Re: [Rd] S4 methods semantics questions

From: John Chambers <jmc_at_r-project.org>
Date: Fri 25 Mar 2005 - 15:26:07 GMT

Byron Ellis wrote:

> Some quick questions about S4 methods.
>
> Does the typing of S4 methods mean that lazy evaluation is no longer
> possible? It seems that you would need to evaluate the arguments to
> determine their type at dispatch.

Yes, it would be a neat trick to know the class of an actual argument without evaluating it ;-)

However, the evaluation proceeds stepwise until a unique method matches, so that arguments not needed to do the dispatch will not yet be evaluated. The order of evaluation is controlled by the signature of the generic, by default all the arguments in order, but specifiable via the signature= argument to setGeneric.

>
> Second, what role, if any, do default arguments play in S4 methods? I
> notice that you can put default arguments into generics but that the
> dispatch is still done on the type of the calling argument rather than
> the default argument, though the default arg is substituted.

Yes, dispatch depends on the call, not on the default expressions for the arguments. If an actual argument is missing, the dispatch tries to match "missing" or "ANY".

> However,
> default values for arguments in method definition seem to be stripped
> or, more likely, overridden at dispatch by the calling argument (i.e.
> "missing").
>
> Some examples:
>
> setGeneric("foo",function(x="bar") standardGeneric("foo"))
> setMethod("foo","missing",function(x) print(x))
>
> >foo()
> [1] "bar"
>
> setGeneric("foo",function(x,y) standardGeneric("foo"))
> setMethod("foo","numeric",function(x,y=2) x+y)
>
> >foo(1)
> Error in foo(1) : argument "y" is missing, with no default

Well, the intent is that defaults are indeed taken from the method, if there is a default there, otherwise from the generic. It looks as if there is a bug in the case that the generic has NO default for that argument (unless, of course, it's a subtle feature, but not that I can think of at the moment).

Your example works as intended if there is a default expression for y in the generic:

R> setGeneric("foo",function(x,y=stop("Need y")) standardGeneric("foo")) [1] "foo"
R> setMethod("foo","numeric",function(x,y=2) x+y) [1] "foo"
R> foo(1)
[1] 3

>
> ---
> Byron Ellis (ellis@stat.harvard.edu)
> "Oook" -- The Librarian
>
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>



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https://stat.ethz.ch/mailman/listinfo/r-devel Received on Sat Mar 26 02:29:25 2005

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