Better explain how pure is different from side effect free

[calvinmetcalf]

It took me a couple readings to realize pure no use of closures. Making it more explicit what the two things do would by helpful. My understanding:

• side effect free: does not write to any variable outside the function and does not modify it's arguments if they are objects
• pure: same as above but also does not read from any external variables besides it's arguments

[antfu]

I agree. But I feel that the problem is that __PURE__ is used on expressions, but not on function declarations, which does not apply to the statement you mentioned?

From my understanding:

• __PURE__ marks expressions as "side-effect-free"
• __NO_SIDE_EFFECTS__ marks functions as "side-effect-free"

So there is actually no "pure function" concept involved.

[robpalme]

The semantic relationship I have always understood is:

• side effect: Calling this function may mutate state outside the function, e.g. mutation of non-primitive arguments
• side cause: This function's return value may depend on state outside the function, e.g. a closure-captured variable
• pure: This function has no side effects and no side causes.

I agree this relation should be clarified somewhere.