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IDE library for Haskell based on the GHC API (GHC 7.0 fork)

branch: ghc_7_0


Scion is a Haskell library that aims to implement those parts of a Haskell IDE which are independent of the particular front-end. Scion is based on the GHC API and Cabal. It provides both a Haskell API and a server for non-Haskell clients such as Emacs and Vim.


(For developer builds see section "Hacking" below.)

Scion requires GHC 6.10.1 or later. All other dependencies should be on Hackage and can be installed using cabal-install. Scion consists of a library and a server which is used by front-ends that are not written in Haskell.

To install the library and server use:

$ cd dir/to/scion
$ cabal install

This will install the executable scion_server in the bin directory of cabal-install, typically $HOME/.cabal/bin.

If you do not want to install the server (and its dependencies), turn off the "server" flag which is enabled by default:

$ cabal install -f-server

In order to use scion with your favourite front-end, see the specific instructions for the front-end below. The Emacs and Vim front-ends are included with Scion and their installation instruction follow below. The necessary files are installed with Scion by default and there is currently no option to turn this off.

Bug Reports

Please send bug reports or feature requests to the Issue tracker.


Since Scion is a library, you should consult the haddock documentation for how to use it. However, you may look at the Emacs frontend for inspiration.


Scion's Emacs mode should be seen as complimentary to the existing Haskell mode. To use it install the Scion server as described above. In the following we'll assume that the server has been install as:

$ ~/.cabal/bin/scion-server

Add the following to your emacs configuration (typically "~/.emacs"):

;; Substitute the desired version for <version>
(add-to-list 'load-path "~/.cabal/share/scion-<version>/emacs")
(require 'scion)

;; if ./cabal/bin is not in your $PATH
(setq scion-program "~/.cabal/bin/scion-server")

(defun my-haskell-hook ()
  ;; Whenever we open a file in Haskell mode, also activate Scion
  (scion-mode 1)
  ;; Whenever a file is saved, immediately type check it and
  ;; highlight errors/warnings in the source.
  (scion-flycheck-on-save 1))

(add-hook 'haskell-mode-hook 'my-haskell-hook)

;; Use ido-mode completion (matches anywhere, not just beginning)
;; WARNING: This causes some versions of Emacs to fail so badly
;; that Emacs needs to be restarted.
(setq scion-completing-read-function 'ido-completing-read)

Scion mode needs to communicate with the external server. By default the server will be started automatically when needed. See "Manually Connecting to Scion" below for how to connect to the server manually.

Scion uses Cabal as a library which in turn might look for external programs such as happy or alex. In order to find these, the PATH environment variable has to be set up correctly.

The scion server process inherits the environment variables from the Emacs process. Depending on your system this may be different than what you would get if you started the server from the shell. To adjust the PATH environment variable from within Emacs, add something like the following to your .emacs:

;; add ~/usr/bin to the PATH
(setenv "PATH" "$HOME/usr/bin:$PATH" t)

Once you have a running and connected Scion server, you can use the commands provided by scion-mode:

  • C-c C-x C-l (scion-load) load the current file with Scion. If the file is within a Cabal project this will prompt to use the settings from one of the components in the package description file. You can still choose to load only the current file.

  • C-c C-o (scion-open-cabal-project) configures a Cabal project and loads the meta-data from a Cabal file. Note that this does not type check or load anything. If you change the Cabal file of a project, call this function to update the session with the new settings.

If loading generates any errors or warnings, a buffer will appear and list them all. Pressing RET on a note will jump to its source location. Pressing q closes the buffer, and C-c C-n (scion-list-compiler-notes) brings it back. Use M-n (scion-next-note-in-buffer) and M-p (scion-previous-note-in-buffer) to navigate within the notes of one buffer.


The following commands offer completion for a few things.

  • C-c i l (haskell-insert-language) asks for a LANGUAGE pragma and adds it to the top of the file.

  • C-c i p (haskell-insert-pragma) inserts a pragma at the current cursor position. (At the moment this doesn't try to make sense of the selected pragma, however.)

  • C-c i m (haskell-insert-module-name) inserts the name of an external module (external), i.e., a module not from the current package.

  • C-c i f (haskell-insert-flag) insert (GHC) command line flag at point. (Really only makes sense within an OPTIONS_GHC pragma.)

Experimental features

The following should work most of the cases.

  • C-c C-. (scion-goto-definition) jumps to the definition of the identifier at point. If there is no identifier at point, offers a list to complete on a particular identifier. This currently only works for identifiers defined within the same project.

  • C-c C-t shows type of identifier at point. This only works if the current file typechecks, but then it also works for local identifiers. For polymorphic function it will show the type to which they are instantiated, e.g.,

    f x = x + (1::Int)

    Calling this command on + will print Int -> Int -> Int instead of Num a => a -> a -> a.

Manually Connecting to Scion

If you set the variable scion-auto-connect to 'ask (the default is 'always), Scion will ask whether to start the server. If you set it to nil you need to manually connect to the server.

You can start the server manually on the command line and then use

M-x scion-connect

to connect to that server. However, most of the time it will be more convenient to start the server from within Emacs:

M-x scion


Vim backend code is hosted on


For discussions about Scion use the scion-lib-devel mailing list.


The main repository for Scion is hosted on Github. Get it via

$ git clone git://

Send patches or pull requests to nominolo (email address at googlemail dot com). Note that, if you fork the project on Github your fork won't take up additional space on your account.


For development it is probably easier to use the GNU Make than Cabal directly. The makefile includes a file called which is not present by default. You can use the provided and edit it:

$ cp
$ edit

After that, the makefile takes care of the rest.

$ make           # configure and build
$ make install   # configure, build, and install

Using an in-place GHC

GHC 6.10.1 has a couple of problems. For example, not all error messages are reported using the GHC API but instead are printed to stdout/stderr. Some parts also call exitWith directly. GHC's HEAD branch has some of these bugs fixed and may contain new features not present in the stable branch. If you want to compile against an inplace GHC, the following steps should work:

  1. On Windows, make sure that Cabal finds the inplace gcc

    $ cd /path/to/ghc
    $ cp `which gcc` ghc/

    (Adjust to version of GCC that GHC was compiled with.)

  2. Set the GHC_PATH variable to the correct path to for your system. Make sure not to set HC, PKG, or HADDOCK, they will automatically be set to point to the inplace versions.

  3. Use make.


The parts of Scion written in Haskell are licensed under the BSD license. The Emacs lisp parts are licensed under the GPL license version 2 or (at your option) any later version.

Known Pitfalls

If you get an error message like this: "scion_server: mkTopLevEnv: not interpreted main:Main" then you should rm [Ss]etup.hi [Ss]etup.o in the project directory.

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