Convert an object from one type to another

convert(from, to) is a built-in generic for converting an object from one type to another. It is special in three ways:

It uses double-dispatch, because conversion depends on both from and to.
It uses non-standard dispatch because to is a class, not an object.
It doesn't use inheritance for the to argument. To understand why, imagine you have written methods to objects of various types to classParent. If you then create a new classChild that inherits from classParent, you can't expect the methods written for classParent to work because those methods will return classParent objects, not classChild objects.

convert() provides two default implementations:

When from inherits from to, it strips any properties that from possesses that to does not (downcasting).
When to is a subclass of from's class, it creates a new object of class to, copying over existing properties from from and initializing new properties of to (upcasting).

If you are converting an object solely for the purposes of accessing a method on a superclass, you probably want super() instead. See its docs for more details.

S3 & S4

convert() plays a similar role to the convention of defining as.foo() functions/generics in S3, and to as()/setAs() in S4.

Usage

convert(from, to, ...)

Arguments

from: An S7 object to convert.
to: An S7 class specification, passed to as_class().
...: Other arguments passed to custom convert() methods. For upcasting, these can be used to override existing properties or set new ones.

Value

Either from coerced to class to, or an error if the coercion is not possible.

Examples

Foo1 <- new_class("Foo1", properties = list(x = class_integer))
Foo2 <- new_class("Foo2", Foo1, properties = list(y = class_double))

# Downcasting: S7 provides a default implementation for coercing an object
# to one of its parent classes:
convert(Foo2(x = 1L, y = 2), to = Foo1)
#> <Foo1>
#>  @ x: int 1

# Upcasting: S7 also provides a default implementation for coercing an object
# to one of its child classes:
convert(Foo1(x = 1L), to = Foo2)
#> <Foo2>
#>  @ x: int 1
#>  @ y: num(0) 
convert(Foo1(x = 1L), to = Foo2, y = 2.5)  # Set new property
#> <Foo2>
#>  @ x: int 1
#>  @ y: num 2.5
convert(Foo1(x = 1L), to = Foo2, x = 2L, y = 2.5)  # Override existing and set new
#> <Foo2>
#>  @ x: int 2
#>  @ y: num 2.5

# For all other cases, you'll need to provide your own.
try(convert(Foo1(x = 1L), to = class_integer))
#> Error : Can't find method for generic `convert()` with dispatch classes:
#> - from: <Foo1>
#> - to  : <integer>
#> 

method(convert, list(Foo1, class_integer)) <- function(from, to) {
  from@x
}
convert(Foo1(x = 1L), to = class_integer)
#> [1] 1

# Note that conversion does not respect inheritance so if we define a
# convert method for integer to foo1
method(convert, list(class_integer, Foo1)) <- function(from, to) {
  Foo1(x = from)
}
convert(1L, to = Foo1)
#> <Foo1>
#>  @ x: int 1

# Converting to Foo2 will still error
try(convert(1L, to = Foo2))
#> Error : Can't find method for generic `convert()` with dispatch classes:
#> - from: <integer>
#> - to  : <Foo2>
#> 
# This is probably not surprising because foo2 also needs some value
# for `@y`, but it definitely makes dispatch for convert() special