From your list of "first-choice languages", C++ is conspicuously missing. That seems rather peculiar, as the type of genericity you are praising Julia for has been one of the core concepts of generic programming in C++ since basically the late nineties with the standardization of the STL (and it's become an increasingly emphasized part of the language throughout its evolution in the last 10 years or so, see C++11 and later). The example you mention, numerical integrators using different number types (complex, arbitrary precision, intervals, etc.), is the type of stuff which would seem a perfect fit for a template-based implementation.