<- replaced by = for default values

NEWS.md

@@ -11,7 +11,7 @@
 * A `%->%` operator has been added. The right operator performs the same
   operation as `%<-%` with the name structure on the right-hand side and
   the values to assign on the left-hand side.
-* `<-` may be used to specify the default value of a variable. A default value
+* `=` may be used to specify the default value of a variable. A default value
   is used when there are an insufficient number of values.
 
 # zeallot 0.0.4

R/operator.R

@@ -43,7 +43,7 @@
 #'
 #' **default values**
 #'
-#' Use `<-` to specify a default value for a variable, \code{c(x, y <- NULL)
+#' Use `=` to specify a default value for a variable, \code{c(x, y = NULL)
 #' \%<-\% tail(1, 2)}.
 #'
 #' @return
@@ -157,12 +157,12 @@
 #'   strsplit(x, " ")[[1]]
 #' }
 #'
-#' c(hour, period <- NA) %<-% parse_time("10:00 AM")
+#' c(hour, period = NA) %<-% parse_time("10:00 AM")
 #'
 #' hour    # "10:00"
 #' period  # "AM"
 #'
-#' c(hour, period <- NA) %<-% parse_time("15:00")
+#' c(hour, period = NA) %<-% parse_time("15:00")
 #'
 #' hour    # "15:00"
 #' period  # NA

R/utils.R

@@ -32,16 +32,29 @@ add_defaults <- function(names, values, env) {
   append(values, evaled)
 }
 
+names2 <- function(x) {
+  if (is.null(names(x))) rep.int("", length(x)) else names(x)
+}
+
 tree <- function(x) {
-  if (length(x) == 1) {
+  if (length(x) == 1 && is.language(x) && !is.symbol(x)) {
     return(x)
   }
 
-  if (x[[1]] == "<-") {
-    return(as.list(x))
+  x <- as.list(x)
+
+  if (length(x) == 1 && length(x[[1]]) <= 1) {
+    if (names2(x) != "") {
+      return(list(as.symbol("="), as.symbol(names(x)), x[[1]]))
+    }
+
+    return(x[[1]])
   }
 
-  append(tree(x[[1]]), lapply(x[-1], tree))
+  append(
+    tree(x[[1]]),
+    lapply(seq_along(x[-1]), function(i) tree(x[-1][i]))
+  )
 }
 
 calls <- function(x) {
@@ -51,7 +64,7 @@ calls <- function(x) {
 
   this <- car(x)
 
-  if (this != "c" && this != "<-") {
+  if (this != "c" && this != "=") {
     stop_invalid_lhs(unexpected_call(this))
   }
 
@@ -60,14 +73,18 @@ calls <- function(x) {
 
 variables <- function(x) {
   if (!is_list(x)) {
+    if (x == "") {
+      stop_invalid_lhs(empty_variable(x))
+    }
+
     if (!is.symbol(x)) {
       stop_invalid_lhs(unexpected_variable(x))
     }
 
     return(as.character(x))
   }
 
-  if (car(x) == "<-") {
+  if (car(x) == "=") {
     var <- as.character(car(cdr(x)))
     default <- car(cdr(cdr(x)))
 
@@ -91,6 +108,10 @@ incorrect_number_of_values <- function() {
   "incorrect number of values"
 }
 
+empty_variable <- function(obj) {
+  paste("found empty variable, check for extraneous commas")
+}
+
 unexpected_variable <- function(obj) {
   paste("expected symbol, but found", class(obj))
 }

cran-comments.md

@@ -10,7 +10,7 @@
 
 ### Notes
 
-* Possibly misspelled word "destructuring" in DESCRIPTION.
+* Possibly misspelled word "destructuring" in DESCRIPTION is intentionally used.
 
 ## Reverse dependencies
 

tests/testthat/test-operator.R

@@ -102,28 +102,28 @@ test_that("%<-% skips multiple values using ...", {
 })
 
 test_that("%<-% assigns default values", {
-  c(a, b <- 1) %<-% c(3)
+  c(a, b = 1) %<-% c(3)
   expect_equal(a, 3)
   expect_equal(b, 1)
 
-  c(d, e <- iris, f <- 3030) %<-% 5
+  c(d, e = iris, f = 3030) %<-% 5
   expect_equal(d, 5)
   expect_equal(e, iris)
   expect_equal(f, 3030)
 })
 
 test_that("%<-% assign default value of NULL", {
-  c(a, b <- NULL) %<-% c(3)
+  c(a, b = NULL) %<-% c(3)
   expect_equal(a, 3)
   expect_equal(b, NULL)
 })
 
 test_that("%<-% default values do not override specified values", {
-  c(a <- 1, b <- 4) %<-% 2
+  c(a = 1, b = 4) %<-% 2
   expect_equal(a, 2)
   expect_equal(b, 4)
 
-  c(d <- 5, e <- 6) %<-% c(8, 9)
+  c(d = 5, e = 6) %<-% c(8, 9)
   expect_equal(d, 8)
   expect_equal(e, 9)
 })
@@ -163,6 +163,13 @@ test_that("%<-% throws error when invalid calls on LHS", {
   )
 })
 
+test_that("%<-% throws error when blank variable names", {
+  expect_error(
+    c(, a) %<-% c(1, 2),
+    "invalid `%<-%` left-hand side, found empty variable, check for extraneous commas"
+  )
+})
+
 test_that('%<-% throws error when invalid "variables" on LHS', {
   expect_error(
     c(mean(1, 2), a) %<-% list(1, 2),

vignettes/unpacking-assignment.Rmd

@@ -329,9 +329,10 @@ It is important to note that although value(s) are skipped they are still
 expected. The next section touches on how to handle missing values.
 
 ## Default values: handle missing values
-You can specify a default value for a left-hand side variable using `<-`. This
-comes in handy when the number of elements returned by a function cannot be
-guaranteed. `tail` for example may return fewer elements than asked for.
+You can specify a default value for a left-hand side variable using `=`, similar
+to specifying the default value of a function argument. This comes in handy when
+the number of elements returned by a function cannot be guaranteed. `tail` for
+example may return fewer elements than asked for.
 
 ```{r}
 nums <- 1:2
@@ -351,7 +352,7 @@ We can fix the problem and resolve the error by specifying a default value for
 `z`.
 
 ```{r}
-c(x, y, z <- NULL) %<-% tail(nums, 3)
+c(x, y, z = NULL) %<-% tail(nums, 3)
 x
 y
 z
@@ -396,6 +397,7 @@ mtcars %>%
   aggregate(. ~ cyl, data = ., FUN = . %>% mean() %>% round(2)) %>%
   transform(kpl = mpg %>% multiply_by(0.4251)) %->% 
   c(cyl, mpg, ...rest)
+
 cyl
 mpg
 rest