S7 class compatibility

[Yunuuuu]

It seems we cannot use S7 class to extend the new class system:

class_new_theme <- S7::new_class("new_theme", ggplot2::class_theme)
S7::convert(ggplot2::theme(), class_new_theme)
#> Error in (structure(function (elements, complete, validate) : argument "elements" is missing, with no default

^{Created on 2025-07-04 with reprex v2.1.0}
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[Yunuuuu]

This works, we must pass the property manually:

class_new_theme <- S7::new_class("new_theme", ggplot2::class_theme)
old_theme <- ggplot2::theme(plot.title = ggplot2::element_text())
class_new_theme(old_theme, old_theme@complete, old_theme@validate)
#> <new_theme> List of 1
#>  $ plot.title: <ggplot2::element_text>
#>   ..@ family       : NULL
#>   ..@ face         : NULL
#>   ..@ italic       : chr NA
#>   ..@ fontweight   : num NA
#>   ..@ fontwidth    : num NA
#>   ..@ colour       : NULL
#>   ..@ size         : NULL
#>   ..@ hjust        : NULL
#>   ..@ vjust        : NULL
#>   ..@ angle        : NULL
#>   ..@ lineheight   : NULL
#>   ..@ margin       : NULL
#>   ..@ debug        : NULL
#>   ..@ inherit.blank: logi FALSE
#>  @ complete: logi FALSE
#>  @ validate: logi TRUE

^{Created on 2025-07-04 with reprex v2.1.0}
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[teunbrand]

It is unclear to me what you're trying to achieve here. Would you mind explaining a bit more?

[Yunuuuu]

Apologies, I missed the message. The main issue we're facing is that it's not possible to directly extend or attach an extensible class to an existing ggplot2 object. If a user provides a ggplot object, our goal is to convert it into our own custom class for further extension. However, the standard S7 conversion S7::convert() method doesn't behave as expected in this case, which limits our ability to build on top of ggplot2 objects in a flexible way.

Would you have any suggestions or workarounds for this limitation?

[teunbrand]

Hmm, seems like S7 only allows to default-convert when all the formal names of the class constructor are included in the properties.
The class_theme() constructor has an elements argument, which is not a formal property, so the default-convert doesn't work.

I honestly feel that this is a limitation on the S7 side of things and maybe they should handle underlying base data in the default convert method.

Meanwhile, you could of course write your own convert() methods.

library(S7)
library(ggplot2)

new_theme_class <- new_class("new_theme", parent = class_theme)
old_theme <- ggplot2::theme(plot.title = ggplot2::element_text())

# Can confirm, it doesn't work out of the box
convert(old_theme, new_theme_class)
#> Error in (structure(function (elements, complete, validate) : argument "elements" is missing, with no default

method(convert, list(from = class_theme, to = new_theme_class)) <- 
  function(from, to) {
    new_theme_class(S7_data(from), complete = from@complete, validate = from@validate)
  }
# Now works
convert(old_theme, new_theme_class)
#> <new_theme> List of 1
#>  $ plot.title: <ggplot2::element_text>
#>   ..@ family       : NULL
#>   ..@ face         : NULL
#>   ..@ italic       : chr NA
#>   ..@ fontweight   : num NA
#>   ..@ fontwidth    : num NA
#>   ..@ colour       : NULL
#>   ..@ size         : NULL
#>   ..@ hjust        : NULL
#>   ..@ vjust        : NULL
#>   ..@ angle        : NULL
#>   ..@ lineheight   : NULL
#>   ..@ margin       : NULL
#>   ..@ debug        : NULL
#>   ..@ inherit.blank: logi FALSE
#>  @ complete: logi FALSE
#>  @ validate: logi TRUE

^{Created on 2025-07-17 with reprex v2.1.1}

[Yunuuuu]

Thanks for the details. The problem is that whenever ggplot2 adds a property, this approach will break the other package.

[teunbrand]

Yeah I get the concern, but if the new property has a default value it should be picked up automatically without breaking the other package. I don't know yet how frequently ggplot2 would need to swap around properties, but I suspect not that often for the theme.

[Yunuuuu]

Would it be possible to add a vignette specifically for extending ggplot2 using the new S7 class system? The current S7 vignette in S7 package is quite general, and I've encountered several issues when trying to create ggplot2 extensions with it. For example, this issue and directly wrapping a ggplot object as a property also fails:.

S7::new_class("CraftBox",
    properties = list(
        plot = ggplot2::class_ggplot
    )
)
#> Error: `ggplot2::ggplot` is not identical to the class with the same @package and @name properties

^{Created on 2025-07-18 with reprex v2.1.0}
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[Yunuuuu]

This may belong in a separate issue, but I’m mentioning it here since it seems related to the S7 package and may ultimately need to be fixed there.

Relevant line in the S7 source: https://github.com/RConsortium/S7/blob/db5964265a88d114fb6de97361060979ad63b3d6/R/class-spec.R#L102

From what I understand, this check requires the super class—e.g., ggplot2::ggplot—to be identical to the class constructor function. This causes an error in cases like the one above. At this point, I’m not sure whether this should be resolved by changing how class_ggplot and ggplot generic function is constructed in ggplot2, or if the identity check in S7 is too strict for common extension use cases.

[teunbrand]

For example, this issue and directly wrapping a ggplot object as a property also fails

Yeah I agree that this is annoying. For now you can use S7::new_S3_class("ggplot2::ggplot") instead, which should accept ggplot objects. I'm not sure to what degree it'll be validated though.

[Yunuuuu]

If we are expected to use S7::new_S3_class("ggplot2::ggplot") to extend the class, then class_theme and class_ggplot probably shouldn’t be exported in the NAMESPACE.

[teunbrand]

In your CraftBox example, you're wrapping the class, not extending it.
There are other reasons for the export of the class objects beside declaring properties with them.

[Yunuuuu]

Yeah, but having to use two different pieces of code to refer to the same class still feels a bit strange to me.

[teunbrand]

I'm not denying it is strange. It is a ducktape solution to a problem that hasn't been resolved yet.