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With modular programming, [[separation of concerns|concerns are separated]] such that modules perform logically discrete functions, interacting through well-defined interfaces. Often modules form a [[directed acyclic graph]] (DAG); in this case a cyclic dependency between modules is seen as indicating that these should be a single module. In the case where modules do form a DAG they can be arranged as a hierarchy, where the lowest-level modules are independent, depending on no other modules, and higher-level modules depend on lower-level ones. A particular program or library is a top-level module of its own hierarchy, but can in turn be seen as a lower-level module of a higher-level program, library, or system.
When creating a modular system, instead of creating a monolithic application (where the smallest component is the whole), several smaller modules are written separately so when they are composed together, they construct the executable application program. Typically, these are also [[Compiler|compiled]] separately, via [[separate compilation]], and then linked by a [[Linker (computing)|linker]]. A [[Just-in-time compilation|just-in-time compiler]] may perform some of this construction "on-the-fly" at [[Run time (program lifecycle phase)|run time]].
These independent functions are commonly classified as either program control functions or specific task functions. Program control functions are designed to work for one program. Specific task functions are closely prepared to be applicable for various programs.
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