I have observed unexpected behavior for isusernametaken function. When we pass a

I just confirmed this on my 7.11 site as well. Hopefully this gets picked up for the next hotfix.

I think this will not change as the behaviour of some older Appian functions is not consistent in some edge cases but running systems depend on this. Just try "=not(null)" and "=if(null, true, false)". So my suggestion is to wrap isusernametaken() this in an expression which does some additional checks.

fn! doesn't have side effects, meaning if should always return expected value. isusernametaken() is a bool, and think of passing null as divided by 0. What should happen, when your only possible return value is a number?

One of the purposes is you should be able to create users based on output of this function, if it returns false for null creating the user fails.

As @stefanh791 mentioned, use a wrapper to check for null.

For more context, from Appian common objects rule!APN_displayUser, it checks for null ahead, as I said earlier, fn! functions should have no side effects, so check for null and handle accordingly:

if(isnull(ri!user), "",
if(isusernametaken(ri!user),
user(ri!user, "firstName") & " " & user(ri!user, "lastName"),
tostring(ri!user)
))

@Shyamb, it is not about fn! , which is simply an optional domain prefix for Appian functions.
I meant that the function isusernametaken returns true even if we pass an empty string. I think it should return false or it should give an error when empty string is passed.
As suggested by Stefan, we can use isnull before passing it to the function.

The behavior is unexpected. It should not return true when passed a null string. I have logged this to our product team for further investigation. The previous suggestions to work around this are great suggestions, so for the time being, I would consider following those.

Thank you Nick.