Building a GHC cross-compiler for Apple iOS targets

Xcode 5 Notes

These instructions are for Xcode 5, which is now what we're aiming to support as Xcode 4 is no longer available.

If you use ghc-7.6.3 on OS X as your bootstrap compiler, you'll need to pass certain options to clang to work around some problems. A wrapper for this, written in Haskell, can be found at

To use it, compile clang-xcode5-wrapper.hs, add it to your path, then edit /usr/local/lib/ghc-7.6.3/settings and change "C compiler command"'s value to "clang-xcode5-wrapper".

(The problem is that 1. GHC tries to run a C pre-processor over Haskell code, and 2. Xcode versions >= 5 use clang instead of gcc, and 3. clang doesn't like Haskell code.).

GHC 7.8 already includes a fix for this, but until that's released you'll have to use this.


1. Read ARM-specific notes

You must use LLVM 3.0's versions of opt and llc — there seem to be issues with other versions. It's easiest to just download the binaries and put them in your path:

See Cross-compiling GHC for more details.

2. Scripts

You will need to check out the scripts at and add the checked out directory to your PATH. You may need to edit these scripts if you are using a different iOS / iOS simulator platform version than the one the scripts are pointed at.

3. Check out GHC

Check out as described at Building and Porting GHC, except:

  • replace libffi with a newer version that fixes some bugs
rm libffi-tarballs/libffi-3.0.11.tar.gz
wget -O libffi-tarballs/libffi-3.0.14.tar.gz
perl boot

4. Create a file

GHC requires you to write a mk/ file — we've integrated the correct configuration into, so just copy/rename that to and uncomment the line:

BuildFlavour  = quick-cross

for a faster build without profiling support, or

BuildFlavour  = perf-cross


5. Configure & build

For iOS:

./configure --target=arm-apple-darwin10 --with-gcc=arm-apple-darwin10-clang
sudo make install

For the iOS simulator:

./configure --target=i386-apple-darwin11 --with-gcc=i386-apple-darwin11-clang
sudo make install

GHC is smart enough to prefix the binaries and libraries with the target name, e.g. arm-apple-darwin10-ghc, so this will install alongside your native GHC without overwriting anything.

6. Make sure your Cabal and cabal-install are new enough

Check out the latest version of Cabal from, and build both Cabal and cabal-install.

You must remove an option that the latest version of Cabal will add to your ~/.cabal/config file:

jobs: $ncpus

which triggers an "external" build method that doesn't yet work with cross-compilation (you might see strange errors like "No cabal file found" or other mysterious failures). Commenting it out should fix it.

The ghc-ios-scripts directory you checked out earlier contains two wrappers called arm-apple-darwin10-cabal and i386-apple-darwin11-cabal. These will pass the right arguments to cabal, so you can do, for example:

arm-apple-darwin10-cabal install text

If you get errors like "Could not find module Prelude" when installing cabal packages, you probably have cabal's library profiling option on, which the quick-cross BuildFlavour above doesn't support. You can either disable it by setting

library-profiling: False

in your ~/.cabal/config file, or by passing the --disable-library-profiling like

i386-apple-darwin11-cabal install text --disable-library-profiling

or rebuild using the perf-cross BuildFlavour from Step 4.

7. Make sure hsc2hs is new enough

The easiest way to do this is as follows, in the ghc build directory:

cd utils/hsc2hs
cabal install

8. Create an Xcode project

Create a new skeleton Xcode project using the wizard, and make sure it runs on your device.

9. Compile your Haskell code

Here's a skeleton haskell.hs to get you started:

{-# LANGUAGE ForeignFunctionInterface #-}

import Foreign
import Foreign.C

foreign import ccall safe "c_main" c_main :: IO ()

main = do
    putStrLn "Haskell start"

The main() function in main.m must be changed to something like this, because Haskell's main now runs first.

int c_main(void)
    int argc = 1;
    char* argv[2];
    argv[0] = "dummy";
    argv[1] = NULL;		
    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));

Now compile it:

arm-apple-darwin10-ghc -staticlib haskell.hs -threaded

Ignore the copious warnings about "truncation and blank padding" and "has no symbols" (until someone fixes them).

This will create (in this example) haskell.a.

(n.b. we're only modifying main for the sake of example — you can also initialize Haskell when the Objective-C runtime loads, as shown in and then call Haskell at will.)

10. Set up your Xcode project for Haskell

Now configure it as follows:

  • Click on the top node in the project tree, then go to the Build Settings tab. Set Dead Code Stripping to No. This is needed because GHC generates "tables next to code", and without this setting, Xcode thinks the tables are dead code and strips them, causing a crash.
  • Also in "Build Settings", change "Architectures" to "Standard Architectures (armv7, armv7s)" (as opposed to "(including 64-bit)") as Apple hasn't yet merged Arm64 support into the LLVM source that we'd need to fully support 64bit iOS devices. The code will still run wonderfully on the 5S/iPad Air/mini Retina in 32bit mode.
  • Click on the top node in the project tree, then go to the Build Phases tab. Click on Link Binary With Libraries to open it then click +. Choose libiconv.dylib then click Add.
  • When you've compiled your Haskell code to a .a (e.g. haskell.a) file, add it to the project anywhere in the hierarchy with Add files to (project) in the right-mouse button menu.

11. Build and run

Run the project again as usual, and Xcode will pick up the haskell.a file and your Haskell code should now run on your iOS device. Anything printed with putStrLn will appear in the Xcode runtime console.

Each time you modify your Haskell code you'll need to re-compile from the command line before re-building in Xcode. It is possible to automate this in Xcode if you wish.

12. Next steps

Take a look at for patched versions of Hackage packages and other useful things. Ask us if you want to join the ghc-ios project on GitHub, and feel free to raise bugs there.

The Haskell-iPhone mailing list is at

Loose ends

Outstanding issues we should fix in rough priority order.

  • Fat binaries (done by lukexi)
  • Cross-compiler for the iOS simulator (done by lukexi)
  • Template Haskell for cross compilers! Could be done by (in order of increasing complexity):
  • Packaging with the wrapper scripts and perhaps release of binaries of official ghc releases
  • Would be nice to not have to disable dead-code removal. (Simon Marlow says "we have special hacks so that you don't have to disable dead-code removal on OS X, in the native code generator and (I presume) in the LLVM backend. Perhaps this just needs to be adapted to work on iOS too?") (done by angerman, part of 7.10)
  • Fix the copious link warnings
  • Stop llvm generating an unnecessary 'bx lr' (return) instruction after the GHC calling convention (which is actually a goto)
  • Programs outputting substantial text to the console can cause hangs in the RTS. (fixed by lukexi in ticket #8307).

Wish List

  • ARM64 support (need support in LLVM, llvmCodeGen, Platform.hs, and calling conventions)
  • SIMD vectorization port of the SSE/AVX SIMD infrastructure
Last modified 4 years ago Last modified on Jul 21, 2015 9:46:20 AM