I want to agree, but the problem is that you now need 10 versions for many operations that are in a string library. Hidden assumptions -- like 'replacing a char

I want to agree, but the problem is that you now need 10 versions for many operations that are in a string library. Hidden assumptions -- like 'replacing a character in a string can never fail' -- may fail (eg in an email address type which only allows only a limited char set). Some constraints can't be expressed without dependent types (say you want to define a URI type that limits the length to 2000 chars total but is composed of variable length components, like the domain name, query strings etc).