How do you set up Haskell + ghc-mod on OS/X?

Question

I first set up Haskell from https://www.haskell.org/platform/ but ghc-mod had problems with sandboxes and to update it I needed a newer cabal which needed a newer ghc. Or something like that. (https://stackoverflow.com/a/28049104/131227 ??)

So I deleted everything, and installed from http://ghcformacosx.github.io/. Then some strange failure setting up a new sandbox was because something didn't properly have a dependancy for happy. (https://github.com/haskell-suite/haskell-src-exts/issues/14)

Ok. Installed happy. Now trying (again) to install ghc-mod and I get a giant mess (below).

Should I use http://www.stackage.org/?
I've seen some people mention ghc-pkg recache or cabal install cabal-install... Are those things I need to do?

Mess:

Resolving dependencies... Configuring ghc-mod-5.2.1.2... Building ghc-mod-5.2.1.2... Failed to install ghc-mod-5.2.1.2 Build log ( /Users/mark/.cabal/logs/ghc-mod-5.2.1.2.log ): Configuring ghc-mod-5.2.1.2... Building ghc-mod-5.2.1.2... Preprocessing library ghc-mod-5.2.1.2...

Language/Haskell/GhcMod/Convert.hs:1:51: Warning:
    -XOverlappingInstances is deprecated: instead use per-instance pragmas OVERLAPPING/OVERLAPPABLE/OVERLAPS [ 1 of 38] Compiling Language.Haskell.GhcMod.Read ( Language/Haskell/GhcMod/Read.hs, dist/build/Language/Haskell/GhcMod/Read.o ) [ 2 of 38] Compiling Language.Haskell.GhcMod.Cabal21 ( Language/Haskell/GhcMod/Cabal21.hs, dist/build/Language/Haskell/GhcMod/Cabal21.o ) [ 3 of 38] Compiling Language.Haskell.GhcMod.Cabal18 ( Language/Haskell/GhcMod/Cabal18.hs, dist/build/Language/Haskell/GhcMod/Cabal18.o ) [ 4 of 38] Compiling Language.Haskell.GhcMod.Cabal16 ( Language/Haskell/GhcMod/Cabal16.hs, dist/build/Language/Haskell/GhcMod/Cabal16.o ) [ 5 of 38] Compiling Language.Haskell.GhcMod.GHCChoice ( Language/Haskell/GhcMod/GHCChoice.hs, dist/build/Language/Haskell/GhcMod/GHCChoice.o ) [ 6 of 38] Compiling Language.Haskell.GhcMod.Error ( Language/Haskell/GhcMod/Error.hs, dist/build/Language/Haskell/GhcMod/Error.o )

Language/Haskell/GhcMod/Error.hs:12:1: Warning:
    Module ‘Control.Monad.Error’ is deprecated:
      Use Control.Monad.Except instead

Language/Haskell/GhcMod/Error.hs:40:10: Warning:
    In the use of type constructor or class ‘Error’
    (imported from Control.Monad.Error, but defined in Control.Monad.Trans.Error):
    Deprecated: "Use Control.Monad.Trans.Except instead"

Language/Haskell/GhcMod/Error.hs:40:10: Warning:
    In the use of type constructor or class ‘Error’
    (imported from Control.Monad.Error, but defined in Control.Monad.Trans.Error):
    Deprecated: "Use Control.Monad.Trans.Except instead" [ 7 of 38] Compiling Language.Haskell.GhcMod.Utils ( Language/Haskell/GhcMod/Utils.hs, dist/build/Language/Haskell/GhcMod/Utils.o )

Language/Haskell/GhcMod/Utils.hs:15:1: Warning:
    The import of ‘Control.Applicative’ is redundant
      except perhaps to import instances from ‘Control.Applicative’
    To import instances alone, use: import Control.Applicative()

Language/Haskell/GhcMod/Utils.hs:45:51: Warning:
    In the use of ‘strMsg’
    (imported from Language.Haskell.GhcMod.Error, but defined in Control.Monad.Trans.Error):
    Deprecated: "Use Control.Monad.Trans.Except instead" [ 8 of 38] Compiling Language.Haskell.GhcMod.Types ( Language/Haskell/GhcMod/Types.hs, dist/build/Language/Haskell/GhcMod/Types.o ) [ 9 of 38] Compiling Language.Haskell.GhcMod.Gap ( Language/Haskell/GhcMod/Gap.hs, dist/build/Language/Haskell/GhcMod/Gap.o )

Language/Haskell/GhcMod/Gap.hs:256:18:
    Not in scope: data constructor ‘ExposePackageId’
    Perhaps you meant ‘ExposePackage’ (imported from DynFlags) cabal: Error: some packages failed to install: ghc-mod-5.2.1.2 failed during the building phase. The exception was: ExitFailure 1

After applying the accepted answer (which seemed to install ghc-mod), running gch-mod gives the following:

> ghc-mod check x.hs
cabal-helper-wrapper: Installing a private copy of Cabal, this might take a
while but will only happen once per Cabal version.

If anything goes horribly wrong just delete this directory and try again:
    /Users/mark/.ghc-mod/cabal-helper

If you want to avoid this automatic installation altogether install version
1.22.0.0 of Cabal manually (into your user or global package-db):
    $ cabal install Cabal-1.22.0.0
...In order, the following would be installed:
filepath-1.3.0.1 (latest: 1.4.0.0) (new version)
directory-1.2.2.1 (new version)
process-1.2.3.0 (reinstall) changes: directory-1.2.2.0 -> 1.2.2.1,
filepath-1.4.0.0 -> 1.3.0.1
Cabal-1.22.0.0 (latest: 1.22.2.0) (new version)
cabal: The following packages are likely to be broken by the reinstalls:
ghc-7.10.1
Cabal-1.22.2.0
Use --force-reinstalls if you want to install anyway.
cabal-helper-wrapper: Installing Cabal version 1.22.0.0 failed.
nYou have two choices now:
- Either you install this version of Cabal in your globa/luser package-db
  somehow
n- Or you can see if you can update your cabal-install to use a different
  version of the Cabal library that we can build with:
    $ cabal install cabal-install --constraint 'Cabal > 1.22.0.0'
nTo check the version cabal-install is currently using try:
    $ cabal --version

ghc-mod: readCreateProcess: /Users/mark/.cabal/libexec/cabal-helper-wrapper "/Users/mark/work/haskell/ixberg/dist" "entrypoints" "source-dirs" "ghc-options" "ghc-src-options" "ghc-pkg-options" "--with-ghc=ghc" "--with-ghc-pkg=ghc-pkg" "--with-cabal=cabal" (exit 1): failed

Strangely, it wants the version of Cabal that is currently installed:

> cabal --version
cabal-install version 1.22.0.0
using version 1.22.0.0 of the Cabal library

Answer 1

Edit (August 2015) Better fix

A better command line tool for installing Haskell binaries and libraries by the awesome FPComplete group called stack means you can install ghc-mod with stack install ghc-mod (outside a project to make it global) and it will just work. The binary will be installed to ~/.local/bin and should be put on your path so your editor can find it.

I will leave the below for anyone who wishes to know what cabal Hell really meant.

Problem With DanielG's Fork

I asked on the Haskell reddit and NihillstDandy explained that the GHC for Mac OS X does not register the Cabal library. This is not true for your install from http://ghcformacosx.github.io/. It does register the Cabal library.

ghc-mod needs the cabal library to work, but it does not compile with the cabal library. Instead it compiles with cabal-helper and that looks to see if you have a registered version of the cabal library. If it does not find any then cabal-helper-wrapper will install a private copy and in this case it tired to install the same version as what cabal-install was compiled with, cabal-1.22.0.0.

When I installed DanielG's Fork it worked for my current project, but after testing it in other projects I am getting the same error you are. This is a problem with the fork and not Haskell (as far as I can tell).

---

Original Answer

So, while I was putting this post together ghc-mod was fixed for GHC-7.10. Thanks to DanielG!

• git clone https://github.com/DanielG/ghc-mod
• cd ghc-mod
• cabal install -j4

And if you get errors that say "setup-Simple-Cabal-1.22.2.0-x86_64-osx-ghc-7.10.1: The program 'happy' version >=1.17 is required but it could not be found."

• cabal install happy -j4

The same for any other problems and it should work. (Although you have happy installed, so it should just work).

---

The rest of the post is on how to downgrade the cabal executable, also known as cabal-install, to version 1.20.0.3. Although you could really use this to downgrade to any version. Since there is a working fork of ghc-mod that works with GHC-7.10 and cabal-1.22 you do not need to downgrade to fix ghc-mod.

I'm just leaving the rest of the post up here since, unfortunately, this is a problem with Haskell as a whole. Halcyon and nix are ways to deal with Haskell's shortcomings in this regard.

---

How to downgrade Cabal in Haskell

When I wrote this post ghc-mod, nor any fork, compiled with GHC-7.10 and versions below 7.10 broke with cabal-1.22 sandboxes.

However, even if you compile ghc-mod with GHC-7.8.4 (the version before 7.10) and cabal-1.20.0.3 you still cannot use ghc-mod inside a sandbox created by cabal-1.22 and above. So the only way to use ghc-mod is to either downgrade your entire system to cabal-1.20.0.3 or don't use sandboxes. This means you can still use GHC-7.10, but you have to use the older version of cabal. That is unless you want to wait until the convener or someone else fixes it for GHC-7.10. Its been over a week already, but some kind Haskell programmer fixed it in a fork (see above).

---

If this is something you still want here is how to compile ghc-mod with GHC-7.8.4 and cabal-1.20.0.3. Also, I replace the executable cabal-1.22 with the older cabal-1.20.0.3. Note, it does not matter what directory you download the sources to.

• download GHC-7.8.4 source for your OS from here
• tar -xf path_to_zipped_source
• cd path_to_ghc-7.8.4

The next steps will install ghc-7.8.4 as ghc-7.8.4 in the same folder as ghc-7.10 is install for you. The -j4 is to tell make to use 4 threads to compile it.

• ./configure
• make install -j4

To get the correct version of cabal we can just ask cabal to get it.

• cabal get cabal-install-1.20.0.3
• cd cabal-install-1.20.0.3

Now to install cabal with ghc-7.8.4. Here cabal-1.22 will pull all the decencies and should give you some warning about installing another version of the cabal library (not to be confused with cabal-install), this should not cause problems.

• cabal --with-compiler=ghc-7.8.4 install

So now you should have cabal-1.20.0.4 installed, but not on your path. It will be in ~/.cabal/bin. We need to make this global, so remove cabal-1.22 (it only removes the symlink).

• rm `which cabal`

And create the symlink to cabal-1.20.0.4. Btw, you will need to remove any sandboxes you made with cabal-1.22 and remake them with cabal-1.20.0.4.

If you do not have realpath you can just type in the full path of cabal. I use it for connivence.

• cd ~/.cabal/bin
• ln -s `realpath cabal` /usr/local/bin

Now this is how I compiled ghc-mod on my mac (you may run into your own problems). I'm not sure why cabal couldn't do it in one call, but this worked in the end.

• cabal --with-compiler=ghc-7.8.4 install happy
• cabal --with-compiler=ghc-7.8.4 install haskell-src-exts-1.16.0.1
• cabal --with-compiler=ghc-7.8.4 install hlint-1.9.19
• cabal --with-compiler=ghc-7.8.4 install ghc-mod

Now you should be able to call ghc-mod from anywhere. To test it just type ghc-mod after you have cabal sandbox init. If it does not throw any errors then your in the clear.

Answer 2

The version of executable cabal that is used to configure this project should be the same as the version of the global Cabal package.

It's easier to install a compatible version of cabal-install than Cabal as follows:

$ ghc-pkg list | grep Cabal
    Cabal-1.22.2.0
$ cabal install cabal-install-1.22.2.0

Or build one in sandbox and replace/shadow the original one.

Then don't forget to re-configure the project with the new cabal:

$ cabal clean && cabal configure

Now the ghc-mod should works.