[Rd] S4 coercion

[Martin Morgan]

Hi Paul -- does this make the problem more explicit?

> ## class hierarchy
> setClass("A", representation=representation(a="numeric"))
[1] "A"
> setClass("B", representation=representation(b="numeric"))
[1] "B"
> setClass("C", contains=c("A", "B"))
[1] "C"

> ## coerce methods -- in some instances implicitly created
> setAs("A", "numeric", function(from) slot(from, "a"))
> setAs("B", "numeric", function(from) slot(from, "b"))

> ## trouble: C equidistant from A, B so no unambiguous nearest 'coerce'
> ## method
> as(new("C"), "numeric")
numeric(0)
Warning message:
Ambiguous method selection for "coerce", target "C#numeric" (the first
of the signatures shown will be used)
    A#numeric
    B#numeric

The solution is to define a method to resolve the conflict, e.g.,

setAs("C", "numeric", function(from) {
    ## avoid breaking the class abstraction
    as(as(from, "A"), "numeric") *
            as(as(from, "B"), "numeric")
})

I don't know how you want to break your particular tie; that has to do
with the classes you're extending.

Martin            

Paul Gilbert <pgilbert at bank-banque-canada.ca> writes:

> I am extending a DBI connection by
>
> setClass("TSPostgreSQLConnection",
> contains=c("PostgreSQLConnection","TSdbOptions"))
>
> but the first time I use this I am getting a warning when it tries to
> coerce  the  TSPostgreSQLConnection to a PostgreSQLConnection. After
> the first use the warning stops,  but the first warning is causing me
> problems when I do automatic checks and set my tests to stop on
> warnings. (I think there should be a correct way to do this that does
> not produce a warning.)  I can trace it back by setting
> options(warn=2) as below.  Do I need to be more specific about how the
> coercion  happens? If so, what is the correct way to coerce the
> TSPostgreSQLConnection into a PostgreSQLConnection? If not, how to a
> get rid of the warning?
>
> Paul Gilbert
>
>
>  > TSdelete("vec", con)
> Error: (converted from warning) Ambiguous method selection for
> "coerce", target "TSPostgreSQLConnection#integer" (the first of the
> signatures shown will be used)
>    PostgreSQLConnection#integer
>    dbObjectId#integer
>  > traceback()
> 15: doWithOneRestart(return(expr), restart)
> 14: withOneRestart(expr, restarts[[1]])
> 13: withRestarts({
>        .Internal(.signalCondition(simpleWarning(msg, call), msg,
>            call))
>        .Internal(.dfltWarn(msg, call))
>    }, muffleWarning = function() NULL)
> 12: .signalSimpleWarning("Ambiguous method selection for \"coerce\",
> target \"TSPostgreSQLConnection#integer\" (the first of the signatures
> shown will be used)\n    PostgreSQLConnection#integer\n
> dbObjectId#integer\n",
>        quote(NULL))
> 11: warning(gettextf(paste("Ambiguous method selection for \"%s\",
> target \"%s\"",
>        "(the first of the signatures shown will be used)\n%s\n"),
>        fdef at generic, .sigLabel(classes), paste("   ", names(methods),
>            collapse = "\n")), domain = NA, call. = FALSE)
> 10: .findInheritedMethods(signature, fdef, mtable = allmethods, table
> = mlist,
>        useInherited = useInherited, verbose = verbose)
> 9: selectMethod("coerce", sig, optional = TRUE, c(from = TRUE, to = FALSE),
>       fdef = coerceFun, mlist = coerceMethods)
> 8: as(obj, "integer")
> 7: isIdCurrent(con)
> 6: postgresqlQuickSQL(conn, statement, ...)
> 5: dbGetQuery(con, paste("SELECT tbl  FROM Meta ", where, ";"))
> 4: dbGetQuery(con, paste("SELECT tbl  FROM Meta ", where, ";"))
> 3: TSdbi:::TSdeleteSQL(serIDs = serIDs, con = con, ...)
> 2: TSdelete("vec", con)
> 1: TSdelete("vec", con)
>  >  str(con)
> Formal class 'TSPostgreSQLConnection' [package "TSPostgreSQL"] with 4 slots
>  ..@ Id     : int [1:2] 21795 1
>  ..@ dbname : chr "test"
>  ..@ vintage: logi FALSE
>  ..@ panel  : logi FALSE
> ====================================================================================
>
> La version française suit le texte anglais.
>
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-- 
Martin Morgan
Computational Biology / Fred Hutchinson Cancer Research Center
1100 Fairview Ave. N.
PO Box 19024 Seattle, WA 98109

Location: Arnold Building M2 B169
Phone: (206) 667-2793