Solidity Library call gives Member not found or not visible after argument-dependent lookup

Question

Using Solidity 0.4.17, I have defined a simple library at ./StringUtils.sol

library StringUtils {
    function isEmptyString(string s) external pure returns (bool) {
        return (keccak256(s) == keccak256(''));
    }
}

Now I am trying to use it in a contract at ./PastryChef.sol

import './StringUtils.sol';


contract PastryChef {
    using StringUtils for StringUtils;

    string name;

    function PastryChef(string _name) public {
        require(!StringUtils.isEmptyString(_name)); // name must not be empty
        name = _name;
    }

}

I get the error

TypeError: Member "isEmptyString" not found or not visible after argument-dependent lookup in type(library StringUtils)
        require(!StringUtils.isEmptyString(_name)); // name must not be empty
                 ^-----------------------^

What am I doing wrong?

Answer 1

I've worked this out. The Library function can not be external but must be either internal or public.

So changing external to internal fixes it.