What is a good gradual language? Siek et al. have previously proposed the refined criteria, which specify certain guarantees about semantic correspondence and preservation of well-typedness and type-safety in the presence of untyped code. However, because of their exclusive focus on syntactic properties, they are not the whole story. Rich semantic properties like parametricity or non-interference, which hold in the static language, should also continue to hold upon gradualisation.

In this paper, we investigate and argue for a new criterion previously hinted at by Devriese et al.: the embedding from the static to the gradual language should be fully abstract. We illustrate in a simple setting that the criterion is useful; it can weed out an erroneous gradualisation that satisfies the refined criteria. We illustrate that it is realistic, by giving a proof sketch for the natural gradualisation of $\operatorname{STLC}_\mu$.