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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 checking 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'' 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). Some examples of the behaviors that can be described by effect systems include: * Reading, writing or allocating memory: the effect kind is ''read'', ''write'', ''allocate'' or ''free'', and the region is the point of the program where allocation was performed (i.e., each program point where allocation is performed is assigned a unique label, and region information is statically propagated along the dataflow). Most functions working with memory will actually be polymorphic in the region variable: for example, a function that swaps two locations in memory will have type ''forall r1 r2, unit ! ''. * Working with resources, such as files: for example, the effect kind may be ''open'', ''read'' and ''close'', and again, the region is the point of the program where the resource is opened. * Control transfers with continuations and long jumps: the effect kind may be ''goto'' (i.e. the piece of code may perform a jump) and ''comefrom'' (i.e. the piece of code may be the target of a jump), and the region denotes the point of the program from which or to which the jump may be performed. * Java's checked exceptions are an example of an effect system: the effect kind is ''throws'' and the region is the type of the exception being thrown. 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. ==References== ; Textbook chapters * * ; Overview papers * 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Effect system」の詳細全文を読む スポンサード リンク
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