Register an S7 class with S4

Description

If you want to use method<- to register an method for an S4 generic with an S7 class, you need to call S4_register() once.

Usage

S4_register(class, env = parent.frame())

Arguments

Value

Nothing; the function is called for its side-effect.

Examples

methods::setGeneric("S4_generic", function(x) {
 standardGeneric("S4_generic")
})
Foo <- new_class("Foo")
S4_register(Foo)
method(S4_generic, Foo) <- function(x) "Hello"
S4_generic(Foo())

Retrieve the S7 class of an object

Description

Given an S7 object, find it's class.

Usage

S7_class(object)

Arguments

Value

An S7 class.

Examples

Foo <- new_class("Foo")
S7_class(Foo())

Get/set underlying "base" data

Description

When an S7 class inherits from an existing base type, it can be useful to work with the underlying object, i.e. the S7 object stripped of class and properties.

Usage

S7_data(object)
S7_data(object, check = TRUE) <- value

Arguments

Value

S7_data() returns the data stored in the base object; ⁠S7_data<-()⁠ is called for its side-effects and returns object invisibly.

Examples

Text <- new_class("Text", parent = class_character)
y <- Text(c(foo = "bar"))
y
S7_data(y)
S7_data(y) <- c("a", "b")
y

Does this object inherit from an S7 class?

Description

• S7_inherits() returns TRUE or FALSE.

• check_is_S7() throws an error if x isn't the specified class.

Usage

S7_inherits(x, class = NULL)
check_is_S7(x, class = NULL, arg = deparse(substitute(x)))

Arguments

Value

• S7_inherits() returns a single TRUE or FALSE.

• check_is_S7() returns nothing; it's called for its side-effects.

Note

Starting with R 4.3.0, base::inherits() can accept an S7 class as the second argument, supporting usage like inherits(x, Foo).

Examples

Foo1 <- new_class("Foo1")
Foo2 <- new_class("Foo2")
S7_inherits(Foo1(), Foo1)
check_is_S7(Foo1())
check_is_S7(Foo1(), Foo1)
S7_inherits(Foo1(), Foo2)
try(check_is_S7(Foo1(), Foo2))
if (getRversion() >= "4.3.0")
 inherits(Foo1(), Foo1)

Base S7 class

Description

The base class from which all S7 classes eventually inherit from.

Usage

S7_object()

Value

The base S7 object.

Examples

S7_object

Standard class specifications

Description

This is used as the interface between S7 and R's other OO systems, allowing you to use S7 classes and methods with base types, informal S3 classes, and formal S4 classes.

Usage

as_class(x, arg = deparse(substitute(x)))

Arguments

Value

A standardised class: either NULL, an S7 class, an S7 union, as new_S3_class, or a S4 class.

Examples

as_class(class_logical)
as_class(new_S3_class("factor"))

S7 wrappers for base types

Description

The following S7 classes represent base types allowing them to be used within S7:

• class_logical

• class_integer

• class_double

• class_complex

• class_character

• class_raw

• class_list

• class_expression

• class_name

• class_call

• class_function

• class_environment (can only be used for properties)

We also include three union types to model numerics, atomics, and vectors respectively:

• class_numeric is a union of class_integer and class_double.

• class_atomic is a union of class_logical, class_numeric, class_complex, class_character, and class_raw.

• class_vector is a union of class_atomic, class_list, and class_expression.

• class_language is a union of class_name and class_call.

Usage

class_logical
class_integer
class_double
class_complex
class_character
class_raw
class_list
class_expression
class_name
class_call
class_function
class_environment
class_numeric
class_atomic
class_vector
class_language

Value

S7 classes wrapping around common base types and S3 classes.

Examples

class_integer
class_numeric
class_factor

S7 wrappers for key S3 classes

Description

S7 bundles S3 definitions for key S3 classes provided by the base packages:

• class_data.frame for data frames.

• class_Date for dates.

• class_factor for factors.

• class_POSIXct, class_POSIXlt and class_POSIXt for date-times.

• class_formula for formulas.

Usage

class_factor
class_Date
class_POSIXct
class_POSIXlt
class_POSIXt
class_data.frame
class_formula

Dispatch on any class

Description

Use class_any to register a default method that is called when no other methods are matched.

Usage

class_any

Examples

foo <- new_generic("foo", "x")
method(foo, class_numeric) <- function(x) "number"
method(foo, class_any) <- function(x) "fallback"
foo(1)
foo("x")

Dispatch on a missing argument

Description

Use class_missing to dispatch when the user has not supplied an argument, i.e. it's missing in the sense of missing() , not in the sense of is.na() .

Usage

class_missing

Value

Sentinel objects used for special types of dispatch.

Examples

foo <- new_generic("foo", "x")
method(foo, class_numeric) <- function(x) "number"
method(foo, class_missing) <- function(x) "missing"
method(foo, class_any) <- function(x) "fallback"
foo(1)
foo()
foo("")

Convert an object from one type to another

Description

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:

1. When from inherits from to, it strips any properties that from possesses that to does not (downcasting).

2. 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

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)
# Upcasting: S7 also provides a default implementation for coercing an object
# to one of its child classes:
convert(Foo1(x = 1L), to = Foo2)
convert(Foo1(x = 1L), to = Foo2, y = 2.5) # Set new property
convert(Foo1(x = 1L), to = Foo2, x = 2L, y = 2.5) # Override existing and set new
# For all other cases, you'll need to provide your own.
try(convert(Foo1(x = 1L), to = class_integer))
method(convert, list(Foo1, class_integer)) <- function(from, to) {
 from@x
}
convert(Foo1(x = 1L), to = class_integer)
# 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)
# Converting to Foo2 will still error
try(convert(1L, to = Foo2))
# This is probably not surprising because foo2 also needs some value
# for `@y`, but it definitely makes dispatch for convert() special

Find a method for an S7 generic

Description

method() takes a generic and class signature and performs method dispatch to find the corresponding method implementation. This is rarely needed because you'll usually rely on the the generic to do dispatch for you (via S7_dispatch() ). However, this introspection is useful if you want to see the implementation of a specific method.

Usage

method(generic, class = NULL, object = NULL)

Arguments

Value

Either a function with class S7_method or an error if no matching method is found.

See Also

method_explain() to explain why a specific method was picked.

Examples

# Create a generic and register some methods
bizarro <- new_generic("bizarro", "x")
method(bizarro, class_numeric) <- function(x) rev(x)
method(bizarro, class_factor) <- function(x) {
 levels(x) <- rev(levels(x))
 x
}
# Printing the generic shows the registered method
bizarro
# And you can use method() to inspect specific implementations
method(bizarro, class = class_integer)
method(bizarro, object = 1)
method(bizarro, class = class_factor)
# errors if method not found
try(method(bizarro, class = class_data.frame))
try(method(bizarro, object = "x"))

Register an S7 method for a generic

Description

A generic defines the interface of a function. Once you have created a generic with new_generic() , you provide implementations for specific signatures by registering methods with ⁠method<-⁠.

The goal is for ⁠method<-⁠ to be the single function you need when working with S7 generics or S7 classes. This means that as well as registering methods for S7 classes on S7 generics, you can also register methods for S7 classes on S3 or S4 generics, and S3 or S4 classes on S7 generics. But this is not a general method registration function: at least one of generic and signature needs to be from S7.

Note that if you are writing a package, you must call methods_register() in your .onLoad. This ensures that all methods are dynamically registered when needed.

Usage

method(generic, signature) <- value

Arguments

Value

The generic, invisibly.

Examples

# Create a generic
bizarro <- new_generic("bizarro", "x")
# Register some methods
method(bizarro, class_numeric) <- function(x) rev(x)
method(bizarro, new_S3_class("data.frame")) <- function(x) {
 x[] <- lapply(x, bizarro)
 rev(x)
}
# Using a generic calls the methods automatically
bizarro(head(mtcars))

Explain method dispatch

Description

method_explain() shows all possible methods that a call to a generic might use, which ones exist, and which one will actually be called.

Note that method dispatch uses a string representation of each class in the class hierarchy. Each class system uses a slightly different convention to avoid ambiguity.

• S7: pkg::class or class

• S4: S4/pkg::class or S4/class

• S3: class

Usage

method_explain(generic, class = NULL, object = NULL)

Arguments

Value

Nothing; this function is called for it's side effects.

Examples

Foo1 <- new_class("Foo1")
Foo2 <- new_class("Foo2", Foo1)
add <- new_generic("add", c("x", "y"))
method(add, list(Foo2, Foo1)) <- function(x, y) c(2, 1)
method(add, list(Foo1, Foo1)) <- function(x, y) c(1, 1)
method_explain(add, list(Foo2, Foo2))

Register methods in a package

Description

When using S7 in a package you should always call methods_register() when your package is loaded. This ensures that methods are registered as needed when you implement methods for generics (S3, S4, and S7) in other packages. (This is not strictly necessary if you only register methods for generics in your package, but it's better to include it and not need it than forget to include it and hit weird errors.)

Usage

methods_register()

Value

Nothing; called for its side-effects.

Examples

.onLoad <- function(...) {
 S7::methods_register()
}

Declare an S3 class

Description

To use an S3 class with S7, you must explicitly declare it using new_S3_class() because S3 lacks a formal class definition. (Unless it's an important base class already defined in base_s3_classes.)

Usage

new_S3_class(class, constructor = NULL, validator = NULL)

Arguments

Value

An S7 definition of an S3 class, i.e. a list with class S7_S3_class.

Method dispatch, properties, and unions

There are three ways of using S3 with S7 that only require the S3 class vector:

• Registering a S3 method for an S7 generic.

• Restricting an S7 property to an S3 class.

• Using an S3 class in an S7 union.

This is easy, and you can usually include the new_S3_class() call inline:

method(my_generic, new_S3_class("factor")) <- function(x) "A factor"
new_class("MyClass", properties = list(types = new_S3_class("factor")))
new_union("character", new_S3_class("factor"))

Extending an S3 class

Creating an S7 class that extends an S3 class requires more work. You'll also need to provide a constructor for the S3 class that follows S7 conventions. This means the first argument to the constructor should be .data, and it should be followed by one argument for each attribute used by the class.

This can be awkward because base S3 classes are usually heavily wrapped for user convenience and no low level constructor is available. For example, the factor class is an integer vector with a character vector of levels, but there's no base R function that takes an integer vector of values and character vector of levels, verifies that they are consistent, then creates a factor object.

You may optionally want to also provide a validator function which will ensure that validate() confirms the validity of any S7 classes that build on this class. Unlike an S7 validator, you are responsible for validating the types of the attributes.

The following code shows how you might wrap the base Date class. A Date is a numeric vector with class Date that can be constructed with .Date().

S3_Date <- new_S3_class("Date",
 function(.data = integer()) {
 .Date(.data)
 },
 function(self) {
 if (!is.numeric(self)) {
 "Underlying data must be numeric"
 }
 }
)

Examples

# No checking, just used for dispatch
Date <- new_S3_class("Date")
my_generic <- new_generic("my_generic", "x")
method(my_generic, Date) <- function(x) "This is a date"
my_generic(Sys.Date())

Define a new S7 class

Description

A class specifies the properties (data) that each of its objects will possess. The class, and its parent, determines which method will be used when an object is passed to a generic.

Learn more in vignette("classes-objects")

Usage

new_class(
 name,
 parent = S7_object,
 package = topNamespaceName(parent.frame()),
 properties = list(),
 abstract = FALSE,
 constructor = NULL,
 validator = NULL
)
new_object(.parent, ...)

Arguments

Value

A object constructor, a function that can be used to create objects of the given class.

Examples

# Create an class that represents a range using a numeric start and end
Range <- new_class("Range",
 properties = list(
 start = class_numeric,
 end = class_numeric
 )
)
r <- Range(start = 10, end = 20)
r
# get and set properties with @
r@start
r@end <- 40
r@end
# S7 automatically ensures that properties are of the declared types:
try(Range(start = "hello", end = 20))
# But we might also want to use a validator to ensure that start and end
# are length 1, and that start is < end
Range <- new_class("Range",
 properties = list(
 start = class_numeric,
 end = class_numeric
 ),
 validator = function(self) {
 if (length(self@start) != 1) {
 "@start must be a single number"
 } else if (length(self@end) != 1) {
 "@end must be a single number"
 } else if (self@end < self@start) {
 "@end must be great than or equal to @start"
 }
 }
)
try(Range(start = c(10, 15), end = 20))
try(Range(start = 20, end = 10))
r <- Range(start = 10, end = 20)
try(r@start <- 25)

Generics in other packages

Description

You need an explicit external generic when you want to provide methods for a generic (S3, S4, or S7) that is defined in another package, and you don't want to take a hard dependency on that package.

The easiest way to provide methods for generics in other packages is import the generic into your NAMESPACE. This, however, creates a hard dependency, and sometimes you want a soft dependency, only registering the method if the package is already installed. new_external_generic() allows you to provide the minimal needed information about a generic so that methods can be registered at run time, as needed, using methods_register() .

Note that in tests, you'll need to explicitly call the generic from the external package with pkg::generic().

Usage

new_external_generic(package, name, dispatch_args, version = NULL)

Arguments

Value

An S7 external generic, i.e. a list with class S7_external_generic.

Examples

MyClass <- new_class("MyClass")
your_generic <- new_external_generic("stats", "median", "x")
method(your_generic, MyClass) <- function(x) "Hi!"

Define a new generic

Description

A generic function uses different implementations (methods) depending on the class of one or more arguments (the signature). Create a new generic with new_generic() then use method<- to add methods to it.

Method dispatch is performed by S7_dispatch(), which must always be included in the body of the generic, but in most cases new_generic() will generate this for you.

Learn more in vignette("generics-methods")

Usage

new_generic(name, dispatch_args, fun = NULL)
S7_dispatch()

Arguments

Value

An S7 generic, i.e. a function with class S7_generic.

Dispatch arguments

The arguments that are used to pick the method are called the dispatch arguments. In most cases, this will be one argument, in which case the generic is said to use single dispatch. If it consists of more than one argument, it's said to use multiple dispatch.

There are two restrictions on the dispatch arguments: they must be the first arguments to the generic and if the generic uses ..., it must occur immediately after the dispatch arguments.

See Also

new_external_generic() to define a method for a generic in another package without taking a strong dependency on it.

Examples

# A simple generic with methods for some base types and S3 classes
type_of <- new_generic("type_of", dispatch_args = "x")
method(type_of, class_character) <- function(x, ...) "A character vector"
method(type_of, new_S3_class("data.frame")) <- function(x, ...) "A data frame"
method(type_of, class_function) <- function(x, ...) "A function"
type_of(mtcars)
type_of(letters)
type_of(mean)
# If you want to require that methods implement additional arguments,
# you can use a custom function:
mean2 <- new_generic("mean2", "x", function(x, ..., na.rm = FALSE) {
 S7_dispatch()
})
method(mean2, class_numeric) <- function(x, ..., na.rm = FALSE) {
 if (na.rm) {
 x <- x[!is.na(x)]
 }
 sum(x) / length(x)
}
# You'll be warned if you forget the argument:
method(mean2, class_character) <- function(x, ...) {
 stop("Not supported")
}

Define a new property

Description

A property defines a named component of an object. Properties are typically used to store (meta) data about an object, and are often limited to a data of a specific class.

By specifying a getter and/or setter, you can make the property "dynamic" so that it's computed when accessed or has some non-standard behaviour when modified. Dynamic properties are not included as an argument to the default class constructor.

See the "Properties: Common Patterns" section in vignette("class-objects") for more examples.

Usage

new_property(
 class = class_any,
 getter = NULL,
 setter = NULL,
 validator = NULL,
 default = NULL,
 name = NULL
)

Arguments

Value

An S7 property, i.e. a list with class S7_property.

Examples

# Simple properties store data inside an object
Pizza <- new_class("Pizza", properties = list(
 slices = new_property(class_numeric, default = 10)
))
my_pizza <- Pizza(slices = 6)
my_pizza@slices
my_pizza@slices <- 5
my_pizza@slices
your_pizza <- Pizza()
your_pizza@slices
# Dynamic properties can compute on demand
Clock <- new_class("Clock", properties = list(
 now = new_property(getter = function(self) Sys.time())
))
my_clock <- Clock()
my_clock@now; Sys.sleep(1)
my_clock@now
# This property is read only, because there is a 'getter' but not a 'setter'
try(my_clock@now <- 10)
# Because the property is dynamic, it is not included as an
# argument to the default constructor
try(Clock(now = 10))
args(Clock)

Define a class union

Description

A class union represents a list of possible classes. You can create it with new_union(a, b, c) or a | b | c. Unions can be used in two places:

• To allow a property to be one of a set of classes, new_property(class_integer | Range). The default default value for the property will be the constructor of the first object in the union. This means if you want to create an "optional" property (i.e. one that can be NULL or of a specified type), you'll need to write (e.g.) NULL | class_integer.

• As a convenient short-hand to define methods for multiple classes. method(foo, X | Y) <- f is short-hand for ⁠method(foo, X) <- f; method(foo, Y) <- foo⁠

S7 includes built-in unions for "numeric" (integer and double vectors), "atomic" (logical, numeric, complex, character, and raw vectors) and "vector" (atomic vectors, lists, and expressions).

Usage

new_union(...)

Arguments

Value

An S7 union, i.e. a list with class S7_union.

Examples

logical_or_character <- new_union(class_logical, class_character)
logical_or_character
# or with shortcut syntax
logical_or_character <- class_logical | class_character
Foo <- new_class("Foo", properties = list(x = logical_or_character))
Foo(x = TRUE)
Foo(x = letters[1:5])
try(Foo(1:3))
bar <- new_generic("bar", "x")
# Use built-in union
method(bar, class_atomic) <- function(x) "Hi!"
bar
bar(TRUE)
bar(letters)
try(bar(NULL))

Get/set a property

Description

• prop(x, "name") / prop@name get the value of the a property, erroring if it the property doesn't exist.

• prop(x, "name") <- value / prop@name <- value set the value of a property.

Usage

prop(object, name)
prop(object, name, check = TRUE) <- value
object@name

Arguments

Value

prop() and @ return the value of the property. ⁠prop<-()⁠ and ⁠@<-⁠ are called for their side-effects and return the modified object, invisibly.

Examples

Horse <- new_class("Horse", properties = list(
 name = class_character,
 colour = class_character,
 height = class_numeric
))
lexington <- Horse(colour = "bay", height = 15, name = "Lex")
lexington@colour
prop(lexington, "colour")
lexington@height <- 14
prop(lexington, "height") <- 15

Property introspection

Description

• prop_names(x) returns the names of the properties

• prop_exists(x, "prop") returns TRUE iif x has property prop.

Usage

prop_names(object)
prop_exists(object, name)

Arguments

Value

prop_names() returns a character vector; prop_exists() returns a single TRUE or FALSE.

Examples

Foo <- new_class("Foo", properties = list(a = class_character, b = class_integer))
f <- Foo()
prop_names(f)
prop_exists(f, "a")
prop_exists(f, "c")

Get/set multiple properties

Description

• props(x) returns all properties.

• props(x) <- list(name1 = val1, name2 = val2) modifies an existing object by setting multiple properties simultaneously.

• set_props(x, name1 = val1, name2 = val2) creates a copy of an existing object with new values for the specified properties.

Usage

props(object, names = prop_names(object))
props(object, check = TRUE) <- value
set_props(object, ..., .check = TRUE)

Arguments

Value

A named list of property values.

Examples

Horse <- new_class("Horse", properties = list(
 name = class_character,
 colour = class_character,
 height = class_numeric
))
lexington <- Horse(colour = "bay", height = 15, name = "Lex")
props(lexington)
props(lexington) <- list(height = 14, name = "Lexington")
lexington

Force method dispatch to use a superclass

Description

super(from, to) causes the dispatch for the next generic to use the method for the superclass to instead of the actual class of from. It's needed when you want to implement a method in terms of the implementation of its superclass.

S3 & S4

super() performs a similar role to NextMethod() in S3 or methods::callNextMethod() in S4, but is much more explicit:

• The super class that super() will use is known when write super() (i.e. statically) as opposed to when the generic is called (i.e. dynamically).

• All arguments to the generic are explicit; they are not automatically passed along.

This makes super() more verbose, but substantially easier to understand and reason about.

super() in S3 generics

Note that you can't use super() in methods for an S3 generic. For example, imagine that you have made a subclass of "integer":

MyInt <- new_class("MyInt", parent = class_integer, package = NULL)

Now you go to write a custom print method:

method(print, MyInt) <- function(x, ...) {
 cat("<MyInt>")
 print(super(x, to = class_integer))
}
MyInt(10L)
#> <MyInt>super(<MyInt>, <integer>)

This doesn't work because print() isn't an S7 generic so doesn't understand how to interpret the special object that super() produces. While you could resolve this problem with NextMethod() (because S7 is implemented on top of S3), we instead recommend using S7_data() to extract the underlying base object:

method(print, MyInt) <- function(x, ...) {
 cat("<MyInt>")
 print(S7_data(x))
}
MyInt(10L)
#> <MyInt>[1] 10

Usage

super(from, to)

Arguments

Value

An S7_super object which should always be passed immediately to a generic. It has no other special behavior.

Examples

Foo1 <- new_class("Foo1", properties = list(x = class_numeric, y = class_numeric))
Foo2 <- new_class("Foo2", Foo1, properties = list(z = class_numeric))
total <- new_generic("total", "x")
method(total, Foo1) <- function(x) x@x + x@y
# This won't work because it'll be stuck in an infinite loop:
method(total, Foo2) <- function(x) total(x) + x@z
# We could write
method(total, Foo2) <- function(x) x@x + x@y + x@z
# but then we'd need to remember to update it if the implementation
# for total(<Foo1>) ever changed.
# So instead we use `super()` to call the method for the parent class:
method(total, Foo2) <- function(x) total(super(x, to = Foo1)) + x@z
total(Foo2(1, 2, 3))
# To see the difference between convert() and super() we need a
# method that calls another generic
bar1 <- new_generic("bar1", "x")
method(bar1, Foo1) <- function(x) 1
method(bar1, Foo2) <- function(x) 2
bar2 <- new_generic("bar2", "x")
method(bar2, Foo1) <- function(x) c(1, bar1(x))
method(bar2, Foo2) <- function(x) c(2, bar1(x))
obj <- Foo2(1, 2, 3)
bar2(obj)
# convert() affects every generic:
bar2(convert(obj, to = Foo1))
# super() only affects the _next_ call to a generic:
bar2(super(obj, to = Foo1))

Validate an S7 object

Description

validate() ensures that an S7 object is valid by calling the validator provided in new_class() . This is done automatically when constructing new objects and when modifying properties.

valid_eventually() disables validation, modifies the object, then revalidates. This is useful when a sequence of operations would otherwise lead an object to be temporarily invalid, or when repeated property modification causes a performance bottleneck because the validator is relatively expensive.

valid_implicitly() does the same but does not validate the object at the end. It should only be used rarely, and in performance critical code where you are certain a sequence of operations cannot produce an invalid object.

Usage

validate(object, recursive = TRUE, properties = TRUE)
valid_eventually(object, fun)
valid_implicitly(object, fun)

Arguments

Value

Either object invisibly if valid, otherwise an error.

Examples

# A range class might validate that the start is less than the end
Range <- new_class("Range",
 properties = list(start = class_double, end = class_double),
 validator = function(self) {
 if (self@start >= self@end) "start must be smaller than end"
 }
)
# You can't construct an invalid object:
try(Range(1, 1))
# And you can't create an invalid object with @<-
r <- Range(1, 2)
try(r@end <- 1)
# But what if you want to move a range to the right?
rightwards <- function(r, x) {
 r@start <- r@start + x
 r@end <- r@end + x
 r
}
# This function doesn't work because it creates a temporarily invalid state
try(rightwards(r, 10))
# This is the perfect use case for valid_eventually():
rightwards <- function(r, x) {
 valid_eventually(r, function(object) {
 object@start <- object@start + x
 object@end <- object@end + x
 object
 })
}
rightwards(r, 10)
# Alternatively, you can set multiple properties at once using props<-,
# which validates once at the end
rightwards <- function(r, x) {
 props(r) <- list(start = r@start + x, end = r@end + x)
 r
}
rightwards(r, 20)