Coercion checking

[kenahoo]

I sometimes want to make sure a certain data.frame has specific columns, and they have specific types. The most polite way of doing this is to coerce the columns to the type I expect, rather than making the caller do it:

# Kind of harsh:
stopifnot(inherits(df, 'data.frame'))
stopifnot(all(c('datetime', 'value') %in% names(df)))
stopifnot(inherits(df$datetime, 'POSIXct'))

# A little kinder:
df <- as.data.frame(df)
stopifnot(all(c('datetime', 'value') %in% names(df)))
df$datetime <- as.POSIXct(df$datetime)

That gets kind of verbose, though. Everything has to be mentioned multiple times. It would be cool if assertr had some sugar for this use case, maybe something like:

df %<>%
  validate(has_all_names('datetime', 'value')) %>%
  coerce('datetime', as.POSIXct)

Any sympathy for that point of view?

[tonyfischetti]

That sounds like it would be helpful sugar but does that fit in with the focus on validation? That might be something better suited for an add-on package. Although, maybe, a is_coerceable predicate might be cool. What do you think?

Thanks for the feedback :)