Add coalescelist interpolation function

config/interpolate_funcs.go

@@ -60,6 +60,7 @@ func Funcs() map[string]ast.Function {
 		"cidrnetmask":  interpolationFuncCidrNetmask(),
 		"cidrsubnet":   interpolationFuncCidrSubnet(),
 		"coalesce":     interpolationFuncCoalesce(),
+		"coalescelist": interpolationFuncCoalesceList(),
 		"compact":      interpolationFuncCompact(),
 		"concat":       interpolationFuncConcat(),
 		"distinct":     interpolationFuncDistinct(),
@@ -318,6 +319,30 @@ func interpolationFuncCoalesce() ast.Function {
 	}
 }
 
+// interpolationFuncCoalesceList implements the "coalescelist" function that
+// returns the first non empty list from the provided input
+func interpolationFuncCoalesceList() ast.Function {
+	return ast.Function{
+		ArgTypes:     []ast.Type{ast.TypeList},
+		ReturnType:   ast.TypeList,
+		Variadic:     true,
+		VariadicType: ast.TypeList,
+		Callback: func(args []interface{}) (interface{}, error) {
+			if len(args) < 2 {
+				return nil, fmt.Errorf("must provide at least two arguments")
+			}
+			for _, arg := range args {
+				argument := arg.([]ast.Variable)
+
+				if len(argument) > 0 {
+					return argument, nil
+				}
+			}
+			return make([]ast.Variable, 0), nil
+		},
+	}
+}
+
 // interpolationFuncConcat implements the "concat" function that concatenates
 // multiple lists.
 func interpolationFuncConcat() ast.Function {

config/interpolate_funcs_test.go

@@ -615,6 +615,33 @@ func TestInterpolateFuncCoalesce(t *testing.T) {
 	})
 }
 
+func TestInterpolateFuncCoalesceList(t *testing.T) {
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
+			{
+				`${coalescelist(list("first"), list("second"), list("third"))}`,
+				[]interface{}{"first"},
+				false,
+			},
+			{
+				`${coalescelist(list(), list("second"), list("third"))}`,
+				[]interface{}{"second"},
+				false,
+			},
+			{
+				`${coalescelist(list(), list(), list())}`,
+				[]interface{}{},
+				false,
+			},
+			{
+				`${coalescelist(list("foo"))}`,
+				nil,
+				true,
+			},
+		},
+	})
+}
+
 func TestInterpolateFuncConcat(t *testing.T) {
 	testFunction(t, testFunctionConfig{
 		Cases: []testFunctionCase{

website/source/docs/configuration/interpolation.html.md

@@ -169,6 +169,9 @@ The supported built-in functions are:
   * `coalesce(string1, string2, ...)` - Returns the first non-empty value from
     the given arguments. At least two arguments must be provided.
 
+  * `coalescelist(list1, list2, ...)` - Returns the first non-empty list from
+    the given arguments. At least two arguments must be provided.
+
   * `compact(list)` - Removes empty string elements from a list. This can be
      useful in some cases, for example when passing joined lists as module
      variables or when parsing module outputs.