Registration of custom column types and parsers

[Enchufa2]

This is a feature request that should not impact current readr's behaviour, but that would open up the possibility of enhancement from third-party packages (cc @edzer).

Motivation

Consider the cases in which a rectangular data source contains, e.g.,

• Quantities with units and/or errors. 1.53(2) m/s.
• Spatial data: POINT(0, 1).

readr::read_csv("
quantity,point
1.53(3) m/s,\"POINT(0,1)\"
5.21(1) m/s,\"POINT(1,5)\"
")
#> # A tibble: 3 x 2
#>   quantity    point     
#>   <chr>       <chr>     
#> 1 quantity    point     
#> 2 1.53(3) m/s POINT(0,1)
#> 3 5.21(1) m/s POINT(1,5)

Currently, those data types are read as character (and other use cases may be stripped to numbers), and the user needs to convert them. The idea would be to allow packages to register custom column types and parsers into readr so that, in this example, if packages quantities and sf were loaded, readr would have automatically generated columns with quantities and sf objects respectively.

Changes required

I could be missing something, but the general changes needed for this would be:

• Move things to inst/include to expose the Collector class, so that other packages can link to readr and derive safely from this class.
• Some mechanism to register
  • (C++) the custom collector into the list of available subclasses (and the means to insert it in a specific position in the chain?) and a parser (guesser) function.
  • (R) custom col_* and parse_*.

The only drawback I can think of is that a package may, e.g., register a parser that catches everything and messes things up. To avoid this issue, readr::read_* may gain a flag to enable external parsers, so that using them requires an action from the user.

If this enhancement is considered, I would be more than happy to work on it.

[msberends]

I'll second this! Would be very welcome if we could extend the functionality of readr with other packages.

Our AMR package is all about antimicrobial resistance (AMR). The new class rsi makes sure columns only contain valid antimicrobial interpretations: resistant (R), susceptible (S) or intermediate (I). This can be forced upon a vector with as.rsi.
Anyway, if data are read from a microbiological laboratory system (in a hospital), all columns with results of antbiotics will contain just R, S or I values. Suchcolumns could be parsed as rsi when the AMR package is loaded 😄