An effect system is a formal system which describes the computational effects of computer programs, such as side effects. In formal logic, a formal system (also called a logical system, a logistic system, or simply a logic Formal systems in mathematics consist In Computer science, a function or expression is said to produce a side effect if it modifies some state in addition to returning a value An effect system can be used to provide a compile-time checking of the possible effects of the program.
An effect system is typically an extension of a type system. In Computer science, a type system defines how a Programming language classifies values and expressions into '''types''', how it can The term "type and effect system" is sometimes used in this case.
References
C. Hankin, F Nielson, H. R. Nielson. Principles of Program Analysis. ISBN 3-540-65410-0