New "compact" command.

This fixes #1453.

0.19.0-release-notes.md

@@ -45,4 +45,8 @@ and compiled, even if it is not executed:
 -   A new `path:join` command and `path:separator` and `path:list-separator`
     variables ([#1562](https://b.elv.sh/1562)).
 
--   A new `runtime:elvish-path` variable ([#1423](https://b.elv.sh/1423)).
+-   A new `runtime:` module that contains paths important for the Elvish runtime
+    ([#1385](https://b.elv.sh/1385), [#1423](https://b.elv.sh/1423)).
+
+-   A new `compact` command that replaces consecutive runs of equal values with
+    a single copy, similar to the Unix `uniq` command.

pkg/eval/builtin_fn_stream.go

@@ -17,6 +17,7 @@ func init() {
 
 		"take":    take,
 		"drop":    drop,
+		"compact": compact,
 
 		"count": count,
 
@@ -240,6 +241,51 @@ func drop(fm *Frame, n int, inputs Inputs) error {
 	return errOut
 }
 
+//elvdoc:fn drop
+//
+// ```elvish
+// compact $inputs?
+// ```
+//
+// Replaces consecutive runs of equal values with a single copy. Similar to the
+// `uniq` command on Unix.
+//
+// Examples:
+//
+// ```elvish-transcript
+// ~> put a a b b c | compact
+// ▶ a
+// ▶ b
+// ▶ c
+// ~> compact [a a b b c]
+// ▶ a
+// ▶ b
+// ▶ c
+// ~> put a b a | compact
+// ▶ a
+// ▶ b
+// ▶ a
+// ```
+
+func compact(fm *Frame, inputs Inputs) error {
+	out := fm.ValueOutput()
+	first := true
+	var errOut error
+	var prev any
+
+	inputs(func(v any) {
+		if errOut != nil {
+			return
+		}
+		if first || !vals.Equal(v, prev) {
+			errOut = out.Put(v)
+			first = false
+			prev = v
+		}
+	})
+	return errOut
+}
+
 //elvdoc:fn count
 //
 // ```elvish

pkg/eval/builtin_fn_stream_test.go

@@ -46,6 +46,14 @@ func TestDrop(t *testing.T) {
 	)
 }
 
+func TestCompact(t *testing.T) {
+	Test(t,
+		That(`put a a b b c | compact`).Puts("a", "b", "c"),
+		That(`put a b a | compact`).Puts("a", "b", "a"),
+		thatOutputErrorIsBubbled("compact [a a]"),
+	)
+}
+
 func TestCount(t *testing.T) {
 	Test(t,
 		That(`range 100 | count`).Puts(100),