Evaluate an S7 call under an interface or trait contract

with(contract, expr) and expr %::% contract evaluate expr in a contract mask. Required generics are shadowed by checking wrappers, so calls to those generics use normal S7 dispatch while checking the optional argument and return specifications stored in an interface requirement or trait method.

Usage

expr %::% contract

Arguments

expr

An expression evaluated in a contract mask. Calls to generics named in the contract are checked.

contract

An interface created by new_interface() or a trait created by new_trait().

Value

The value of expr, after any optional return check.

Examples

local({
  draw <- S7::new_generic("draw", "x", function(x, color) {
    S7::S7_dispatch()
  })
  Circle <- S7::new_class("TypedCircle", properties = list(r = S7::class_double))
  S7::method(draw, Circle) <- function(x, color) paste(color, x@r)
  Drawable <- new_interface(
    "TypedDrawable",
    generics = list(draw = interface_requirement(
      draw,
      args = list(color = S7::class_character),
      returns = S7::class_character
    ))
  )
  with(Drawable, draw(Circle(r = 2), color = "red"))
  checked_draw <- with(Drawable, function(x) draw(x, color = "red"))
  checked_draw(Circle(r = 2))
  draw(Circle(r = 2), color = "red") %::% Drawable
})
#> [1] "red 2"