Frans Brom asks us to consider the expressive-communicative function of legislation, where what some law does is to give voice and legal standing to some commonly held moral value, in contrast to two other ostentive roles for legislation, as a way to codify the dominant morality of a society, and as a way to modify behaviour in certain ways the legislators deem necessary. I will respond that I'm unconvinced that this expressive-communicative function (ECF), as Brom's short piece presents it at least, is distinct from the two other functions.
It is worth quoting Brom on the ECF at a bit of length:
"The interactive paradigm of legislation shows that law can form a point of reference in moral discussions. This is especially the case when legislation is based upon a societal agreement the impact of which is not sufficiently conceptualised in moral terms. Once this agreement is restated in legislation, the law becomes an authoritative source fr the concrete normative implications of this agreement. One could even say that the societal agreement is recreated into a moral framework for the public domain."
This is all well and good, but I think Brom needs to do more to seperate the ECF from legal codification and modification. His view seems to be that the ECF of a law is to give a moral framework wherein some value is given creedence, which in turn provides normative guidance that would be lacking if we were still arguing about what that value comes down to. I say that the first part of this falls under codification and the second under modification, that is, the ECF reduces to the cases where the codification of some moral position is such to modify behaviour in the jurisdiction away from other positions, which is hardly a strange effect of codification.
If codification happens, it is always the codification of one position among many. That position needn't be one which is actually endorsed by anybody (nor is it likely to be exactly like any non-legal framework, but never mind that). The fact that such-and-such a framework is the product of a code of laws carries with it the consequence that that framework now works upon everybody in the jurisdiction. Being under the effect of some law can't help but modify behaviour in its jurisdiction to the effect of encouraging actions in line with the codified framework (leaving aside the interesting case of dead-letter laws, which don't concern us here because they also fail to authoritively express the position of anybody, because they fail to carry authority). And so much the worse for different frameworks to the extent that they differ from the one enshrined in law.
So the normative implications of ECF are covered by the classical codification and modification views as well. There still is an issue of what exactly the target of the codification is, especially in the type of cases Brom draws attention to, where there isn't widespread agreement on exactly what the endorsed value consists in, like the intrinsic value of non-human animals in Dutch animal welfare law. An important point about vague concepts like these, which David Lewis reminds us of, is that while it's unclear where their applications start and end, it isn't that anything goes. Just as the boundary between blue and green is unclear and yet there are blues that aren't at all green and vice versa, so there are many things that might be an animal's intrinsic value and things which clearly couldn't. Pointedly, the type of view where animals have ethical value only in as far as they are valuable to people simply couldn't qualify. Thus, the position being codified is actually a range of what could count under the vague term. This means that you can still have sensible disagreement in the courts about, say, whether a law protecting the inherent worth of animals prohibits conditions where they are kept in good health but in extremely restrictive settings, since it's vague whether an animal moving in its distinctive ways is part of its inherent worth. But it stops acts which egregriously disregard the well-being of the animals involved. Once again it is important to note that the view in law needn't be the view anybody had prior to the passing of legislation, so the fact that the law covers a range of positions rather than any single one isn't a problem. It can gain support from everybody across that range (though that support is likely to be qualified) in that they agree that something like the law needs to be enacted.
Thus, the classical functions of codification and modification can do everything Brom recommends the ECF for: it can posit an authoritative framework with normative implications, and can do so in a way to accomodate vague values rather than precise positions.