attrio | Attributes for plain old Ruby objects | Application Framework library

If you want to do it "properly", then you need to wrap all non-deterministic (non-pure) function calls in IO or IOEither (depending on whether they can or cannot fail).

So first let's define which function calls are "pure" and which are not. The easiest I find to think of it is like so - if function ALWAYS gives the same output for the same input and doesn't cause any observable side-effects, then it's pure.

"Same output" doesn't mean referential equality, it means structural/behaviour equality. So if your function returns another function, this returned function might not be the same function object, but it must behave the same (for the original function to be considered pure).

So in these terms, the following is true:

• cherio.load is pure
• $ is pure
• .get is not pure
• .find is not pure
• .attr is not pure
• .map is pure
• .filter is pure

Now let's create wrappers for all non-pure function calls: