3.4 Using the libraries in your applications
3.4.1 Portability and the Standard ML Basis library
Some parts of the Standard ML Basis library are specific to the host platform. These are compiled Basis library units for structures and functors. Harlequin's implementations for UNIX and Windows both store these compiled units in a directory called system so that you can make portable references to them, as follows:
require "system.unit";