as.fractions is clearly in the MASS library, but searching for its documentation always redirects me to the docs for the fractions function, which makes no mention of as.fractions. So, where, if anywhere, is as.fractions documented?
It doesn't have any separate documentation because it is just a tiny wrapper around fractions. The entire function is
function (x)
if (is.fractions(x)) x else fractions(x)
So if the object you are passing is already of class "fractions" then the function does nothing at all. If it isn't a "fractions" object, it is exactly the same as calling fractions. In other words, as.fractions is just another name for fractions