Turned routines to preprocess user inputs into S3 methods.

Still needs to be tested.

R/R/process_user_input.r

@@ -0,0 +1,53 @@
+process_user_input <- function (x) {
+  UseMethod("process_user_input", x)
+}
+
+process_user_input.default <- function(x, ...) {
+  stop(
+    deparse(substitute(x)),
+    " must be a 'data.frame',",
+    " a 'response',",
+    " or a 'character' vector containing JSON.",
+    call. = FALSE
+  )
+}
+
+process_user_input.data.frame <- function(x, ...) {
+    x
+}
+
+process_user_input.character <- function(x, ...) {
+
+  if (!requireNamespace("jsonlite", quietly = TRUE)) {
+    stop(
+      "The package 'jsonlite' is needed to convert ",
+      deparse(substitute(x)),
+      " into a data frame.",
+      "\n       Either install 'jsonlite' or pass ",
+      deparse(substitute(x)),
+      " as a 'data.frame'.",
+      call. = FALSE
+    )
+  }
+
+  jsonlite::fromJSON(x)
+
+}
+
+process_user_input.response <- function(x, ...) {
+
+  if (!requireNamespace("httr", quietly = TRUE)) {
+    stop(
+      "The package 'httr' is needed to convert ",
+      deparse(substitute(x)),
+      " into a data frame.",
+      "\n       Either install 'httr' or pass ",
+      deparse(substitute(x)),
+      " as a 'data.frame'.",
+      call. = FALSE
+    )
+  }
+
+  httr::content(x, as = "text")
+
+}