Improve addition method

Fixes #286

NEWS.md

@@ -1,5 +1,9 @@
 # glue (development version)
 
+* `+` now works in more situations, and gives errors when one side isn't a 
+  character vector. It no longer automatically applies glue interpolation to
+  both sides; you'll need to do that yourself as needed (#286).
+
 * Add `$(C_VISIBILITY)` to compiler flags to hide internal symbols from the dll (#284 @lionel-).
 
 # glue 1.6.2

R/glue.R

@@ -337,7 +337,14 @@ as.character.glue <- function(x, ...) {
 
 #' @export
 `+.glue` <- function(e1, e2) {
-  glue(e1, e2, .envir = parent.frame())
+  if (!is.character(e1)) {
+    stop("LHS must be a character vector.")
+  }
+  if (!is.character(e2)) {
+    stop("RHS must be a character vector.")
+  }
+
+  as_glue(paste0(e1, e2))
 }
 
 #' @importFrom methods setOldClass

tests/testthat/_snaps/glue.md

@@ -1,3 +1,14 @@
+# + method requires character vectors
+
+    Code
+      as_glue("a") + 1
+    Error <simpleError>
+      RHS must be a character vector.
+    Code
+      1 + as_glue("a")
+    Error <simpleError>
+      LHS must be a character vector.
+
 # unterminated comment
 
     Code

tests/testthat/test-glue.R

@@ -509,9 +509,17 @@ test_that("throws informative error if interpolating a function", {
 
 test_that("+ method for glue works", {
   expect_identical(glue("foo") + "bar", as_glue("foobar"))
+  expect_identical(glue("x = ") + "{x}", as_glue("x = {x}"))
 
-  x <- 1
-  expect_identical(glue("x = ") + "{x}", glue("x = {x}"))
+  x <- c("a", "b", "c")
+  expect_identical("(" + as_glue(x) + ")", paste0("(", x, ")"))
+})
+
+test_that("+ method requires character vectors", {
+  expect_snapshot(error = TRUE, {
+    as_glue("a") + 1
+    1 + as_glue("a")
+  })
 })
 
 test_that("unterminated quotes are error", {