There take on what they call capabitilites is very interesting. Basically anything that would make a function non-pure seems to be declared explicitely.

A computational effect or an “effectful” computation is one which relies on or changes elements that are outside of its immediate environment. Some examples of effectful actions that a function might take are:

• writing to a database
• throwing an exception
• making a network call
• getting a random number
• altering a global variable