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Persistent GHC powered background server for FAST Haskell development tools


hdevtools is a backend for text editor plugins, to allow for things such as syntax and type checking of Haskell code, and retrieving type information, all directly from within your text editor.

The advantage that hdevtools has over competitors, is that it runs silently in a persistent background process, and therefore is able to keeps all of your Haskell modules and dependent libraries loaded in memory. This way, when you change only a single source file, only it needs to be reloaded and rechecked, instead of having to reload everything.

This makes hdevtools very fast for checking syntax and type errors (runs just as fast as the :reload command in GHCi).

In fact, syntax and type checking is so fast, that you can safely enable auto checking on every save. Even for huge projects, checking is nearly instant.

Once you start using hdevtools and you get used to having your errors shown to you instantly (without having to switch back and forth between GHCi and your editor), and shown directly on your code, in your editor (without having to wait forever for GHC to run) you will wonder how you ever lived without it.

In addition to checking Haskell source code for errors, hdevtools has tools for getting info about identifiers, and getting type information for snippets of code.

Text Editor Integration

hdevtools is designed to be integrated into text editors. The list of current editor plugins that supply this integration is below.

But before you do anything, you must first install hdevtools itself. The easiest way is from Hackage via cabal install:

$ cabal install hdevtools

Then you should install one or more of the following editor plugins:

Vim - Syntastic

Syntastic is a popular syntax checking plugin for Vim, and is the recommended Vim integration of hdevtools syntax and type checking. Recent versions of Syntastic(since Sep. 2012) have builtin support for hdevtools.

Simply install hdevtools (as above) and Syntastic, and it will automatically check your Haskell files.

Syntastic will respect the g:hdevtools_options variable (the same one as used by vim-hdevtools, see below). See the section "Specifying GHC Options" below for details how to use it.

Vim - vim-hdevtools

In addition to Syntastic, it is recommended that you also use vim-hdevtools for additional functionality.

vim-hdevtools offers integration with the rest of the hdevtools tools, including retrieving info about the identifier under the cursor, and getting the type of the code under the cursor. Refer to its documentation for more details.

Emacs - flycheck

Flycheck is a modern batteries-included syntax checker for Emacs, and there is a flycheck-hdevtools checker available.

Manual Editor Integration for any Editor

Most editors allow you to run a make command, and will then parse the output for errors and show line numbers, allowing you to jump between errors.

The hdevtools check command is suitable for such usage.

For example, in Vim something like this will work:

:let &makeprg='hdevtools check %'

(Vim will replace the % character with the name of the current file). Then you can run


And Vim will invoke hdevtools to check the current file for errors, and then show a list of them and allow jumping to them.

See the "Command Line Usage" section below for more information.

Command Line Usage

Note: When using one of the above editor plugins, you don't really need to know this.

Available Commands and Help

For the list of commands available, run:

$ hdevtools --help

To get help for a specific command, run:

$ hdevtools [COMMAND] --help

For example:

$ hdevtools check --help

The hdevtools background process

The first time hdevtools runs a command, it will spawn a background process that will remain running forever. You can check the status of this background process by running:

$ hdevtools --status

You can shutdown the background process by running:

$ hdevtools --stop-server

Communication with the background process is done through a unix socket file. The default name is .hdevtools.sock, in the current directory. This allows you to use hdevtools with multiple projects simultaneously, without the background processes getting in the way of each other.

You can use a different socket file name with the --socket option, which should be used for each invocation of hdevtools. Remember that when telling hdevtools to check a Haskell file, paths are relative to the path of the background process, not your current directory. This can cause problems, and therefore it is recommended that you leave the socket file as the default, and always run hdevtools from the same directory.

Specifying GHC Options

For most non-trivial projects, you will need to tell hdevtools about additional GHC options that your project requires.

All hdevtools commands accept a -g flag for this purpose.

For example:

  • Your project source code is in the directory src
  • You want to use the GHC option -Wall
  • You want to hide the package transformers to prevent conflicts

Invoke hdevtools with something like this:

$ hdevtools check -g -isrc -g -Wall -g -hide-package -g transformers Foo.hs

Notice that a -g flag is inserted before each GHC option. Don't try to string multiple GHC options together after a single -g flag:

This won't work:

$ hdevtools check -g '-hide-package transformers' Foo.hs

The Vim plugins allow setting GHC options in the g:hdevtools_options variable. For example, for the above project, put the following in your .vimrc:

let g:hdevtools_options = '-g -isrc -g -Wall -g -hide-package -g transformers'

In general, you will need to pass to hdevtools the same GHC options that you would pass to GHCi.


hdevtools was inspired by ghcmod, but has the advantage that due to its client-server architecture it is much faster.