As per wiki, pure function is the one with following characteristics:
- Its return value is the same for the same arguments (no variation with local static variables, non-local variables, mutable reference arguments or input streams from I/O devices).
- Its evaluation has no side effects (no mutation of local static variables, non-local variables, mutable reference arguments or I/O streams).
Thus a pure function is a computational analogue of a mathematical function. Some authors, particularly from the imperative language community, use the term “pure” for all functions that just have the above property.
I am trying to translate that to PHP. Here are the thoughts I have on pure functions:
- All arguments are passed by value and not by reference.
- No arguments with &
- No objects passed as arguments as objects are passed by reference
- They don’t access state of the class
- They don’t access anything with $this
- The don’t access global or super globals
- They don’t use $_ variables
- They don’t have global keyword
- They don’t access I/O, network calls, etc.
- No file operations
- No network calls, database calls, etc.
- They don’t have static calls
- When there are static calls, their results may change
- There are no side effects
- State of the class shouldn’t change, i.e., it should’t update members of the class
- Output is solely dependent on input
Let me know your thoughts and correct me if I am missing something.