inclusive list.range and iterator.range

5 files changed, 47 insertions, 33 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b5adc01..d471cee 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@
   ranges when compiled to JavaScript.
 - Fixed a bug where the `list` module's `contains`, `any`, and `all` could
   exhaust the stack when compiling to JavaScript.
+- `list.range` and `iterator.range` return values are now inclusive of both start and end bounds. 
 
 ## v0.22.1 - 2022-06-27
 
diff --git a/src/gleam/iterator.gleam b/src/gleam/iterator.gleam
index 92729a1..6290b05 100644
--- a/src/gleam/iterator.gleam
+++ b/src/gleam/iterator.gleam
@@ -1,6 +1,8 @@
+import gleam/int
 import gleam/list
 import gleam/map.{Map}
 import gleam/option.{None, Option, Some}
+import gleam/order
 
 // Internal private representation of an Iterator
 type Action(element) {
@@ -447,29 +449,40 @@ pub fn cycle(iterator: Iterator(a)) -> Iterator(a) {
 ///
 /// ```gleam
 /// > range(from: 1, to: 5) |> to_list
-/// [1, 2, 3, 4]
+/// [1, 2, 3, 4, 5]
 ///
 /// > range(from: 1, to: -2) |> to_list
-/// [1, 0, -1]
+/// [1, 0, -1, -2]
 ///
 /// > range(from: 0, to: 0) |> to_list
-/// []
+/// [0]
 /// ```
 ///
 pub fn range(from start: Int, to stop: Int) -> Iterator(Int) {
-  let increment = case start < stop {
-    True -> 1
-    False -> -1
-  }
-
-  let next_step = fn(current) {
-    case current == stop {
-      True -> Done
-      False -> Next(current, current + increment)
-    }
+  case int.compare(start, stop) {
+    order.Eq -> once(fn() { start })
+    order.Gt ->
+      unfold(
+        from: start,
+        with: fn(current) {
+          case current < stop {
+            False -> Next(current, current - 1)
+            True -> Done
+          }
+        },
+      )
+
+    order.Lt ->
+      unfold(
+        from: start,
+        with: fn(current) {
+          case current > stop {
+            False -> Next(current, current + 1)
+            True -> Done
+          }
+        },
+      )
   }
-
-  unfold(start, next_step)
 }
 
 fn do_find(continuation: fn() -> Action(a), f: fn(a) -> Bool) -> Result(a, Nil) {
diff --git a/src/gleam/list.gleam b/src/gleam/list.gleam
index 1a9d5ce..a46278a 100644
--- a/src/gleam/list.gleam
+++ b/src/gleam/list.gleam
@@ -1038,13 +1038,13 @@ pub fn sort(list: List(a), by compare: fn(a, a) -> Order) -> List(a) {
 ///
 /// ```gleam
 /// > range(0, 0)
-/// []
+/// [0]
 ///
 /// > range(0, 5)
-/// [0, 1, 2, 3, 4]
+/// [0, 1, 2, 3, 4, 5]
 ///
 /// > range(1, -5)
-/// [1, 0, -1, -2, -3, -4]
+/// [1, 0, -1, -2, -3, -4, -5]
 /// ```
 ///
 pub fn range(from start: Int, to stop: Int) -> List(Int) {
@@ -1053,7 +1053,7 @@ pub fn range(from start: Int, to stop: Int) -> List(Int) {
 
 fn tail_recursive_range(start: Int, stop: Int, acc: List(Int)) -> List(Int) {
   case int.compare(start, stop) {
-    order.Eq -> reverse(acc)
+    order.Eq -> reverse([stop, ..acc])
     order.Gt -> tail_recursive_range(start - 1, stop, [start, ..acc])
     order.Lt -> tail_recursive_range(start + 1, stop, [start, ..acc])
   }
diff --git a/test/gleam/iterator_test.gleam b/test/gleam/iterator_test.gleam
index 8f035d9..976e209 100644
--- a/test/gleam/iterator_test.gleam
+++ b/test/gleam/iterator_test.gleam
@@ -222,19 +222,19 @@ pub fn range_test() {
     |> should.equal(expected)
   }
 
-  test(0, 0, [])
-  test(1, 1, [])
-  test(-1, -1, [])
-  test(0, 1, [0])
-  test(0, 5, [0, 1, 2, 3, 4])
-  test(1, -5, [1, 0, -1, -2, -3, -4])
+  test(0, 0, [0])
+  test(1, 1, [1])
+  test(-1, -1, [-1])
+  test(0, 1, [0, 1])
+  test(0, 5, [0, 1, 2, 3, 4, 5])
+  test(1, -5, [1, 0, -1, -2, -3, -4, -5])
 }
 
 pub fn drop_test() {
   iterator.range(0, 10)
   |> iterator.drop(5)
   |> iterator.to_list
-  |> should.equal([5, 6, 7, 8, 9])
+  |> should.equal([5, 6, 7, 8, 9, 10])
 }
 
 type Cat {
@@ -392,7 +392,7 @@ pub fn last_test() {
 
   iterator.range(1, 10)
   |> iterator.last
-  |> should.equal(Ok(9))
+  |> should.equal(Ok(10))
 }
 
 pub fn empty_test() {
diff --git a/test/gleam/list_test.gleam b/test/gleam/list_test.gleam
index 566aa94..1ea4f42 100644
--- a/test/gleam/list_test.gleam
+++ b/test/gleam/list_test.gleam
@@ -472,22 +472,22 @@ pub fn index_map_test() {
 
 pub fn range_test() {
   list.range(0, 0)
-  |> should.equal([])
+  |> should.equal([0])
 
   list.range(1, 1)
-  |> should.equal([])
+  |> should.equal([1])
 
   list.range(-1, -1)
-  |> should.equal([])
+  |> should.equal([-1])
 
   list.range(0, 1)
-  |> should.equal([0])
+  |> should.equal([0, 1])
 
   list.range(0, 5)
-  |> should.equal([0, 1, 2, 3, 4])
+  |> should.equal([0, 1, 2, 3, 4, 5])
 
   list.range(1, -5)
-  |> should.equal([1, 0, -1, -2, -3, -4])
+  |> should.equal([1, 0, -1, -2, -3, -4, -5])
 
   // This should not overflow the stack
   list.range(1, 100_000)