google · copybara-service · Feb 10, 2025 · Feb 10, 2025
diff --git a/interpreter/operator_dispatcher.go b/interpreter/operator_dispatcher.go
@@ -601,6 +601,13 @@ func (i *interpreter) unaryOverloads(m model.IUnaryExpression) ([]convert.Overlo
 				Result:   i.evalPopulationStdDevQuantity,
 			},
 		}, nil
+	case *model.Tail:
+		return []convert.Overload[evalUnarySignature]{
+			{
+				Operands: []types.IType{&types.List{ElementType: types.Any}},
+				Result:   evalTail,
+			},
+		}, nil
 	default:
 		return nil, fmt.Errorf("unsupported Unary Expression %v", m.GetName())
 	}

diff --git a/interpreter/operator_list.go b/interpreter/operator_list.go
@@ -203,6 +203,24 @@ func evalSkip(m model.IBinaryExpression, listObj, indexObj result.Value) (result
 	return result.New(result.List{Value: list[index:], StaticType: staticType})
 }
 
+// Tail(argument List<T>) List<T>
+// https://cql.hl7.org/09-b-cqlreference.html#tail
+func evalTail(m model.IUnaryExpression, listObj result.Value) (result.Value, error) {
+	if result.IsNull(listObj) {
+		return result.New(nil)
+	}
+	list, err := result.ToSlice(listObj)
+	if err != nil {
+		return result.Value{}, err
+	}
+	staticType := listObj.GolangValue().(result.List).StaticType
+	// If the list is empty, return an empty list.
+	if len(list) == 0 {
+		return result.New(result.List{StaticType: staticType})
+	}
+	return result.New(result.List{Value: list[1:], StaticType: staticType})
+}
+
 // Indexer(argument List<T>, index Integer) T
 // [](argument List<T>, index Integer) T
 // https://cql.hl7.org/09-b-cqlreference.html#indexer-1

diff --git a/model/model.go b/model/model.go
@@ -838,6 +838,9 @@ type CalculateAge struct {
 	Precision DateTimePrecision
 }
 
+// Tail ELM Expression https://cql.hl7.org/09-b-cqlreference.html#tail.
+type Tail struct{ *UnaryExpression }
+
 // BinaryExpression is a CQL expression that has two operands. The ELM representation may have
 // additional operands (ex BinaryExpressionWithPrecision).
 type BinaryExpression struct {
@@ -1476,5 +1479,8 @@ func (m *Median) GetName() string { return "Median" }
 // GetName returns the name of the system operator.
 func (s *Skip) GetName() string { return "Skip" }
 
+// GetName returns the name of the system operator.
+func (t *Tail) GetName() string { return "Tail" }
+
 // GetName returns the name of the system operator.
 func (m *PopulationStdDev) GetName() string { return "PopulationStdDev" }
diff --git a/parser/operators.go b/parser/operators.go
@@ -103,6 +103,9 @@ func (v *visitor) resolveFunction(libraryName, funcName string, operands []model
 	case *model.Skip:
 		listType := resolved.WrappedOperands[0].GetResultType().(*types.List)
 		t.Expression = model.ResultType(&types.List{ElementType: listType.ElementType})
+	case *model.Tail:
+		listType := resolved.WrappedOperands[0].GetResultType().(*types.List)
+		t.Expression = model.ResultType(&types.List{ElementType: listType.ElementType})
 	case *model.Union:
 		listTypeLeft := resolved.WrappedOperands[0].GetResultType().(*types.List)
 		listTypeRight := resolved.WrappedOperands[1].GetResultType().(*types.List)
@@ -1646,6 +1649,16 @@ func (p *Parser) loadSystemOperators() error {
 				}
 			},
 		},
+		{
+			name: "Tail",
+			operands: [][]types.IType{
+				{&types.List{ElementType: types.Any}}},
+			model: func() model.IExpression {
+				return &model.Tail{
+					UnaryExpression: &model.UnaryExpression{},
+				}
+			},
+		},
 		{
 			name:     "Union",
 			operands: [][]types.IType{{&types.List{ElementType: types.Any}, &types.List{ElementType: types.Any}}},

diff --git a/parser/operators_test.go b/parser/operators_test.go
@@ -1304,6 +1304,16 @@ func TestBuiltInFunctions(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "Tail",
+			cql:  "Tail({1, 2, 3})",
+			want: &model.Tail{
+				UnaryExpression: &model.UnaryExpression{
+					Operand:    model.NewList([]string{"1", "2", "3"}, types.Integer),
+					Expression: model.ResultType(&types.List{ElementType: types.Integer}),
+				},
+			},
+		},
 		{
 			name: "Union",
 			cql:  "Union({1}, {'hi'})",

diff --git a/tests/enginetests/operator_list_test.go b/tests/enginetests/operator_list_test.go
@@ -819,6 +819,93 @@ func TestSkip(t *testing.T) {
 	}
 }
 
+func TestTail(t *testing.T) {
+	tests := []struct {
+		name                 string
+		cql                  string
+		wantModel            model.IExpression
+		wantResult           result.Value
+		wantSourceExpression model.IExpression
+		wantSourceValues     []result.Value
+	}{
+		{
+			name: "Tail({1, 2}) = {2}",
+			cql:  "Tail({1, 2})",
+			wantModel: &model.Tail{
+				UnaryExpression: &model.UnaryExpression{
+					Operand:    model.NewList([]string{"1", "2"}, types.Integer),
+					Expression: model.ResultType(&types.List{ElementType: types.Integer}),
+				},
+			},
+			wantResult: newOrFatal(t, result.List{Value: []result.Value{newOrFatal(t, int32(2))}, StaticType: &types.List{ElementType: types.Integer}}),
+			wantSourceExpression: &model.Tail{
+				UnaryExpression: &model.UnaryExpression{
+					Operand:    model.NewList([]string{"1", "2"}, types.Integer),
+					Expression: model.ResultType(&types.List{ElementType: types.Integer}),
+				},
+			},
+		},
+		{
+			name:       "Tail({1}) = {}",
+			cql:        "Tail({1})",
+			wantResult: newOrFatal(t, result.List{Value: []result.Value{}, StaticType: &types.List{ElementType: types.Integer}}),
+		},
+		{
+			name:       "Tail(List<Integer>{}) = {}",
+			cql:        "Tail(List<Integer>{})",
+			wantResult: newOrFatal(t, result.List{Value: []result.Value{}, StaticType: &types.List{ElementType: types.Integer}}),
+		},
+		{
+			name:       "Tail(null as List<Integer>) = null",
+			cql:        "Tail(null as List<Integer>)",
+			wantResult: newOrFatal(t, nil),
+		},
+		{
+			name:       "Tail(null as List<Integer>) = null",
+			cql:        "Tail(null as List<Integer>)",
+			wantResult: newOrFatal(t, nil),
+		},
+		{
+			name: "Tail({@2010, @2011, @2012}) = {@2011, @2012}",
+			cql:  "Tail({@2010, @2011, @2012})",
+			wantResult: newOrFatal(t, result.List{
+				Value: []result.Value{
+					newOrFatal(t, result.Date{Date: time.Date(2011, time.January, 1, 0, 0, 0, 0, defaultEvalTimestamp.Location()), Precision: model.YEAR}),
+					newOrFatal(t, result.Date{Date: time.Date(2012, time.January, 1, 0, 0, 0, 0, defaultEvalTimestamp.Location()), Precision: model.YEAR}),
+				},
+				StaticType: &types.List{ElementType: types.Date},
+			}),
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			p := newFHIRParser(t)
+			parsedLibs, err := p.Libraries(context.Background(), wrapInLib(t, tc.cql), parser.Config{})
+			if err != nil {
+				t.Fatalf("Parse returned unexpected error: %v", err)
+			}
+			if diff := cmp.Diff(tc.wantModel, getTESTRESULTModel(t, parsedLibs)); tc.wantModel != nil && diff != "" {
+				t.Errorf("Parse diff (-want +got):\n%s", diff)
+			}
+
+			results, err := interpreter.Eval(context.Background(), parsedLibs, defaultInterpreterConfig(t, p))
+			if err != nil {
+				t.Fatalf("Eval returned unexpected error: %v", err)
+			}
+			gotResult := getTESTRESULTWithSources(t, results)
+			if diff := cmp.Diff(tc.wantResult, gotResult, protocmp.Transform()); diff != "" {
+				t.Errorf("Eval returned diff (-want +got)\n%v", diff)
+			}
+			if diff := cmp.Diff(tc.wantSourceExpression, gotResult.SourceExpression(), protocmp.Transform()); tc.wantSourceExpression != nil && diff != "" {
+				t.Errorf("Eval SourceExpression diff (-want +got)\n%v", diff)
+			}
+			if diff := cmp.Diff(tc.wantSourceValues, gotResult.SourceValues(), protocmp.Transform()); tc.wantSourceValues != nil && diff != "" {
+				t.Errorf("Eval SourceValues diff (-want +got)\n%v", diff)
+			}
+		})
+	}
+}
+
 func TestIndexerList(t *testing.T) {
 	tests := []struct {
 		name       string

diff --git a/tests/spectests/exclusions/exclusions.go b/tests/spectests/exclusions/exclusions.go
@@ -349,7 +349,6 @@ func XMLTestFileExclusionDefinitions() map[string]XMLTestFileExclusions {
 				"ProperIn",
 				"ProperlyIncludes",
 				"ProperlyIncludedIn",
-				"Tail",
 				"Take",
 				"Union",
 			},
@@ -369,6 +368,7 @@ func XMLTestFileExclusionDefinitions() map[string]XMLTestFileExclusions {
 				"EqualNullNull",
 				// TODO: b/346880550 - These test appear to have incorrect assertions.
 				"SkipAll",
+				"TailOneElement",
 			},
 		},
 		"CqlQueryTests.xml": XMLTestFileExclusions{