Any of with, if and case can be used in pure functions without affecting their purity, just like conditionals like if or switch in other languages.

• case is not a macro but a special form: it gets compiled as an erlang case expression (and it cannot introduce any side effect)
• both with and if are macros that get expanded as case at compile time, so there are literally the same thing in terms of resulting code

So when you write:

if foo() do
  bar()
end

It is exactly the same as writing the code it would expand into:

case foo() do
  x when x in [false, nil] -> nil
  _ -> bar()
end

In this example, the purity of this expression only depends on the purity of foo/0 and bar/0.