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. Only names resolved through the mask are checked; namespace-qualified calls and calls inside separately defined helpers are not instrumented.

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.

Details

Checks force dispatch and typed arguments before the generic body runs. Generic defaults retain their lexical scope and share ordinary R promises. If a typed argument has only a method default, that default is evaluated before dispatch and supplied to the generic. Such defaults should be pure expressions of arguments and lexical bindings, without relying on method-body locals or missing() for that argument.

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"