One of NULL, a cols() specification, or
a string. See vignette("readr") for more details.
If NULL, all column types will be inferred from guess_max rows of the
input, interspersed throughout the file. This is convenient (and fast),
but not robust. If the guessed types are wrong, you'll need to increase
guess_max or supply the correct types yourself.
Column specifications created by list() or cols() must contain
one column specification for each column. If you only want to read a
subset of the columns, use cols_only().
Alternatively, you can use a compact string representation where each
character represents one column:
c = character
i = integer
n = number
d = double
l = logical
f = factor
D = date
T = date time
t = time
? = guess
_ or - = skip
By default, reading a file without a column specification will print a
message showing what readr guessed they were. To remove this message,
set show_col_types = FALSE or set options(readr.show_col_types = FALSE).