns the
|
||
line entered by the user. The line will be put in the ~answer~ variable.
|
||
- ~let guessedNumber = readMaybe (toS answer)~: there are a few things to tell about this line.
|
||
|
||
If you open GHCI and ask the type for each interresting symbol here is what you get:
|
||
|
||
#+BEGIN_SRC
|
||
λ :t getLine
|
||
getLine :: IO Text
|
||
|
||
λ :t toS
|
||
toS :: StringConv a b => a -> b
|
||
|
||
λ :t readMaybe
|
||
readMaybe :: Read a => GHC.Base.String -> Maybe a
|
||
#+END_SRC
|
||
|
||
- ~answer~ comes from ~getLine :: IO Text~ so ~answer~ should have the type ~Text~.
|
||
- Now we want to read this ~Text~ and see if this is a number and compare it to another ~Int~.
|
||
- To transform the number we don't use a function ~textToInt~ we simply use a
|
||
quite generic function ~readMaybe~ that take some ~String~ and try to
|
||
transform that to some type. For our specific case, the compiler is able to
|
||
figure out the type we want to transform the text into is ~Int~. Take the
|
||
time to digest that: ~Int~ is specified in the type signature of the
|
||
~guessNum~ function so the compiler could discover that ~readMaybe~ should
|
||
return a ~Maybe Int~. How does he do that? Let's follow:
|
||
1. see a ~n == nbToFound~ so we can deduce ~n~ and ~nbToFound~ have the same type.
|
||
2. Reading the type signature of the function it is clear ~nbToFound~ is of
|
||
type ~Int~ (it's the second argument of a function with type ~Int -> Int -> IO ()~)
|
||
3. Then ~n~ is generated from a pattern matching; the case ~Just n~ which
|
||
could be the the result of the ~readMaybe~ function. So we can deduce
|
||
that the ~a~ in the type signature of ~readMaybe~ is ~Int~ for this specific case.
|
||
- so ~guessedNumber :: Maybe Int~, if the user enter something that cannot be
|
||
transformed in number from a string then ~guessedNumber~ would be equal
|
||
to ~Nothing~ and we ask the user to enter a number. If the user entered a
|
||
number the type will be ~Just n~ were ~n~ will be an ~Int~.
|
||
- We compare the ~guessedNumber~ to the number to found ~nbToFound~.
|
||
- If the user found the right number we stop here by displaying the number of try.
|
||
- If the user hasn't found the number, depending on its value we tell the user
|
||
it's either too low or too high and we call the same function, this time, we
|
||
increment the number of try.
|
||
|
||
The full program is then:
|
||
|
||
#+BEGIN_SRC haskell
|
||
#!/usr/bin/env stack
|
||
{- stack script
|
||
--resolver lts-11.6
|
||
--install-ghc
|
||
--package protolude
|
||
--package random
|
||
-}
|
||
{-# LANGUAGE NoImplicitPrelude #-}
|
||
{-# LANGUAGE OverloadedStrings #-}
|
||
|
||
import Protolude
|
||
|
||
import System.Random (randomRIO)
|
||
|
||
main :: IO ()
|
||
main = guessNumber
|
||
|
||
-- | Choose a random number and ask the user to find it.
|
||
guessNumber :: IO ()
|
||
guessNumber = do
|
||
n <- randomRIO (0,100)
|
||
putText "I've choosen a number bettween 0 and 100"
|
||
putText "Can you guess which number it was?"
|
||
guessNum 0 n
|
||
|
||
-- | Given a number of try the user already made and the number to find
|
||
-- ask the user to find it.
|
||
guessNum :: Int -> Int -> IO ()
|
||
guessNum nbTry nbToFound = do
|
||
putText "What is your guess?"
|
||
answer <- getLine
|
||
let guessedNumber = readMaybe (toS answer)
|
||
case guessedNumber of
|
||
Nothing -> putText "Please enter a number"
|
||
Just n ->
|
||
if n == nbToFound
|
||
then putText ("You found it in " <> show (nbTry + 1) <> " tries.")
|
||
else do
|
||
if n < nbToFound
|
||
then putText "Your answer is too low, try a higher number"
|
||
else putText "Your answer is too high, try a lower number"
|
||
guessNum (nbTry + 1) nbToFound
|
||
#+END_SRC
|
||
|
||
which once executed:
|
||
|
||
#+BEGIN_SRC
|
||
> ./guess_number.hs
|
||
I've choosen a number bettween 0 and 100
|
||
Can you guess which number it was?
|
||
What is your guess?
|
||
50
|
||
Your answer is too low, try a higher number
|
||
What is your guess?
|
||
75
|
||
Your answer is too low, try a higher number
|
||
What is your guess?
|
||
90
|
||
Your answer is too high, try a lower number
|
||
What is your guess?
|
||
83
|
||
Your answer is too low, try a higher number
|
||
What is your guess?
|
||
87
|
||
You found it in 5 tries.
|
||
#+END_SRC
|
||
|
||
**** TO-CLEAN What did we learn so far?
|
||
|
||
So up until now, if you followed. You should be able to "reproduce" and make
|
||
minimal changes.
|
||
But I am certain than it still be difficult to make some changes.
|
||
It is time to learn some general principles.
|
||
I know it might be a bit repetitive but its important to be certain to ingest
|
||
those informations.
|
||
|
||
A generic function of type ~IO ()~ typically =main= should look like:
|
||
|
||
#+BEGIN_SRC haskell
|
||
f :: IO a
|
||
f = do
|
||
α <- f1
|
||
β <- f2
|
||
γ <- f3
|
||
δ <- f4
|
||
f5
|
||
#+END_SRC
|
||
|
||
where each expression =fi= is of type =IO a= for some =a=.
|
||
You can use any value =α=, =β=, etc‥ as a parameter.
|
||
In order to be valid.
|
||
The last expression must have the same type as =f=.
|
||
so here =f5 :: IO a=.
|
||
|
||
Now if I give you the following functions:
|
||
|
||
- ~getLine :: IO Text~ that read a line from stdin.
|
||
- ~putText :: Text -> IO ()~ that read a line from stdin.
|
||
|
||
With that you have the ability to read stdin and print things.
|
||
|
||
- ~if τ then f1 else f2~ where =τ :: Bool= and the type of ~f1~ and ~f2~ must be the
|
||
same. Generally this is denoted by: =:type f1 ~ :type f2= and that type
|
||
will be the same as the entire ~if ‥ then ‥ else ‥~ expression.
|
||
- you can compare things that can be compared with ~<~, ~<=~, ~>~, ~>=~, ~==~, ~/=~ (different).
|
||
- you can concatenate things that could be concatenated (like Text) with ~<>~
|
||
- you can transform things as Text with ~show~ in particular numbers.
|
||
|
||
So that is a few number of component but they are all composable.
|
||
And so far we only needed that to write our first programs.
|
||
|
||
Haskell libs will provide you with a lot more base functions but also a lot more
|
||
composition functions.
|
||
|
||
*** TODO Command Line Application
|
||
|
||
Another thing you might want to achieve at first is to retrieve arguments for a
|
||
command line application.
|
||
|
||
**** TO-CLEAN Basic
|
||
|
||
The simplest way to retrieve parameters to a command line is to use the ~getArgs~ function.
|
||
|
||
#+BEGIN_SRC haskell
|
||
getArgs :: IO [String]
|
||
#+END_SRC
|
||
|
||
Here is a minimal example.
|
||
|
||
#+BEGIN_SRC haskell
|
||
#!/usr/bin/env stack
|
||
-- stack --resolver lts-11.6 script
|
||
{-# LANGUAGE OverloadedStrings #-}
|
||
{-# LANGUAGE NoImplicitPrelude #-}
|
||
import Protolude
|
||
import System.Environment (getArgs)
|
||
|
||
main :: IO ()
|
||
main = do
|
||
arguments <- getArgs
|
||
case head arguments of
|
||
Just filename -> die ("The first argument is: " <> toS filename)
|
||
Nothing -> die "Please enter a filename"
|
||
#+END_SRC
|
||
|
||
#+BEGIN_SRC
|
||
> ./cmdline-basic.sh foo
|
||
The first argument is: foo
|
||
> ./cmdline-basic.sh
|
||
Please enter a filename
|
||
#+END_SRC
|
||
|
||
If you have a very basic command line it could be good enough.
|
||
But if you plan to have more things to configure you can consider
|
||
to use a library to parse options.
|
||
|
||
**** TODO Option Parsing
|
||
|
||
For that we will use the =optparse-generic= package.
|
||
|
||
|
||
#+BEGIN_SRC haskell
|
||
#!/usr/bin/env stack
|
||
{- stack script
|
||
--resolver lts-11.6
|
||
--install-ghc
|
||
--package protolude
|
||
--package optparse-generic
|
||
-}
|
||
{-# LANGUAGE NoImplicitPrelude #-}
|
||
{-# LANGUAGE OverloadedStrings #-}
|
||
import Protolude
|
||
import System.Environment (getArgs)
|
||
|
||
main :: IO ()
|
||
main = do
|
||
arguments <- getArgs
|
||
case head arguments of
|
||
Just filename -> die ("The first argument is: " <> toS filename)
|
||
Nothing -> die "Please enter a filename"
|
||
#+END_SRC
|
||
|
||
*** TODO File Access
|
||
*** TODO Daemons & Logging
|
||
** TODO Intermediate
|
||
*** TO-CLEAN Stack template
|
||
|
||
☞ As a first projet a lot of new concept will be introduced. Don't be
|
||
discouraged by that.
|
||
|
||
Let's create a project with a sane and modern file organisation.
|
||
|
||
I made a stack templates largely inspired by =tasty-travis= template. It will
|
||
provide a bootstrap for organizing your application with tests, benchmarks and
|
||
continuous integration.
|
||
|
||
This template provide a file organisation for your projects.
|
||
|
||
Mainly do jump into programmin you could theoretically just download the binary
|
||
of the main Haskell compiler GHC to your compiler and compile each file with
|
||
=ghc myfile.hs=. But let's face it. It's not suitable for real project which
|
||
need more informations about it.
|
||
|
||
So let's start with a sane professional organisation for your files.
|
||
|
||
#+BEGIN_COMMENT
|
||
****** TODO modify the URL to use a better URL: torrent / IPFS
|
||
#+END_COMMENT
|
||
|
||
#+BEGIN_SRC shell
|
||
stack new guess https://git.io/vbpej
|
||
#+END_SRC
|
||
|
||
After that, this should generate a new ~guess~ directory with the following
|
||
files:
|
||
|
||
#+BEGIN_SRC
|
||
> tree
|
||
.
|
||
├── CHANGELOG.md
|
||
├── LICENSE
|
||
├── README.md
|
||
├── Setup.hs
|
||
├── guess.cabal
|
||
├── package.yaml
|
||
├── src
|
||
│ └── Lib.hs
|
||
├── src-benchmark
|
||
│ └── Main.hs
|
||
├── src-doctest
|
||
│ └── Main.hs
|
||
├── src-exe
|
||
│ └── Main.hs
|
||
├── src-test
|
||
│ └── Main.hs
|
||
├── stack.yaml
|
||
└── tutorial.md
|
||
|
||
5 directories, 13 files
|
||
#+END_SRC
|
||
|
||
Most of your source code should be in the =src= directory. Generally =src-exe=
|
||
should be a minimal code that could handle the =main= function to start your
|
||
application. We'll talk about other parts later in the book but most other file
|
||
should be quite straightforward.
|
||
|
||
Edit the file =src-exe/Main.hs=
|
||
|
||
The file contains:
|
||
|
||
#+BEGIN_SRC haskell
|
||
import Protolude
|
||
|
||
import Lib (inc)
|
||
|
||
main :: IO ()
|
||
main = print (inc 41)
|
||
#+END_SRC
|
||
|
||
To compile it do a
|
||
|
||
#+BEGIN_SRC
|
||
> stack build
|
||
> stack exec -- guess-exe
|
||
42
|
||
#+END_SRC
|
||
|
||
So that program print 42 and stop.
|
||
|
||
*** TODO DB Access
|
||
**** NoSQL (Redis looks easy)
|
||
**** Stream DB (Kafka or NATS, etc...)
|
||
**** SQL (SQLite & Postgres)
|
||
Not sure about that part. Perhaps this should move in the Production section
|
||
*** TODO REST API
|
||
**** TODO Servant
|
||
**** TODO JSON manipulation
|
||
**** TODO Swagger-UI
|
||
** TODO Intermediate Conclusion
|
||
|
||
Congratulation for going this far. Now you should be able to work in Haskell at
|
||
least as well as in any other programming language.
|
||
|
||
Now there are different directions:
|
||
|
||
- learning more libraries
|
||
- learn to optimise code to make it as fast as C
|
||
- learn to understand details of the compilation and Haskell
|
||
- learn tips and tricks
|
||
- learn more about abstractions and type classes
|
||
- learn parallel and concurrent programming
|
||
- learn to deploy like a pro using nix
|
||
|
||
The order in which to learn all thoses things can be very different for everty need.
|
||
#+TODO: TODO TO-CLEAN TO-REVIEW | DONE
|
||
#+LANGUAGE: en
|
||
#+KEYWORDS: haskell
|
||
#+PROPERTY: header-args :output-dir HWP :mkdirp yes :tangle-mode (identity #o755)
|
||
|
||
* TODO Make Production Quality Products
|
||
** TODO Dhall /Maybe/
|
||
** TODO Error Handling
|
||
|
||
** TODO Unit testing / doctests
|
||
** TODO Generative Testing
|
||
** TODO Enhance reproductibility with docker
|
||
** TODO Enhance reproductibility with nix
|
||
** TODO How to deploy?
|
||
|
||
There are plenty of ways de deploy
|
||
|
||
*** TODO Trashy and easy
|
||
|
||
Compile in docker and copy the binary.
|
||
|
||
*** TODO With =nix= and =nixops=
|
||
#+TODO: TODO TO-CLEAN TO-REVIEW | DONE
|
||
#+LANGUAGE: en
|
||
#+KEYWORDS: haskell
|
||
#+PROPERTY: header-args :output-dir HWP :mkdirp yes :tangle-mode (identity #o755)
|
||
|
||
* TODO Enhance Code Quality
|
||
** TODO Type tricks
|
||
|
||
|
||
*** TODO New Types
|
||
*** TODO Phantom Types
|
||
** TODO Useful Abstractions
|
||
*** TODO Monoid
|
||
*** TODO Functors
|
||
*** TODO Applicative
|
||
*** TODO Monads
|
||
*** TODO Arrows
|
||
*** TODO Foldable
|
||
*** TODO Traversable
|
||
** TODO Useful Abstractions for Applications
|
||
*** TODO No organisation, everything in IO
|
||
*** TODO Handler Pattern
|
||
|
||
See the talk from Jasper Van Der Jeught
|
||
|
||
In my opinion as efficient as MTL, Free, Effects, but with more verbosity and
|
||
repetitions.
|
||
|
||
*** TODO Custom Monad
|
||
*** TODO MTL
|
||
*** TODO Free
|
||
*** TODO Effects
|
||
** TODO Lenses
|
||
|
||
This will only be an introduction for being an user of the library.
|
||
|
||
** TODO Generics and lens-generic
|
||
** TODO Code organisation
|
||
*** TODO No organisation, everything in IO
|
||
*** TODO Custom Monad
|
||
*** TODO Handler Pattern
|
||
*** TODO MTL
|
||
*** TODO Free
|
||
*** TODO Effects
|
||
** TODO Recognize some classical abstractions
|
||
*** TODO Algebra
|
||
*** TODO Catamorphisms
|
||
*** TODO Free & Interpreters
|
||
#+TODO: TODO TO-CLEAN TO-REVIEW | DONE
|
||
#+LANGUAGE: en
|
||
#+KEYWORDS: haskell
|
||
#+PROPERTY: header-args :output-dir HWP :mkdirp yes :tangle-mode (identity #o755)
|
||
|
||
* TODO Appendices
|
||
|
||
** TODO Papers You Should have read
|