Add support for indexing into variables

.gitignore

@@ -1 +1,3 @@
 .DS_Store
+.idea
+*.iml

ast/ast.go

@@ -52,4 +52,5 @@ const (
 	TypeString
 	TypeInt
 	TypeFloat
+	TypeList
 )

ast/index.go

@@ -0,0 +1,68 @@
+package ast
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Index represents an indexing operation into another data structure
+type Index struct {
+	Target Node
+	Key    Node
+	Posx   Pos
+}
+
+func (n *Index) Accept(v Visitor) Node {
+	return v(n)
+}
+
+func (n *Index) Pos() Pos {
+	return n.Posx
+}
+
+func (n *Index) String() string {
+	return fmt.Sprintf("Index(%s, %s)", n.Target, n.Key)
+}
+
+func (n *Index) Type(s Scope) (Type, error) {
+	variableAccess, ok := n.Target.(*VariableAccess)
+	if !ok {
+		return TypeInvalid, fmt.Errorf("target is not a variable")
+	}
+
+	variable, ok := s.LookupVar(variableAccess.Name)
+	if !ok {
+		return TypeInvalid, fmt.Errorf("unknown variable accessed: %s", variableAccess.Name)
+	}
+	if variable.Type != TypeList {
+		return TypeInvalid, fmt.Errorf("invalid index operation into non-indexable type: %s", variable.Type)
+	}
+
+	list := variable.Value.([]Variable)
+
+	// Ensure that the types of the list elements are homogenous
+	listTypes := make(map[Type]struct{})
+	for _, v := range list {
+		if _, ok := listTypes[v.Type]; ok {
+			continue
+		}
+		listTypes[v.Type] = struct{}{}
+	}
+
+	if len(listTypes) != 1 {
+		typesFound := make([]string, len(listTypes))
+		i := 0
+		for k, _ := range listTypes {
+			typesFound[0] = k.String()
+			i++
+		}
+		types := strings.Join(typesFound, ", ")
+		return TypeInvalid, fmt.Errorf("list %q does not have homogenous types. found %s", variableAccess.Name, types)
+	}
+
+	return list[0].Type, nil
+}
+
+func (n *Index) GoString() string {
+	return fmt.Sprintf("*%#v", *n)
+}

ast/index_test.go

@@ -0,0 +1,110 @@
+package ast
+
+import (
+	"testing"
+)
+
+func TestIndexType_string(t *testing.T) {
+	i := &Index{
+		Target: &VariableAccess{Name: "foo"},
+		Key: &LiteralNode{
+			Typex: TypeInt,
+			Value: 1,
+		},
+	}
+
+	scope := &BasicScope{
+		VarMap: map[string]Variable{
+			"foo": Variable{
+				Type: TypeList,
+				Value: []Variable{
+					Variable{
+						Type:  TypeString,
+						Value: "Hello",
+					},
+					Variable{
+						Type:  TypeString,
+						Value: "World",
+					},
+				},
+			},
+		},
+	}
+
+	actual, err := i.Type(scope)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	if actual != TypeString {
+		t.Fatalf("bad: %s", actual)
+	}
+}
+
+func TestIndexType_int(t *testing.T) {
+	i := &Index{
+		Target: &VariableAccess{Name: "foo"},
+		Key: &LiteralNode{
+			Typex: TypeInt,
+			Value: 1,
+		},
+	}
+
+	scope := &BasicScope{
+		VarMap: map[string]Variable{
+			"foo": Variable{
+				Type: TypeList,
+				Value: []Variable{
+					Variable{
+						Type:  TypeInt,
+						Value: 34,
+					},
+					Variable{
+						Type:  TypeInt,
+						Value: 54,
+					},
+				},
+			},
+		},
+	}
+
+	actual, err := i.Type(scope)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	if actual != TypeInt {
+		t.Fatalf("bad: %s", actual)
+	}
+}
+
+func TestIndexType_nonHomogenous(t *testing.T) {
+	i := &Index{
+		Target: &VariableAccess{Name: "foo"},
+		Key: &LiteralNode{
+			Typex: TypeInt,
+			Value: 1,
+		},
+	}
+
+	scope := &BasicScope{
+		VarMap: map[string]Variable{
+			"foo": Variable{
+				Type: TypeList,
+				Value: []Variable{
+					Variable{
+						Type:  TypeString,
+						Value: "Hello",
+					},
+					Variable{
+						Type:  TypeInt,
+						Value: 43,
+					},
+				},
+			},
+		},
+	}
+
+	_, err := i.Type(scope)
+	if err == nil {
+		t.Fatalf("expected error")
+	}
+}

ast/scope.go

@@ -33,6 +33,12 @@ func NewVariable(v interface{}) (result Variable, err error) {
 	return
 }
 
+// String implements Stringer on Variable, displaying the type and value
+// of the Variable.
+func (v Variable) String() string {
+	return fmt.Sprintf("{Variable (%s): %+v}", v.Type, v.Value)
+}
+
 // Function defines a function that can be executed by the engine.
 // The type checker will validate that the proper types will be called
 // to the callback.

ast/scope_test.go

@@ -37,3 +37,18 @@ func TestBasicScopeLookupVar(t *testing.T) {
 		t.Fatal("should find foo")
 	}
 }
+
+func TestVariableStringer(t *testing.T) {
+	expected := "{Variable (TypeInt): 42}"
+	variable := &Variable{
+		Type:  TypeInt,
+		Value: 42,
+	}
+
+	actual := variable.String()
+
+	if actual != expected {
+		t.Fatalf("variable string formatting:\nExpected: %s\n     Got: %s\n",
+			expected, actual)
+	}
+}

ast/variable_access_test.go

@@ -34,3 +34,20 @@ func TestVariableAccessType_invalid(t *testing.T) {
 		t.Fatal("should error")
 	}
 }
+
+func TestVariableAccessType_list(t *testing.T) {
+	c := &VariableAccess{Name: "baz"}
+	scope := &BasicScope{
+		VarMap: map[string]Variable{
+			"baz": Variable{Type: TypeList},
+		},
+	}
+
+	actual, err := c.Type(scope)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	if actual != TypeList {
+		t.Fatalf("bad: %s", actual)
+	}
+}

check_types.go

@@ -61,6 +61,9 @@ func (v *TypeCheck) visit(raw ast.Node) ast.Node {
 	case *ast.Call:
 		tc := &typeCheckCall{n}
 		result, err = tc.TypeCheck(v)
+	case *ast.Index:
+		tc := &typeCheckIndex{n}
+		result, err = tc.TypeCheck(v)
 	case *ast.Concat:
 		tc := &typeCheckConcat{n}
 		result, err = tc.TypeCheck(v)
@@ -285,6 +288,55 @@ func (tc *typeCheckVariableAccess) TypeCheck(v *TypeCheck) (ast.Node, error) {
 	return tc.n, nil
 }
 
+type typeCheckIndex struct {
+	n *ast.Index
+}
+
+func (tc *typeCheckIndex) TypeCheck(v *TypeCheck) (ast.Node, error) {
+
+	value, err := tc.n.Key.Type(v.Scope)
+	if err != nil {
+		return nil, err
+	}
+	if value != ast.TypeInt {
+		return nil, fmt.Errorf("key of an index must be an int, was %s", value)
+	}
+
+	// Ensure we have a VariableAccess as the target
+	varAccessNode, ok := tc.n.Target.(*ast.VariableAccess)
+	if !ok {
+		return nil, fmt.Errorf("target of an index must be a VariableAccess node, was %T", tc.n.Target)
+	}
+
+	// Get the variable
+	variable, ok := v.Scope.LookupVar(varAccessNode.Name)
+	if !ok {
+		return nil, fmt.Errorf("unknown variable accessed: %s", varAccessNode.Name)
+	}
+	if variable.Type != ast.TypeList {
+		return nil, fmt.Errorf("invalid index operation into non-indexable type: %s", variable.Type)
+	}
+
+	list := variable.Value.([]ast.Variable)
+
+	// Ensure that the types of the list elements are homogenous
+	listTypes := make(map[ast.Type]struct{})
+	for _, v := range list {
+		if _, ok := listTypes[v.Type]; ok {
+			continue
+		}
+		listTypes[v.Type] = struct{}{}
+	}
+
+	if len(listTypes) != 1 {
+		return nil, fmt.Errorf("list %q does not have homogenous types (%s)", varAccessNode.Name)
+	}
+
+	// This is the type since the list is homogenous in type
+	v.StackPush(list[0].Type)
+	return tc.n, nil
+}
+
 func (v *TypeCheck) ImplicitConversion(
 	actual ast.Type, expected ast.Type, n ast.Node) ast.Node {
 	if v.Implicit == nil {