Whole-program optimization is a compilation technique in which optimizations operate over the entire program. This allows the compiler many optimization opportunities that are not available when analyzing modules separately (as with separate compilation). Most of MLton's optimizations are whole-program optimizations. Because MLton compiles the whole program at once, it can perform optimization across module boundaries. As a consequence, MLton often reduces or eliminates the run-time penalty that arises with separate compilation of SML features such as functors, modules, polymorphism, and higher-order functions. MLton takes advantage of having the entire program to perform transformations such as: defunctorization, monomorphisation, higher-order control-flow analysis, inlining, unboxing, argument flattening, redundant-argument removal, constant folding, and representation selection. Whole-program compilation is an integral part of the design of MLton and is not likely to change.