Ignore return values of generators (#57)

r-lib · Nov 5, 2024 · 9dc9bc8 · 9dc9bc8
1 parent 1235934
commit 9dc9bc8
Show file tree

Hide file tree

Showing 10 changed files with 154 additions and 145 deletions.
diff --git a/NEWS.md b/NEWS.md
@@ -11,6 +11,10 @@
   are now cleaned up when terminated early, either because of an
   error or because of a `break` (#52).
 
+* Implicit and explicit return values of generators are no longer
+  yielded. This is consistent with Javascript and Python and simplifies
+  certain idioms (#51).
+
 * Generators and async functions assigned in namespaces no
   longer produce R CMD check notes about visible bindings (#40).
 

diff --git a/R/generator.R b/R/generator.R
@@ -29,16 +29,18 @@
 #' @export
 #' @examples
 #' # A generator statement creates a generator factory. The
-#' # following generator yields two times and then returns `"c"`:
+#' # following generator yields three times and then returns `"d"`.
+#' # Only the yielded values are visible to the callers.
 #' generate_abc <- generator(function() {
 #'   yield("a")
 #'   yield("b")
-#'   "c"
+#'   yield("c")
+#'   "d"
 #' })
 #'
-#' # Or equivalently:
+#' # Equivalently:
 #' generate_abc <- generator(function() {
-#'   for (x in letters[1:3]) {
+#'   for (x in c("a", "b", "c")) {
 #'     yield(x)
 #'   }
 #' })
@@ -103,13 +105,15 @@ generator <- function(fn) {
   assert_lambda(substitute(fn))
   generator0(fn)
 }
+
 #' @rdname generator
 #' @param expr A yielding expression.
 #' @export
 gen <- function(expr) {
   fn <- new_function(NULL, substitute(expr), caller_env())
   generator0(fn)()
 }
+
 generator0 <- function(fn, type = "generator") {
   state_machine <- NULL
   fmls <- formals(fn)

diff --git a/R/parser.R b/R/parser.R
@@ -568,7 +568,7 @@ strip_explicit_return <- function(expr) {
 }
 return_call <- function(info) {
   if (is_null(info$async_ops)) {
-    quote(return(last_value()))
+    quote(return(exhausted()))
   } else {
     quote(return(as_promise(last_value())))
   }
@@ -996,8 +996,8 @@ try_catch_states <- function(preamble,
   depth <- machine_depth(counter)
   try_catch_depth <- depth + 1L
 
-  # Handlers can't be evaluated until runtime. We store them in a list
-  # dynamically.
+  # Handlers can't be evaluated until runtime. We store their expressions in a
+  # list. They are evaluated when the user enters the `tryCatch()` state.
   handler_body <- expr({
     !!!preamble %&&% list(user_call(preamble))
     handlers[[!!try_catch_depth]] <- user(base::list(!!!handlers_exprs))

diff --git a/man/generator.Rd b/man/generator.Rd
diff --git a/tests/testthat/_snaps/generator.md b/tests/testthat/_snaps/generator.md
@@ -28,7 +28,7 @@
               state[[1L]] <- 3L
           }, `3` = {
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -64,7 +64,7 @@
               state[[1L]] <- 3L
           }, `3` = {
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())

diff --git a/tests/testthat/_snaps/parser-block.md b/tests/testthat/_snaps/parser-block.md
@@ -21,7 +21,7 @@
               state[[1L]] <- 3L
           }, `3` = {
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -60,7 +60,7 @@
                   "after2"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -93,7 +93,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -141,7 +141,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -187,7 +187,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -222,7 +222,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -251,7 +251,7 @@
               state[[1L]] <- 3L
           }, `3` = {
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -291,7 +291,7 @@
               state[[1L]] <- 5L
           }, `5` = {
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -336,7 +336,7 @@
                   "after2"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -377,7 +377,7 @@
                   "after2"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -416,7 +416,7 @@
                   "after2"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -481,7 +481,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -520,7 +520,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -559,7 +559,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())

diff --git a/tests/testthat/_snaps/parser-if.md b/tests/testthat/_snaps/parser-if.md
@@ -83,7 +83,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -174,7 +174,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -271,7 +271,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -347,7 +347,7 @@
                       "if-after"
                   })
                   exhausted <- TRUE
-                  return(last_value())
+                  return(exhausted())
               }, `4` = {
                   break
               })
@@ -367,7 +367,7 @@
                       "foo"
                   })
                   exhausted <- TRUE
-                  return(last_value())
+                  return(exhausted())
               }, `2` = {
                   break
               })
@@ -441,7 +441,7 @@
                       state[[3L]] <- 3L
                   }, `3` = {
                       exhausted <- TRUE
-                      return(last_value())
+                      return(exhausted())
                   }, `4` = {
                       break
                   })
@@ -476,7 +476,7 @@
                       "foo"
                   })
                   exhausted <- TRUE
-                  return(last_value())
+                  return(exhausted())
               }, `2` = {
                   break
               })
@@ -589,7 +589,7 @@
                         "if-2-after"
                       })
                       exhausted <- TRUE
-                      return(last_value())
+                      return(exhausted())
                   }, `4` = {
                       break
                   })
@@ -624,7 +624,7 @@
                       FALSE
                   })
                   exhausted <- TRUE
-                  return(last_value())
+                  return(exhausted())
               }, `2` = {
                   break
               })
@@ -694,7 +694,7 @@
                       "if-after"
                   })
                   exhausted <- TRUE
-                  return(last_value())
+                  return(exhausted())
               }, `4` = {
                   break
               })
@@ -725,7 +725,7 @@
                       "else-after"
                   })
                   exhausted <- TRUE
-                  return(last_value())
+                  return(exhausted())
               }, `4` = {
                   break
               })
@@ -844,7 +844,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -932,7 +932,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -1026,7 +1026,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -1120,7 +1120,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -1209,7 +1209,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())
@@ -1430,7 +1430,7 @@
                   "after"
               })
               exhausted <- TRUE
-              return(last_value())
+              return(exhausted())
           })
           exhausted <- TRUE
           invisible(exhausted())