[tsuyoshi]

Another issue, comparing the MLton FFI and the Ocaml FFI, is that the FFI in MLton is somewhat slower. When I converted an Ocaml program to SML, I discovered that a small portion written in C ran much faster when I rewrote it in SML. In Ocaml, writing portions in C is almost always a performance win, just because the Ocaml compiler doesn't optimize very well. In MLton there's a bit of overhead just for the FFI (possibly because the code generator allocates registers very differently from how it's done in C). That overhead goes away if you use the C code generator instead, but usually the native code generator performs better anyway.

If I remember correctly, it has been proposed to rewrite the MLton floating point conversion in pure SML for precisely this reason.