Better err msg for read-only active bindings

R/clone.R

@@ -158,7 +158,7 @@ generator_funs$clone_method <- function(deep = FALSE) {
 
   if (length(active_copies) > 0) {
     for (name in names(active_copies)) {
-      makeActiveBinding(name, active_copies[[name]], public_bind_env)
+      make_active_binding(name, active_copies[[name]], public_bind_env)
     }
   }
 

R/new.R

@@ -118,7 +118,7 @@ generator_funs$new <- function(...) {
   # Set up active bindings ------------------------------------------
   if (!is.null(active)) {
     for (name in names(active)) {
-      makeActiveBinding(name, active[[name]], public_bind_env)
+      make_active_binding(name, active[[name]], public_bind_env)
     }
   }
 
@@ -223,7 +223,7 @@ encapsulate({
     list2env2(public_methods, envir = super_bind_env)
     list2env2(private_methods, envir = super_bind_env)
     for (name in names(active)) {
-      makeActiveBinding(name, active[[name]], super_bind_env)
+      make_active_binding(name, active[[name]], super_bind_env)
     }
 
     # Return an object with all the information needed to merge down

R/utils.R

@@ -37,3 +37,26 @@ encapsulate({
     x[!funcs]
   }
 })
+
+#' Wraps \code{\link[base]{makeActiveBinding}}, providing a better
+#' error message for read-only bindings.
+#' @param sym A string naming the variable to bind to.
+#' @param fun A function to bind to the variable.
+#' @param env An environment to create the binding in.
+#' @return An active binding.
+#' @examples
+#' fn <- function() 1
+#' make_active_binding("var", fn, globalenv())
+#' var
+#' try(var <- 123)
+#' @noRd
+make_active_binding <- function(sym, fun, env) {
+  new_fun <- if(length(formals(fun)) == 0L) {
+    function(value) {
+      if(!missing(value)) {
+        stop(sym, " is read-only.")
+      } else fun()
+    }
+  } else fun
+  makeActiveBinding(sym, new_fun, env)
+}