Add a new "conj" command for appending values to a list.

0.19.0-release-notes.md

@@ -5,6 +5,9 @@ This is the draft release notes for 0.19.0, scheduled to be released around
 
 -   A new `doc` module provides access to the documentation of builtin modules.
 
+-   A new `conj` command "appends" values to a list, and has a guaranteed time
+    complexity independent of the size of the list.
+
 -   A new `inexact-num` converts its argument to an inexact number.
 
     It is functionally identical to the now deprecated `float64` command since

pkg/eval/builtin_fn_container.d.elv

@@ -31,6 +31,23 @@ fn ns {|map| }
 # ```
 fn make-map {|input?| }
 
+# Outputs a list created from adding values in `$more` to the end of `$list`.
+#
+# The output is the same as `[$@list $more...]`, but the time complexity is
+# guaranteed to be O(m), where m is the number of values in `$more`.
+#
+# Examples:
+#
+# ```elvish-transcript
+# ~> conj [] a
+# ▶ [a]
+# ~> conj [a b] c d
+# ▶ [a b c d]
+# ```
+#
+# Etymology: [Clojure](https://clojuredocs.org/clojure.core/conj).
+fn conj {|list @more| }
+
 # Output a slightly modified version of `$container`, such that its value at `$k`
 # is `$v`. Applies to both lists and to maps.
 #

pkg/eval/builtin_fn_container.go

@@ -17,6 +17,7 @@ func init() {
 
 		"make-map": makeMap,
 
+		"conj":   conj,
 		"assoc":  assoc,
 		"dissoc": dissoc,
 
@@ -74,6 +75,13 @@ func makeMap(input Inputs) (vals.Map, error) {
 	return m, errMakeMap
 }
 
+func conj(li vals.List, more ...any) vals.List {
+	for _, val := range more {
+		li = li.Conj(val)
+	}
+	return li
+}
+
 func assoc(a, k, v any) (any, error) {
 	return vals.Assoc(a, k, v)
 }

pkg/eval/builtin_fn_container_test.go

@@ -41,6 +41,15 @@ func TestMakeMap(t *testing.T) {
 	)
 }
 
+func TestConj(t *testing.T) {
+	Test(t,
+		That("conj [] a").Puts(vals.MakeList("a")),
+		That("conj [a b]").Puts(vals.MakeList("a", "b")),
+		That("conj [a b] c").Puts(vals.MakeList("a", "b", "c")),
+		That("conj [a b] c d").Puts(vals.MakeList("a", "b", "c", "d")),
+	)
+}
+
 func TestAssoc(t *testing.T) {
 	Test(t,
 		That(`put (assoc [0] 0 zero)[0]`).Puts("zero"),