Build a Go-like structural interface on top of S7 — new

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.

Value

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

Details

Embedding retains every parent requirement. Requirements sharing an alias or a generic name must have identical generics and type specifications; incompatible collisions are errors. Both aliases and generic names refer to the checked operation inside with(). Reusing a requirement across parents is valid, including diamond-shaped inheritance.

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.

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)
})
#>      interface requirement    ok                               message
#> draw     Shape        draw FALSE Can't find method for `draw(<Rect>)`.