[R-pkg-devel] tibbles are not data frames

Yes, basically tibbles violate the substitution principle. A lot of
other packages do, probably base R as well, although it is sometimes
hard to say, because there is no clear object hierarchy.

Let's take a step back, and see how you can check for a data frame argument.

1. Weak check.

is.data.frame(arg)

This essentially means that you trust subclasses of data.frame to
adhere to the substitution principle. While this is nice in theory, a
lot packages (including both major packages implementing subclasses of
data.frame!) do not always adhere. So this is not really a safe
solution.

Base R does this as well, sometimes, e.g. aggregate.data.frame has:

    if (!is.data.frame(x))
        x <- as.data.frame(x)

which is essentially equivalent to the weak check, since it leaves
data.frame subclasses untouched.

2. Strong "check".

arg <- as.data.frame(arg)

This is safer, because it does not rely on subclass implementors. It
also has the additional benefit that your code is polymorphic: it
works with any input, as long as it can be converted to a data frame.

Base R also uses this often, e.g. in merge.data.frame:

    nx <- nrow(x <- as.data.frame(x))
    ny <- nrow(y <- as.data.frame(y))

Gabor

Disclaimer: I do not represent the tibble authors in any way.

On Tue, Sep 26, 2017 at 11:21 AM, David Hugh-Jones

[R-pkg-devel] tibbles are not data frames

Thread (48 messages)