~> (the optimistic operator) is used when you want to specify a version ‘up to next major | minor | patch‘. For example:
~> 0.1.2 will get you a version up to 0.2 (but not including 0.2 and higher)
~> 0.1 will get you a version up to 1.0 (but not including 1.0 and higher)
~> 0 will get you a version of 0 and higher (same as if it was omitted)
where 0.1.2 would mean ‘I want this exact version’