Code should be split into functions/methods that are pure and ones that have side effects

[BernhardPosselt]

For terminology: https://vimeo.com/160326750

The basic idea is:

• pure functions are very easy and fast to unit test
• functions with side effects need slower integration tests and are harder to setup

Classic use case for this is converting data (e.g. https://github.com/nextcloud/news/blob/master/lib/Service/FeedService.php#L317) which means: when converting data, split it out into a separate, one method class and inject them in the constructor (thats fine because functions suck in php)

Other usecases usually include calculating a result, validation or grouping structural things (e.g. Functors (e.g. map for lists), Monads (e.g. Promises, Optionals, Streams, etc)

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[stale]

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