Additionally, invoking BLAS/LAPACK only happens for dense arrays of specific types (single/double precision real/complex numbers). The great thing in Julia is that almost all of the array and linear algebra functionality is implemented completely generically, so that algorithms can be written using these higher-level concepts, and the low-level functionality will (very efficiently) dispatch to BLAS/LAPACK if possible; if not, the generic implementations still allow your algorithm to work for e.g. your user-defined number type.