Maybe R should have optional "training wheels" that produce a warning every time an implicit conversion happens. In the OP's case, it would warn that a data fra

Maybe R should have optional "training wheels" that produce a warning every time an implicit conversion happens. In the OP's case, it would warn that a data frame was implicitly being converted to a matrix, and maybe also warn that the numeric vectors within were being converted to character in order to get slotted into that matrix.