train_continuous now preserves the class of the input (#167)

Fixes #166

NEWS.md

@@ -32,6 +32,9 @@
 * `log_breaks()` provides usable breaks even with very small ranges
   (@billdenney, #168)
 
+* `train_continuous()` now maintains the class of inputs when they are not
+  numeric (@billdenney, #166).
+
 # scales 1.0.0
 
 ## New Features

R/scale-continuous.r

@@ -38,7 +38,12 @@ train_continuous <- function(new, existing = NULL) {
   if (is.factor(new) || !typeof(new) %in% c("integer", "double")) {
     stop("Discrete value supplied to continuous scale", call. = FALSE)
   }
-  suppressWarnings(range(existing, new, na.rm = TRUE, finite = TRUE))
+  # range(NULL, x) strips attributes
+  if (is.null(existing)) {
+    suppressWarnings(range(new, na.rm = TRUE, finite = TRUE))
+  } else {
+    suppressWarnings(range(existing, new, na.rm = TRUE, finite = TRUE))
+  }
 }
 
 # Map values for a continuous palette.

tests/testthat/test-scale.r

@@ -19,3 +19,40 @@ test_that("NA.value works for discrete", {
   expect_that(dscale(x, pal, "grey50")[1], equals("grey50"))
   expect_that(dscale(x, pal, "grey50")[5], equals("grey50"))
 })
+
+test_that("train_continuous stops on discrete values", {
+  expect_error(train_continuous(LETTERS[1:5]),
+    regexp = "Discrete value supplied"
+  )
+})
+
+test_that("train_continuous maintains class with `existing=NULL`.", {
+  expect_equal(
+    train_continuous(1:5),
+    c(1, 5)
+  )
+  my_date <- as.POSIXct("2018-01-01")
+  expect_equal(
+    train_continuous(my_date),
+    c(my_date, my_date)
+  )
+})
+
+test_that("train_continuous changes class to the class of `existing` when not NULL.", {
+  my_date <- as.POSIXct("2018-01-01")
+  expect_equal(
+    train_continuous(my_date, existing = 1),
+    c(1, my_date)
+  )
+  expect_equal(
+    train_continuous(1, existing = my_date),
+    c(as.POSIXct(1, origin = "1970-01-01"), my_date)
+  )
+})
+
+test_that("train_continuous with new=NULL maintains existing range.", {
+  expect_equal(
+    train_continuous(NULL, existing = c(1, 5)),
+    c(1, 5)
+  )
+})