3.318 Like Frege and Russell, I take a proposition to be a function of the expressions contained in it.
So propositions are being treated here as functions. But it does not follow, Anscombe points out (p. 103), that Wittgenstein thinks propositions just are functions. We can speak of 8 as a function of 2, she notes, without meaning that 8 just is a function and nothing else, or that thinking of it this way is the right way to think of it. So Wittgenstein is not here saying anything incompatible with Frege’s view that a proposition is not a function.