[R-sig-DB] dbWriteTable and dbReadTable generics

[David James]

Sure, that would do it nicely.

Regards,

David


On Mon, Oct 21, 2013 at 9:00 PM, Hadley Wickham <h.wickham using gmail.com> wrote:

> > I do agree that having row.names=, append=, and overwrite= makes sense,
> > because these arguments are common and quite natural to most current DBI
> > method implementations of these generic functions.  However, adding them
> > explicitly to the generic definition implies that methods can be
> dispatched
> > on the *class* of those same arguments, which I'm not clear whether it's
> > helpful.
>
> Not necessarily - you can use the signature argument to setGeneric to
> say which arguments can be part of the method signature. Simple
> example below:
>
> setGeneric("f", function(x, y, z) standardGeneric("f"))
> setMethod("f", signature(z = "numeric"), function(x, y, z) z * 2)
>
> f(,, 2)
>
> setGeneric("g", function(x, y, z) standardGeneric("g"),
>   signature = "x")
> setMethod("g", signature(z = "numeric"), function(x, y, z) z * 2)
>
>
> > Here, I seem to see two similar but not-quite equal objectives:
> >
> > (1) defining an interface more strictly (i.e., the DBI dictates that
> > dbWriteTables, etc., must have a more complete signature including
> > row.names=, etc.). Perfectly reasonable, yet setMethod for the various
> > implementations may (?) need to be revised (if so, not a big a deal).
> >
> > (2) defining the signature on which methods are be dispatched.
> >
> > I don't think that (1) is wrong (after all, the current use of name= is
> > precisely this case) or that (1) and (2) are mutually exclusive. However,
> > the original intention was clearly (2): methods are to be dispatched
> only on
> > the conn= and name= arguments, other arguments (...) may be passed from
> the
> > generic to the method(s), but not used for dispatching.  I still feel
> that
> > the current definition expresses this intention most clearly and
> > unambigiously.  Adding row.names=, etc., for the sake of a more complete
> > interface may imply behavior we (I)  haven't anticipated, e.g., what
> error
> > do users get when they incorrectly supply "row.names = t" (t as the
> > transpose as opposed to T for TRUE)?  Overall, not a particular big
> issue.
>
> So what do you think of the following? I think it does a better job of
> meeting those goals than the current code.
>
> setGeneric("dbWriteTable",
>   def = function(conn, name, value, row.names = FALSE,
>                        overwrite = FALSE, append = FALSE, ...) {
>      standardGeneric("dbWriteTable")
>   }, valueClass = "logical", signature = "conn"
> )
>
> setGeneric("dbReadTable",
>   def = function(conn, name, row.names = FALSE, ...) {
>     standardGeneric("dbReadTable")
>   },
>   valueClass = "data.frame", signature = "conn"
> )
>
> Hadley
>
> --
> Chief Scientist, RStudio
> http://had.co.nz/
>

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