Either column objects created by col_*, or their
abbreviated character names. If you're only overriding a few columns,
it's best to refer to columns by name. If not named, the column types
must match the column names exactly.
.default
Any named columns not explicitly overridden in ...
will be read with this column type.
Examples
cols(a = col_integer())
cols_only(a = col_integer())
# You can also use the standard abreviations
cols(a = "i")
cols(a = "i", b = "d", c = "_")
providing 
.