Decorator Definition

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Decorators have the following qualities:

• Syntactic Compatibility. The decorated function can replace undecorated functions without modification of the codebase. This leads to two requirements:
  • The decorator doesn't add new input arguments, including new input args with default values.
  • The decorator doesn't change the returned data structure in case of successful execution of the undecorated function.

Decorators may alter the behaviour of a function in one or more ways:

• Strengthening function preconditions. E.g. validate_arguments() throws new expections.
• Weakening function postconditions. E.g. purrr::saftly + purrr::compose + Null object
• Report additional information for function execution. E.g. print info in console or open intermediate results with View.
• Modification of return values (not structure). E.g. rounding numeric values.

Decorators violate several SOLID principles:

• Single-responsibility principle (see excerpt)
• Liskov substitution principle (because they can strengthen input conditions)

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Violating LSP

• By itself, purrr::saftly violates LSP. This is because the function changes the output of a successful execution into a list containing the results.