Effect system
In computing, an effect system is a formal system which describes the computational effects of computer programs, such as side effects. An effect system can be used to provide a compile-time check of the possible effects of the program. The effect system extends the notion of type to have an "effect" component, which comprises an effect kind and a region. The effect kind describes "what" is being done, and the region describes "with what" (parameters) it is being done. An effect system is typically an extension of a type system. The term "type and effect system" is  sometimes used in this case. Often, a type of a value is denoted together with its effect as "type ! effect", where both the type component and the effect component mention certain regions (for example, a type of a mutable memory cell is parameterized by the label of the memory region in which the cell resides). The term "algebraic effect" follows from the type system. Effect systems may be used to prove the external purity of certain internally impure definitions: for example, if a function internally allocates and modifies a region of memory, but the function's type does not mention  the region, then the corresponding effect may be erased from the function's effect. Examples. Some examples of the behaviors that can be described by effect systems include: From a programmer's point of view, effects are useful as it allows for separating the implementation ("how") of specific actions from the specification of what actions to perform. For example, an "ask name" effect can read from either the console, pop a window, or just return a default value. The control flow can be described as a blend of "yield" (in that the execution continues) and "throw" (in that an unhandled  effect propagates down until handled).