address knitr installation dependency (#790)

Author: simonpcouch

Diff for: NEWS.md

@@ -1,5 +1,7 @@
 # parsnip (development version)
 
+* Fixed installation failures due to undocumented knitr installation dependency (#785).
+
 # parsnip 1.0.1
 
 * Enabled passing additional engine arguments with the xgboost `boost_tree()` engine. To supply engine-specific arguments that are documented in `xgboost::xgb.train()` as arguments to be passed via `params`, supply the list elements directly as named arguments to `set_engine()`. Read more in `?details_boost_tree_xgboost` (#787).

Diff for: R/aaa.R

@@ -80,3 +80,13 @@ function(results, object) {
   }
   res
 }
+
+combine_words <- function(x) {
+  if (isTRUE(length(x) > 2)) {
+    last <- ", and "
+  } else {
+    last <- " and "
+  }
+
+  glue::glue_collapse(x, ", ", last = last)
+}

Diff for: R/engine_docs.R

@@ -224,7 +224,7 @@ make_engine_list <- function(mod) {
         dplyr::pull(mode) %>%
         unique() %>%
         sort() %>%
-        knitr::combine_words()
+        combine_words()
       notes <- paste0(
         notes, " ",
         cli::symbol$sup_2, " Requires a parsnip extension package for ",

Diff for: man/rmd/aaa.Rmd

@@ -119,7 +119,7 @@ descr_models <- function(mod, eng) {
   } else {
     txt <- "are multiple modes:"
   }
-  paste("For this engine, there", txt, knitr::combine_words(res))
+  paste("For this engine, there", txt, combine_words(res))
 }
 
 uses_extension <- function(mod, eng, mod_mode) {

Diff for: tests/testthat/_snaps/misc.md

@@ -1,3 +1,31 @@
+# combine_words helper works
+
+    Code
+      combine_words(1)
+    Output
+      1
+
+---
+
+    Code
+      combine_words(1:2)
+    Output
+      1 and 2
+
+---
+
+    Code
+      combine_words(1:3)
+    Output
+      1, 2, and 3
+
+---
+
+    Code
+      combine_words(1:4)
+    Output
+      1, 2, 3, and 4
+
 # model type functions message informatively with unknown implementation
 
     Code

Diff for: tests/testthat/test_misc.R

@@ -62,6 +62,14 @@ test_that('S3 method dispatch/registration', {
 
 })
 
+# ------------------------------------------------------------------------------
+test_that("combine_words helper works", {
+  expect_snapshot(combine_words(1))
+  expect_snapshot(combine_words(1:2))
+  expect_snapshot(combine_words(1:3))
+  expect_snapshot(combine_words(1:4))
+})
+
 # ------------------------------------------------------------------------------
 
 test_that('control class', {