Mass and right hand side of ODE computed by the same nested function

I want to solve a system of coupled ordinary differential equations of the form

M(y) y'(t) = f(y)

without inverting the matrix M. Since both, M and f, are computed from the same elements, it seems most efficient to do this in a common function,

function [M, f] = myfun(...)

How can I pass this on to ode45?

Thanks, Michael