R7 does not seem to work with class unions

[sanchit-saini]

> bar <- new_generic(name = "bar", signature = c("x", "y"))
> setClassUnion("char_OR_NULL", c("character", "NULL"))
> method(bar, list("char_OR_NULL", "numeric")) <- function(x, y, ...) paste0("foo-", x, ":", y)
> bar("foo", 1)
Error: No method found!
> bar(NULL, 1)
Error: No method found!

[hadley]

Reprex:

library(R7)

bar <- new_generic(name = "bar", signature = c("x", "y"))
setClassUnion("char_OR_NULL", c("character", "NULL"))
method(bar, list("char_OR_NULL", "numeric")) <- function(x, y, ...) paste0("foo-", x, ":", y)
bar("foo", 1)
#> Error: No method found!
bar(NULL, 1)
#> Error: No method found!

^{Created on 2021-10-22 by the reprex package (v2.0.1)}

[jimhester]

It does work with R7 unions FWIW, e.g.

library(R7)

bar <- new_generic(name = "bar", signature = c("x", "y"))
chr_or_null <- new_union("character", "NULL")
method(bar, chr_or_null) <- function(x, y, ...) paste0("foo-", x, ":", y)
bar("foo", 1)
#> [1] "foo-foo:1"
bar(NULL, 1)
#> [1] "foo-:1"

^{Created on 2021-10-26 by the reprex package (v2.0.1)}

I am not sure it should work with S4 class unions?

[hadley]

If (e.g.) Bioc wanted to shift from S4 to R7, I suspect it would be useful to be able to switch generics while leaving the class structure as is.

Should method(bar, list("char_OR_NULL", "numeric")) <- warn that char_OR_NULL isn't known class?

[mmaechler]

I agree it would be nice and possibly important to have "glue functionality" providing compatibility. If this (partial S4 support) seems a bit unclean, might it be a possibilty to have the functionality "modularized" in an extra S4toR7 or R7forS4 package which would depend on methods and R7 ?

[hadley]

Revisit after #134

[hadley]

With #134

library(R7)
char_OR_NULL <- setClassUnion("char_OR_NULL", c("character", "NULL"))

foo <- new_generic("foo", dispatch_args = "x")
method(foo, char_OR_NULL) <- function(x) "char_or_NULL"

foo("foo", 1)
#> Error: Can't find method for generic `foo()` with classes:
#> - x: <character>
foo(NULL, 1)
#> Error: Can't find method for generic `foo()` with classes:
#> - x: <NULL>

^{Created on 2022-01-26 by the reprex package (v2.0.1)}

I think the easiest way to deal with this will be to convert S4 unions to R7 unions, so the same method dispatch logic can apply. Does this seem like the right way to get the classes that an S4 union applies to?

Foo <- setClass("Foo", slots = "x")
setClassUnion("optionalFoo", c("Foo", "NULL"))

union <- getClass("optionalFoo")
subclasses <- Filter(function(x) x@distance == 1, union@subclasses)
lapply(subclasses, function(x) getClass(x@subClass))
#> $`NULL`
#> Class "NULL" [package "methods"]
#> 
#> No Slots, prototype of class "NULL"
#> 
#> Extends: "OptionalFunction", "optionalMethod", "optionalFoo"
#> 
#> Known Subclasses: 
#> Class ".NULL", directly, with explicit coerce
#> 
#> $Foo
#> Class "Foo" [in ".GlobalEnv"]
#> 
#> Slots:
#>           
#> Name:    x
#> Class: ANY
#> 
#> Extends: "optionalFoo"

^{Created on 2022-01-26 by the reprex package (v2.0.1)}

Translating S4 unions to R7 unions also means that we need to be able to translate the S4 wrappers of base types to the R7 wrappers of base types, i.e. we need to be able to translate getClass("character") to "character". From a quick inspection, it doesn't look like you can determine this from the class metadata, so I guess we just need to do some matching based on @name? (assuming the @package is "methods").

[lawremi]

The overall approach sounds good to me. For finding the members of the union, it may be better to recurse instead in order to handle nested unions.

[hadley]

Now

library(R7)
char_OR_NULL <- setClassUnion("char_OR_NULL", c("character", "NULL"))

foo <- new_generic("foo", dispatch_args = "x")
method(foo, char_OR_NULL) <- function(x) "char_or_NULL"

foo("foo")
#> [1] "char_or_NULL"
foo(NULL)
#> [1] "char_or_NULL"

^{Created on 2022-02-08 by the reprex package (v2.0.1)}