Apply @coatless suggestion.

.ci/macos-steps.yaml

@@ -27,7 +27,7 @@ steps:
 
     if [ "$(binding)" == "R" ]; then
       if [ "a$(R.version)" != "a" ]; then
-        brew install r
+        brew cask install r
       fi
       brew cask install gfortran
       Rscript -e "install.packages(c('Rcpp', 'RcppArmadillo', 'RcppEnsmallen', 'BH', 'roxygen2', 'testthat'), repos = 'http://cran.us.r-project.org')"

src/mlpack/bindings/R/CMakeLists.txt

@@ -51,9 +51,8 @@ if (BUILD_R_BINDINGS)
     set(R_NOT_FOUND_MSG "${R_NOT_FOUND_MSG}\n    - testthat")
   endif ()
 
-  ## We need to check here if R is even available.  Although actually
-  ## technically, I'm not sure if we even need to know!  For the tests though we
-  ## do.  So it's probably a good idea to check.
+  ## We need to check here if R and other dependencies is even available, as
+  ## it is require to build R-bindings.
   if (FORCE_BUILD_R_BINDINGS)
     if (NOT R_FOUND OR NOT R_RCPP OR NOT R_RCPPARMADILLO OR NOT R_RCPPENSMALLEN
         OR NOT R_BH OR NOT R_ROXYGEN2 OR NOT R_TESTTHAT)