new_interface: Build a Go-like structural interface on top of S7

In s7contract: Behavioral Contracts and Generative Laws for 'S7'

Build a Go-like structural interface on top of S7

Description

new_interface() models the method-list part of Go interfaces as a list of required S7 generics. An interface is just a named set of required generics, and a class or object satisfies it when S7 can find a method for every required generic.

Usage

new_interface(
  name,
  generics = list(),
  parents = list(),
  package = NULL,
  methods = NULL
)

interface_requirement(
  generic,
  name = NULL,
  args = list(),
  returns = S7::class_any
)

Arguments

name	For new_interface(), the interface name. For interface_requirement(), the requirement name; it defaults to the generic name when omitted.
generics	For new_interface(), a named list of S7 generics or interface_requirement() objects. These are generic functions because S7 methods are registered separately on generics.
parents	Optional interface or list of interfaces to embed.
package	Optional package name used only for display.
methods	Compatibility alias for generics.
generic	An S7 generic function.
args	Optional named list of S7 classes, interfaces, or traits for runtime argument checking with with() or ⁠%::%⁠. Arguments named in args are also checked against generic and method formals during conformance checks. Dispatch arguments other than the first can use S7 classes or unions to refine multiple-dispatch requirements. Every concrete combination in those unions must have a compatible method.
returns	Optional S7 class, interface, or trait for runtime return checking with with() or ⁠%::%⁠; defaults to S7::class_any.

Details

This mirrors Go's basic interfaces defined only by methods. Define small interfaces at the point where consuming code needs a behavior. Define S7 classes, generics, and methods normally; then let consumers name the protocol they accept. Up-front interfaces are also useful for package protocols, abstract data types, or recursive protocols.

Go's full post-1.18 type-set language is outside this model, including tilde type terms, unions of concrete types, or pointer/value receiver rules.

Value

new_interface() returns an S7 object of class s7_interface. interface_requirement() returns an S7 object of class s7_interface_requirement.

Examples

local({
  area <- S7::new_generic("area", "x")
  draw <- S7::new_generic("draw", "x")

  Circle <- S7::new_class(
    "Circle",
    properties = list(r = S7::class_double)
  )
  Rect <- S7::new_class(
    "Rect",
    properties = list(w = S7::class_double, h = S7::class_double)
  )

  S7::method(area, Circle) <- function(x) pi * x@r^2
  S7::method(draw, Circle) <- function(x) sprintf("circle(r = %s)", x@r)
  S7::method(area, Rect) <- function(x) x@w * x@h

  Drawable <- new_interface("Drawable", generics = list(draw = draw))
  Shape <- new_interface("Shape", generics = list(area = area), parents = Drawable)

  implements(Circle, Shape)
  missing_requirements(Rect, Shape)
})

s7contract documentation built on Sept. 10, 2026, 1:09 a.m.