Make it easy to catch mutations in the data by emitting col_types string #314
Description
One of the nice features of readr is that it introspects column types and gives you data frames with columns of "appropriate" data types. This is cool. It does so heuristically by reading a bunch of rows (but not all) and guessing.
This is very nice for interactive use, however I think it could be made into a neat feature to catch data schema mutation.
I would like to propose a new function which reads a whole CSV file, and from the entire file returns a col_types string. A user can then take that string and put it in their production script's read_csv ca;;, so that in the future, if a new data file comes in which has different data types, instead of (for example) silently returning a data frame which has chars where a date once was, it will now give a proper error.