111 lines
4.6 KiB
Markdown
111 lines
4.6 KiB
Markdown
---
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title: Using GHC IDE for Haskell Error Checking and Autocompletion
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date: 2020-01-06T17:07:25-08:00
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tags: ["Haskell", "Language Server Protocol"]
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---
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Last year, when I took Oregon State University's CS 381 class, I ended up setting
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up my editor with the Haskell IDE engine. This made it possible
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to detect errors, view types, and have good autocompletion within the editor itself.
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Recently, I've found that GHC IDE works better for my projects, so instead
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of butchering the original article, I'll just quickly write an updated version here,
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referencing the old one when necessary.
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By the end of the article, your editor should be able to detect errors and
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properly autocomplete Haskell code, somewhat like in the below screenshot:
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### Downloading and Installing GHC IDE
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GHC IDE is a Haskell-based program that uses the
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{{< sidenote "right" "lsp-note" "language server protocol" >}}
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You don't really need to know what the language server protocol (LSP) is
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to use GHC IDE. If you are nonetheless interested, I wrote a little
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bit about it <a href="{{< relref "/blog/haskell_language_server" >}}#prelude-language-server-protocol">in the previous iteration of this post.</a>
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If you want more information, check out the <a href="https://microsoft.github.io/language-server-protocol/">official Microsoft page on LSP.</a>
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{{< /sidenote >}} to communicate with any editor that supports it. Editors
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with support the the LSP include Atom, Visual Studio Code, Emacs, and Vim. Thus,
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You can get a good Haskell development environment without tying yourself to one
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application or service.
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We first want to download the GHC IDE. To do this, you need to have
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[Git](https://git-scm.com/) installed. Once you have that, in your Git bash (on Windows)
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or in your terminal (maxOS, Linux), type the command:
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```
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git clone https://github.com/digital-asset/ghcide.git
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```
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To install GHC IDE, you can use either `cabal` (which is typically the `cabal-install` package,
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and is required normally for this class) or `stack` (a build tool). For `cabal`:
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```
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cabal install
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```
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And for `stack`:
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```
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stack install
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```
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This will create an executable in your `~/.local/bin` directory. By default, this
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is not usable from other programs, such as Vim, so you should add this directory
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to your path. On Linux and macOS, this is done by adding the following line
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to your `.bashrc` file (or equivalent):
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```
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export PATH=$PATH:/home/<yourusername>/.local/bin
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```
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On Windows, this is done by
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{{< sidenote "right" "path-note" "editing your PATH variable." >}}
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If you need to know how to change your <code>PATH</code>, I wrote
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about it briefly in the <a href="{{< relref "/blog/haskell_language_server" >}}
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#installation-of-v0-5-0-0-windows-systems">previous iteration of this post.</a>
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{{< /sidenote >}} I don't run Windows,
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so I don't know where `cabal install` will place the executable, but I do know
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where the executable will appear if you use `stack install` - in the directory
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given by:
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```
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stack path --local-bin
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```
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Adding that to your path should be sufficient to use GHC IDE.
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### Setting up Your Editor
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This is where the paths diverge. I personally use (Neo)vim, but for the sake
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of completeness, I'll go over installation for Atom and VSCode (I'm not including
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Emacs because I know nothing about configuring Emacs).
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#### Atom
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There appears to be an Atom extension specifically for GHC IDE:
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[ide-haskell-ghcide](https://atom.io/packages/ide-haskell-ghcide). It doesn't
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have a lot of configuration options, and will certainly require GHC IDE to
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be in your path. However, since both GHC IDE and the Haskell IDE engine
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use the Language Server Protocol, the more mature [ide-haskell-hie](https://atom.io/packages/ide-haskell-hie) extension may work, as well. In fact, since `ide-haskell-ghcide` is so young,
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I'd recommend trying `ide-haskell-hie` first, configuring the settings (found under
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_Settings > Packages > (Search ide-haskell-hie) > Settings_)
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to use the following full path:
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```
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<output of stack path --local-bin>/ghcide
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```
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#### VSCode
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The team behind GHC IDE maintains an official VSCode extension found
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[here](https://marketplace.visualstudio.com/items?itemName=DigitalAssetHoldingsLLC.ghcide).
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Installing it, when you have GHC IDE also installed, should be sufficient to get
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VSCode to autocomplete and error check.
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#### (Neo)vim
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My original recommendations for (neo)vim remain unchanged, with the exception
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of using `ghcide` instead of `hie` in the `serverCommands` variable. You
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can find the original instructions
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[here](https://danilafe.com/blog/haskell_language_server/#neovim).
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### Conclusion
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I hope that using GHC IDE, you'll be able to have a significantly more pleasant
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Haskell experience in CS 381. Enjoy!
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