What is the Effect pattern in the context of functional programming?
- The program contains, in its type, what kinds of effects it performs, and what it returns, e.g. what type of value it will result in.
- What external effects it requires (if any). We separate the execution from the description of the effects, opting to perform the execution at the outermost layer so we can keep referential transparency inside.
It’s basically a description of a type of computation.
Examples of types that conform to this pattern - Option, Either
One of the most popular implementation of this pattern for side effects (communication with the outside world) is the IO type.
The IO type is a description a side-effecting computation - it represents a value that is delayed and will be
calculated only when explicitly evaluated, as opposed to Future, which is evaluated when initialized.
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