How to create a package namespace without saving any files! ...Wait, why would you though

make_package_stub.R

#' Generate a minimal fake package namespace
#'
#' Mocks up a tiny package namespace and monkey-patches it into the current R
#' sessions's namespace registry. This abuses some R internals and has high
#' potential to break things for the remainder of your session. Use it with 
#' great caution, or maybe not at all.
#'
#' The intended use case was to provide nonfunctional skeletons of selected
#' functions from packages that are not installed, solely so that they could
#' then be replaced by test stubs. Embarrassingly soon after writing this
#' (within minutes!), I learned that mockery::stub works for uninstalled 
#' functions without any need for a fake package, so this function is now of
#' entirely academic interest.
#'
#' The core logic is mostly taken from base::loadNamespace, but don't blame
#' the R-core team for any use or abuse of this function.
#'
#' @param name The name to assign to the namespace
#' @param functions named vector of function definitions to export. 
#' @param version a version string, for those who track their monkey-patches 
#'        very carefully
#' @param load,attach logical: load namespace and attach to the search path
#'        after registering it?
#'
#' @author Chris Black
#'
#' @examples 
#' \dontrun{
#'   f = function()notReal::g()
#'   f() # Error in loadNamespace(name) : there is no package called 'notReal'
#'
#'   if(length(find.package("notReal")) == 0){
#'     makeNamespaceStub("notReal", c(g=function()1))
#'     on.exit(.Internal(unregisterNamespace("notReal")))
#'   }
#'
#'   f() # 1
#'   mockery::stub(f, "notReal::g", function()2)
#'   f() # 2
#' }
#'
make_package_stub <- function(name, functions, version="0.0.1", load = FALSE, attach = FALSE){
  import_env <- new.env(parent = .BaseNamespaceEnv, hash = TRUE)
  attr(import_env, "name") <- paste0("imports:", name)
  pkg_env <- new.env(parent = import_env, hash = TRUE)
  
  info <- new.env(hash = TRUE, parent = baseenv())
  pkg_env$.__NAMESPACE__. <- info
  info$spec <- c(name = name, version = version)
  setNamespaceInfo(pkg_env, "imports", list(base = TRUE))
  
  export_env <- new.env(parent = baseenv())
  for (i in seq_along(functions)){
      assign(names(functions)[[i]], functions[[i]], envir = pkg_env)   
  }
  setNamespaceInfo(pkg_env, "exports", export_env)
  
  data_env <- new.env(parent = baseenv(), hash = TRUE)
  attr(data_env, "name") <- paste0("lazydata:", name)
  setNamespaceInfo(pkg_env, "lazydata", data_env)
  setNamespaceInfo(pkg_env, "dynlibs", NULL)
  setNamespaceInfo(pkg_env, "S3methods", matrix(NA_character_, 0L, 3L))
  pkg_env$.__S3MethodsTable__. <- new.env(hash = TRUE, parent = baseenv())
  
  .Internal(registerNamespace(name, pkg_env))
  namespaceExport(pkg_env, names(functions))
  lockEnvironment(pkg_env, TRUE)
  lockEnvironment(parent.env(pkg_env), TRUE)

  if(load){
      loadNamespace(name)
  }
  if(attach){
      attachNamespace(name)
  }

  invisible(pkg_env)
}