[R-pkg-devel] match.arg With S4 Methods and Missing Inputs

[Martin Maechler]

>>>>> Georgi Boshnakov 
>>>>>     on Mon, 8 Nov 2021 09:46:00 +0000 writes:

    > You need to define the generic with a default value for
    > this parameter. Methods can then have a different default
    > value for it. 
    > I remember reading this in S4's documentation but don't remember where.

    > Georgi Boshnakov

interesting ... and would make quite some sense.

Can others confirm / disprove ?

Even as co-author of the "using S4 almost everywhere" package 'Matrix'
I wouldn't have known this.

If this is seen to be true (I don't have time for checking just now),
I think it's something we really *should* add to one or more of
the related help pages.

Martin Maechler


    > ________________________________

    > Sent: Monday, November 8, 2021 5:37:18 AM
    > To: Dario Strbenac <dstr7320 using uni.sydney.edu.au>
    > Cc: r-package-devel using r-project.org <r-package-devel using r-project.org>
    > Subject: Re: [R-pkg-devel] match.arg With S4 Methods and Missing Inputs

    >> From the line `function(A, B) standardGeneric("SetOfParams")`, A and B will
    > always have default values of R_MissingArg
    > Providing default values within the methods does nothing since A and B have
    > already been initialized before arriving at the method.
    > You could do something like:


    > if (missing(A))
    > A <- ...
    > if (missing(B))
    > B <- ...


    > within each method, and that would emulate having default values for A and
    > B.

    > On Mon, Nov 8, 2021 at 12:00 AM Dario Strbenac <dstr7320 using uni.sydney.edu.au>
    > wrote:

    >> Good day,
    >> 
    >> How can a parameter take a default value from a vector of permitted ones,
    >> if it is missing?
    >> 
    >> setClassUnion("characterOrMissing", c("character", "missing"))
    >> setClassUnion("integerOrMissing", c("integer", "missing"))
    >> setClass("SetOfParams", representation(A = "characterOrMissing", B =
    >> "integer"))
    >> setGeneric("SetOfParams", function(A, B) standardGeneric("SetOfParams"))
    >> 
    >> setMethod("SetOfParams", c("missing", "missing"), function() # Empty constructor
    >> {
    >> new("SetOfParams", A = "M", B = 100L)
    >> })
    >> 
    >> setMethod("SetOfParams", c("characterOrMissing", "integerOrMissing"),
    >> function(A = c("L", "M", "N"), B = 100L)
    >> {
    >> A <- match.arg(A)
    >> new("SetOfParams", A = A, B = B)
    >> })
    >> 
    >> SetOfParams(B = 500L)
    >> Error in match.arg(A) : argument "A" is missing, with no default.
    >> 
    >> How can I avoid the error about A having no default? I thought I specified
    >> it so that it does have one, which match.arg would set for me if the user
    >> did not specify one.
    >> 
    >> --------------------------------------
    >> Dario Strbenac
    >> University of Sydney
    >> Camperdown NSW 2050
    >> Australia
    >> ______________________________________________
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