It has been very long time since my last blog post. During this period I have become big enthusiast of functional programming, especially using Haskell language. In this and next posts I am going to show that Haskell can be very pleasant to use and with proper tools we are able to develop applications without unnecessary burden.
This post is not an introduction to Haskell language. It is meant to describe how to setup Haskell with stack build tool and spacemacs as an editor. I am also planning to write a post about Haskell basics and its usage in my little project in series of the next posts.
The only necessary prerequisite is having the most recent version of Emacs installed on your system.
New project build/management tool – stack
What is more, stack can also download and setup locally Haskell compiler in version required by your project.
stack in action
stack new hello-haskell cd hello-haskell stack setup stack build stack exec hello-haskell
If you would like to play around with your project’s code you should type stack ghci in your terminal – this will launch Haskell interactive console – ghci – in version specified in project configuration.
Another stack command worth mentioning is stack test which executes test suites declared in test/ directory.
Dependencies and project settings are placed in hello-haskell.cabal file. It is standard cabal configuration file where we can add desired libraries, set project version, licence, link to the repository and so on. I suggest reading some cabal documentation if you have any doubts but in my opinion this file is very easy and straightforward to edit.
Settings specific for stack are placed in stack.yaml file. Most important option is resolver – which influences version of GHC compiler and libraries your project will be using.
There is one thing you might encounter while setting up project dependencies. What if you need library that is not present in any of stack resolvers? Well, then we must go to stack.yaml file and edit or add section:
extra-deps: - Vec-1.0.5
With this information stack will download and build desired package from hackage repositories. In my case I needed Vec library so I added it on a list with full name containing version number.
Powerful editor in new edition – spacemacs
In my opinion, it is really great feature as we can use this editor in the way we like more or is more convenient to us. Whether we are Vim-lovers or Emacs-fans or we want to mix them both – spacemacs allows to work in whatever style we like. I personally use mostly Vim-like mode with only few of original Emacs commands and with spacemacs shortcuts for many actions.
spacemacs is based on layers which add additional functionalities to editor. It can enrich our development environment with syntax completion, git integration, code completion and integration with build tools for many languages.
One of these layers is haskell layer. It supports this language quite well with syntax checking, code suggestions, built-in REPL and code templates for common patterns.
I refer to the official documentation for detailed installation instruction on various platform. After we are ready and spacemacs is on our disk, we can proceed.
Entire spacemacs configuration is placed in .spacemacs file in your home directory. This file is written in Lisp-like language and contains many options to change or add. Here is my current .spacemacs file on what this post section is based:
https://gist.github.com/rafalnowak/202aba0ee7986515345b
In dotspacemacs-configuration-layers we need to add haskell layer (I also recommend setting auto-completion and syntax-checking layers as well). In order to get layer to work properly, we need to install some additional packages:
stack install stylish-haskell hlint hasktags
(add-hook 'haskell-mode-hook 'turn-on-haskell-indentation) (add-to-list 'exec-path "~/.local/bin/")
It makes spacemacs aware of Haskell indentation style and adds binaries installed by stack to path. It is important as we want to make our editor able to run Haskell tools.
Full description, as well as platform specific problems, are listed in Haskell layer documentation: https://github.com/syl20bnr/spacemacs/tree/master/layers/%2Blang/haskell There is also a list of useful shortcuts used by this layer.
One essential note: if you wish to use spacemacs with ghc-mod integration, you will need to install ghc-mod at least in version 5.4.0.0. Previous versions do not work properly with Haskell layer and stack. To install ghc-mod in this version you must add cabal-helper-0.6.1.0 to your extra-deps section in stack.yaml and run
stack install ghc-mod
Which should proceed now without problems.
After this configuration we are ready to use all power of Haskell and stack in our projects. We will also have solid support from editor. If you have followed steps above, you will see that spacemacs is colouring Haskell syntax, checking its correctness and giving you code completion tips. There is also interactive console for Haskell available under SPC m s s keys combination which makes quick testing of new functions possible.
Unfortunately, there are some disadvantages of spacemacs. For me, the biggest drawdown is its responsivity. Sometimes during code completion or syntax checking it can hang application for a second or less.
Summary
As we could see, Haskell with stack and spacemacs is really powerful yet still simple to use. With stack we can achieve reproducible builds with specific compiler and libraries versions as well as easy project management. spacemacs allows us to create code quickly with support for Haskell syntax, build tools and code completion.
In my next post I am going to describe my experiences with my first bigger Haskell project – functional ray tracer I have been working on recently – https://github.com/rafalnowak/RaytracaH
