MLton runs fine on Darwin (and on Mac OS X).
Notes
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MLton requires the GMP library, which is available via Homebrew, MacPorts, and Fink. n 
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For Intel-based Macs, MLton targets the AMD64 architecture on Darwin 10 (Mac OS X Snow Leopard) and higher and targets the x86 architecture on Darwin 8 (Mac OS X Tiger) and Darwin 9 (Mac OS X Leopard). 
Known issues
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Executables that save and load worlds on Darwin 11 (Mac OS X Lion) and higher should be compiled with -link-opt -fno-PIE; see MLtonWorld for more details.
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ProfilingTime may give inaccurate results on multi-processor machines. The SIGPROFsignal, used to sample the profiled program, is supposed to be delivered 100 times a second (i.e., at 10000us intervals), but there can be delays of over 1 minute between the delivery of consecutiveSIGPROFsignals. A more complete description may be found here and here.