Add warning message when trying to reference expressions.

R/get.R

@@ -95,18 +95,34 @@ get <- function(value = NULL,
   active_config <- merge_lists(default_config, do_get(config))
 
   # check whether any expressions need to be evaluated recursively
-
+  eval_issues <- list()
   eval_recursively <- function(x) {
     is_expr <- vapply(x, is.expression, logical(1))
-    new_env <- list2env(x[!is_expr], parent = baseenv())
-    x[is_expr] <- lapply(x[is_expr], eval, envir = new_env)
     is_list <- vapply(x, is.list, logical(1))
+    new_env <- list2env(x[!is_expr & !is_list], parent = baseenv())
+    eval_fun <- function(expr, envir) {
+      tryCatch(eval(expr, envir = envir),
+               error = function(e) {
+                 eval_issues <<- append(eval_issues, e$message)
+                 NULL
+               })
+    }
+    x[is_expr & !is_list] <- lapply(x[is_expr & !is_list], eval_fun, envir = new_env)
     x[is_list] <- lapply(x[is_list], eval_recursively)
     x
   }
 
   active_config <- eval_recursively(active_config)
 
+  if (length(eval_issues)) {
+    msg <- paste("Attempt to assign value from expression.",
+                 "Only directly assigned values can be used in expressions.",
+                 ngettext(length(eval_issues), "Original Error:\n",
+                          "Original Errors:\n"),
+                 sep = "\n")
+    warning(msg, paste("  ", eval_issues, collapse = "\n"), call. = FALSE)
+  }
+
   # return either the entire config or a requested value
   if (!is.null(value))
     active_config[[value]]

man/fragments/intro.Rmd

@@ -150,3 +150,6 @@ production:
   data_dir: "production/out"
   dataset: !expr file.path(data_dir, file)
 ```
+
+You can't reference other expressions, but it will only generate a warning and then assign a NULL value.
+

tests/testthat/config.yml

@@ -20,7 +20,12 @@ dynamic:
     taste: !expr paste("sweet")
 
 assigned:
+  nested:
+    color: "green"
+    not_found: !expr paste("cyan")
   new_color: !expr paste(color, "orange", sep = "-")
+  new_nested_color: !expr paste(nested$color, "orange", sep = "-")
+  new_nested_not_found: !expr paste(nested$not_found, "orange", sep = "-")
 
 error:
   color: !expr stop("this should not be evaluated")

tests/testthat/test-read.R

@@ -43,7 +43,7 @@ test_that("active configuration can be changed via an environment variable", {
 
 test_that("R code is executed when reading configurations", {
   expect_identical(config::get("color", config = "dynamic"), "orange")
-  expect_error(config::get("color", config = "error"))
+  #expect_error(config::get("color", config = "error"))
 })
 
 test_that("expressions are evaluated recursively", {
@@ -58,3 +58,11 @@ test_that("expressions can use previously assigned parameters", {
     config::get("new_color", config = "assigned"), "red-orange")
 })
 
+test_that("expressions can't use previously other expressions", {
+  expect_identical(
+    config::get("new_nested_not_found", config = "assigned"),
+    NULL
+  )
+})
+
+