= Implementation module
An <implementation module> supplies the executable procedures and private declarations behind a <Modula-2> module interface. Its associated <definition module> determines which declarations clients may import. This separation lets clients depend on an interface while the implementation selects a representation.
For example, a counter module can export a procedure `Next` while storing a private running count. Its <implementation module> defines how `Next` updates and returns that count; clients need not know the private variable's name. Replacing the storage mechanism is possible when the exported behavior remains compatible. The GNU Modula-2 manual illustrates separate definition and implementation source <files>, then compiles implementation modules before <linking (computing)> them into a <software library>.<Building a shared library in the GNU Modula-2 documentation>[§2.14, a.def/a.mod and compilation/linking example]{ref}
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