Implement [] and Indexer operators for List<T> and Strings.

interpreter/operator_dispatcher.go

@@ -849,6 +849,17 @@ func (i *interpreter) binaryOverloads(m model.IBinaryExpression) ([]convert.Over
 				Result:   i.evalSplit,
 			},
 		}, nil
+	case *model.Indexer:
+		return []convert.Overload[evalBinarySignature]{
+			{
+				Operands: []types.IType{types.String, types.Integer},
+				Result:   i.evalIndexerString,
+			},
+			{
+				Operands: []types.IType{&types.List{ElementType: types.Any}, types.Integer},
+				Result:   i.evalIndexerList,
+			},
+		}, nil
 	default:
 		return nil, fmt.Errorf("unsupported Binary Expression %v", m.GetName())
 	}

interpreter/operator_list.go

@@ -121,3 +121,25 @@ func evalSingletonFrom(m model.IUnaryExpression, listObj result.Value) (result.V
 		return result.Value{}, fmt.Errorf("singleton from requires a list of length 0 or 1, but got length %d", len(list))
 	}
 }
+
+// Indexer(argument List<T>, index Integer) T
+// [](argument List<T>, index Integer) T
+// https://cql.hl7.org/09-b-cqlreference.html#indexer-1
+// Indexer is also defined for String, see operator_string.go for that implementation.
+func (i *interpreter) evalIndexerList(m model.IBinaryExpression, lObj, rObj result.Value) (result.Value, error) {
+	if result.IsNull(lObj) || result.IsNull(rObj) {
+		return result.New(nil)
+	}
+	list, err := result.ToSlice(lObj)
+	if err != nil {
+		return result.Value{}, err
+	}
+	idx, err := result.ToInt32(rObj)
+	if err != nil {
+		return result.Value{}, err
+	}
+	if idx < 0 || idx >= int32(len(list)) {
+		return result.New(nil)
+	}
+	return list[idx], nil
+}

interpreter/operator_string.go

@@ -219,6 +219,27 @@ func (i *interpreter) evalCombine(m model.INaryExpression, operands []result.Val
 	return result.New(resultBuilder.String())
 }
 
+// Indexer(argument String, index Integer) String
+// https://cql.hl7.org/09-b-cqlreference.html#indexer
+// Indexer is also defined for List<T>, see operator_list.go for that implementation.
+func (i *interpreter) evalIndexerString(m model.IBinaryExpression, lObj, rObj result.Value) (result.Value, error) {
+	if result.IsNull(lObj) || result.IsNull(rObj) {
+		return result.New(nil)
+	}
+	str, err := result.ToString(lObj)
+	if err != nil {
+		return result.Value{}, err
+	}
+	idx, err := result.ToInt32(rObj)
+	if err != nil {
+		return result.Value{}, err
+	}
+	if idx < 0 || idx >= int32(len(str)) {
+		return result.New(nil)
+	}
+	return result.New(string([]rune(str)[idx]))
+}
+
 // convert a quantity value to a string
 func quantityToString(q result.Quantity) string {
 	f := strconv.FormatFloat(q.Value, 'f', -1, 64)

model/model.go

@@ -932,6 +932,9 @@ type Union struct{ *BinaryExpression }
 // it always takes two arguments.
 type Split struct{ *BinaryExpression }
 
+// Indexer ELM Expression https://cql.hl7.org/04-logicalspecification.html#indexer.
+type Indexer struct{ *BinaryExpression }
+
 // BinaryExpressionWithPrecision represents a BinaryExpression with a precision property.
 type BinaryExpressionWithPrecision struct {
 	*BinaryExpression
@@ -1371,3 +1374,6 @@ func (a *Split) GetName() string { return "Split" }
 
 // GetName returns the name of the system operator.
 func (a *Combine) GetName() string { return "Combine" }
+
+// GetName returns the name of the system operator.
+func (i *Indexer) GetName() string { return "Indexer" }

parser/expressions.go

@@ -122,6 +122,8 @@ func (v *visitor) VisitExpression(tree antlr.Tree) model.IExpression {
 		m = v.VisitTypeExtentExpressionTermContext(t)
 	case *cql.ElementExtractorExpressionTermContext:
 		m = v.VisitElementExtractorExpressionTerm(t)
+	case *cql.IndexedExpressionTermContext:
+		m = v.VisitIndexedExpressionTermContext(t)
 
 		// All cases that have a single child and recurse to the child are handled below. For example in
 		// the CQL grammar the only child of QueryExpression is Query, so QueryExpression can be handled

parser/operator_expressions.go

@@ -669,3 +669,12 @@ func (v *visitor) VisitSuccessorExpressionTerm(ctx *cql.SuccessorExpressionTermC
 	}
 	return m
 }
+
+func (v *visitor) VisitIndexedExpressionTermContext(ctx *cql.IndexedExpressionTermContext) model.IExpression {
+	baseExpr := ctx.ExpressionTerm()
+	m, err := v.parseFunction("", "Indexer", []antlr.Tree{baseExpr, ctx.Expression()}, false)
+	if err != nil {
+		return v.badExpression(err.Error(), ctx)
+	}
+	return m
+}

parser/operator_expressions_test.go

@@ -999,6 +999,32 @@ func TestOperatorExpressions(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "Indexer [] syntax for List<T>",
+			cql:  "{1, 2, 3}[0]",
+			want: &model.Indexer{
+				BinaryExpression: &model.BinaryExpression{
+					Expression: model.ResultType(types.Integer),
+					Operands: []model.IExpression{
+						model.NewList([]string{"1", "2", "3"}, types.Integer),
+						model.NewLiteral("0", types.Integer),
+					},
+				},
+			},
+		},
+		{
+			name: "Indexer [] syntax for String",
+			cql:  "'abc'[0]",
+			want: &model.Indexer{
+				BinaryExpression: &model.BinaryExpression{
+					Expression: model.ResultType(types.String),
+					Operands: []model.IExpression{
+						model.NewLiteral("abc", types.String),
+						model.NewLiteral("0", types.Integer),
+					},
+				},
+			},
+		},
 	}
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {

parser/operators.go

@@ -116,6 +116,18 @@ func (v *visitor) resolveFunction(libraryName, funcName string, operands []model
 		// Last(List<T>) T is a special case because the ResultType is not known until invocation.
 		listType := resolved.WrappedOperands[0].GetResultType().(*types.List)
 		t.Expression = model.ResultType(listType.ElementType)
+	case *model.Indexer:
+		switch opType := resolved.WrappedOperands[0].GetResultType().(type) {
+		case types.System:
+			if opType != types.String {
+				return nil, fmt.Errorf("internal error - expected Indexer(String, Integer) overload during parsing, but got Indexer(%v, _)", resolved.WrappedOperands[0].GetResultType())
+			}
+			t.Expression = model.ResultType(types.String)
+		case *types.List:
+			t.Expression = model.ResultType(opType.ElementType)
+		default:
+			return nil, fmt.Errorf("internal error -- upsupported Indexer operand types")
+		}
 	case *model.Predecessor:
 		t.Expression = model.ResultType(resolved.WrappedOperands[0].GetResultType())
 	case *model.Successor:
@@ -920,6 +932,19 @@ func (p *Parser) loadSystemOperators() error {
 				}
 			},
 		},
+		{
+			name: "Indexer",
+			operands: [][]types.IType{
+				{types.String, types.Integer},
+				{&types.List{ElementType: types.Any}, types.Integer},
+			},
+			model: func() model.IExpression {
+				return &model.Indexer{
+					// The result type is set in the resolveFunction().
+					BinaryExpression: &model.BinaryExpression{},
+				}
+			},
+		},
 		// DATE AND TIME OPERATORS - https://cql.hl7.org/09-b-cqlreference.html#datetime-operators-2
 		{
 			name:     "Add",

parser/operators_test.go

@@ -687,6 +687,32 @@ func TestBuiltInFunctions(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "Indexer functional form for List<T>",
+			cql:  "Indexer({1}, 0)",
+			want: &model.Indexer{
+				BinaryExpression: &model.BinaryExpression{
+					Expression: model.ResultType(types.Integer),
+					Operands: []model.IExpression{
+						model.NewList([]string{"1"}, types.Integer),
+						model.NewLiteral("0", types.Integer),
+					},
+				},
+			},
+		},
+		{
+			name: "Indexer functional form for String",
+			cql:  "Indexer('abc', 0)",
+			want: &model.Indexer{
+				BinaryExpression: &model.BinaryExpression{
+					Expression: model.ResultType(types.String),
+					Operands: []model.IExpression{
+						model.NewLiteral("abc", types.String),
+						model.NewLiteral("0", types.Integer),
+					},
+				},
+			},
+		},
 		// DATE AND TIME OPERATORS - https://cql.hl7.org/09-b-cqlreference.html#datetime-operators-2
 		{
 			name: "After",

tests/enginetests/operator_list_test.go

@@ -467,3 +467,87 @@ func TestListOperatorSingletonFrom_Error(t *testing.T) {
 		})
 	}
 }
+
+func TestIndexerList(t *testing.T) {
+	tests := []struct {
+		name       string
+		cql        string
+		wantModel  model.IExpression
+		wantResult result.Value
+	}{
+		{
+			name: "Indexer [] syntax for List<Integer>",
+			cql:  "{1, 2}[1]",
+			wantModel: &model.Indexer{
+				BinaryExpression: &model.BinaryExpression{
+					Expression: model.ResultType(types.Integer),
+					Operands: []model.IExpression{
+						model.NewList([]string{"1", "2"}, types.Integer),
+						model.NewLiteral("1", types.Integer),
+					},
+				},
+			},
+			wantResult: newOrFatal(t, 2),
+		},
+		{
+			name: "Indexer [] syntax for List<String>",
+			cql:  "{'a', 'b'}[1]",
+			wantModel: &model.Indexer{
+				BinaryExpression: &model.BinaryExpression{
+					Expression: model.ResultType(types.String),
+					Operands: []model.IExpression{
+						model.NewList([]string{"a", "b"}, types.String),
+						model.NewLiteral("1", types.Integer),
+					},
+				},
+			},
+			wantResult: newOrFatal(t, "b"),
+		},
+		{
+			name:       "Indexer functional form",
+			cql:        "Indexer({1, 2}, 1)",
+			wantResult: newOrFatal(t, 2),
+		},
+		{
+			name:       "Indexer with index too large",
+			cql:        "{1, 2}[100]",
+			wantResult: newOrFatal(t, nil),
+		},
+		{
+			name:       "Indexer with index smaller than 0",
+			cql:        "{1, 2}[-100]",
+			wantResult: newOrFatal(t, nil),
+		},
+		{
+			name:       "Indexer on null",
+			cql:        "(null as List<Integer>)[1]",
+			wantResult: newOrFatal(t, nil),
+		},
+		{
+			name:       "Indexer with null index",
+			cql:        "{1, 2}[null as Integer]",
+			wantResult: newOrFatal(t, nil),
+		},
+	}
+	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)
+			}
+			if diff := cmp.Diff(tc.wantResult, getTESTRESULT(t, results), protocmp.Transform()); diff != "" {
+				t.Errorf("Eval diff (-want +got)\n%v", diff)
+			}
+
+		})
+	}
+}

tests/enginetests/operator_string_test.go

@@ -389,3 +389,81 @@ func TestCombine(t *testing.T) {
 		})
 	}
 }
+
+func TestIndexerString(t *testing.T) {
+	tests := []struct {
+		name       string
+		cql        string
+		wantModel  model.IExpression
+		wantResult result.Value
+	}{
+		{
+			name: "[] Indexer",
+			cql:  "'abc'[1]",
+			wantModel: &model.Indexer{
+				BinaryExpression: &model.BinaryExpression{
+					Expression: model.ResultType(types.String),
+					Operands: []model.IExpression{
+						model.NewLiteral("abc", types.String),
+						model.NewLiteral("1", types.Integer),
+					},
+				},
+			},
+			wantResult: newOrFatal(t, "b"),
+		},
+		{
+			name: "Indexer functional form",
+			cql:  "Indexer('abc', 1)",
+			wantModel: &model.Indexer{
+				BinaryExpression: &model.BinaryExpression{
+					Expression: model.ResultType(types.String),
+					Operands: []model.IExpression{
+						model.NewLiteral("abc", types.String),
+						model.NewLiteral("1", types.Integer),
+					},
+				},
+			},
+			wantResult: newOrFatal(t, "b"),
+		},
+		{
+			name:       "Indexer with index too large",
+			cql:        "'abc'[100]",
+			wantResult: newOrFatal(t, nil),
+		},
+		{
+			name:       "Indexer with index smaller than 0",
+			cql:        "'abc'[-100]",
+			wantResult: newOrFatal(t, nil),
+		},
+		{
+			name:       "Indexer on null",
+			cql:        "(null as String)[1]",
+			wantResult: newOrFatal(t, nil),
+		},
+		{
+			name:       "Indexer with null index",
+			cql:        "'abc'[null as Integer]",
+			wantResult: newOrFatal(t, nil),
+		},
+	}
+	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)
+			}
+			if diff := cmp.Diff(tc.wantResult, getTESTRESULT(t, results), protocmp.Transform()); diff != "" {
+				t.Errorf("Eval diff (-want +got)\n%v", diff)
+			}
+		})
+	}
+}