MLton's -target flag directs MLton to cross compile an application for another platform. By default, MLton is only able to compile for the machine it is running on. In order to use MLton as a cross compiler, you need to do two things.

1. Install the GCC cross-compiler tools on the host so that GCC can compile to the target.

2. Cross compile the MLton runtime system to build the runtime libraries for the target.

To make the terminology clear, we refer to the host as the machine MLton is running on and the target as the machine that MLton is compiling for.

To build a GCC cross-compiler toolset on the host, you can use the script bin/build-cross-gcc, available in the MLton sources, as a template. The value of the target variable in that script is important, since that is what you will pass to MLton's -target flag. Once you have the toolset built, you should be able to test it by cross compiling a simple hello world program on your host machine.

% gcc -b i386-pc-cygwin -o hello-world hello-world.c

You should now be able to run hello-world on the target machine, in this case, a Cygwin machine.

Next, you must cross compile the MLton runtime system and inform MLton of the availability of the new target. The script bin/add-cross from the MLton sources will help you do this. Please read the comments at the top of the script. Here is a sample run adding a Solaris cross compiler.

% add-cross sparc-sun-solaris sun blade
Making runtime.
Building print-constants executable.
Running print-constants on blade.

Running add-cross uses ssh to compile the runtime on the target machine and to create print-constants, which prints out all of the constants that MLton needs in order to implement the Basis Library. The script runs print-constants on the target machine (blade in this case), and saves the output.

Once you have done all this, you should be able to cross compile SML applications. For example,

mlton -target i386-pc-cygwin hello-world.sml

will create hello-world, which you should be able to run from a Cygwin shell on your Windows machine.

Cross-compiling alternatives

Building and maintaining cross-compiling gcc's is complex. You may find it simpler to use mlton -keep g to generate the files on the host, then copy the files to the target, and then use gcc or mlton on the target to compile the files.