Declarative post conditions

post-condition.R

`%?%` <- function(value, predicate) {
  list(value = value, predicate = predicate(value))
}

# Implicitly, `assert` is either a boolean or a value-predicate pair
`%because%` <- function(assert, reason) {
  if (is.logical(assert)) {
    if (!assert) stop(reason, call. = FALSE)
    return(invisible(TRUE))
  }
  if (!assert$predicate) stop(reason, call. =  FALSE)
  invisible(assert$value)
}

# Simple assertion (returns TRUE, invisibly)
is.function(identity) %because% "`identity` should be a function"

# Verify and assign
f <- identity %?% is.function %because% "`f` must be a function"
stopifnot(identical(f, identity))

# Error: `f` must be a function
f <- NULL %?% is.function %because% "`f` must be a function"