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Incomputing, an effect system is a formal system that 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.[1]
Some examples of the behaviors that can be described by effect systems include:
forall r1 r2, unit ! {read r1, read r2, write r1, write r2}.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).[2]
CanThrow capability.[9]throws — is available, there is no way to resume with a value, and they cannot be used with functions (only methods) unless the function implements a custom @FunctionalInterface.[10]{{cite book}}: CS1 maint: multiple names: authors list (link)
A benchmark comparing the performance of different free monad implementations.
