diff --git a/Makefile b/Makefile
index 6082f83f..ffba0024 100644
--- a/Makefile
+++ b/Makefile
@@ -64,4 +64,4 @@ else
 	git tag -a v$(RELEASE_VERSION) -m "New $(RELEASE_VERSION) version" && \
 	git push origin v$(RELEASE_VERSION) && \
 	goreleaser
-endif
\ No newline at end of file
+endif
diff --git a/cli/browser/flags.go b/cli/browser/flags.go
index 47c6875a..da0fadf2 100644
--- a/cli/browser/flags.go
+++ b/cli/browser/flags.go
@@ -78,11 +78,11 @@ func (flags Flags) List() []string {
 			continue
 		}
 
-		switch val.(type) {
+		switch v := val.(type) {
 		case int:
-			arg = fmt.Sprintf("--%s=%d", arg, val.(int))
+			arg = fmt.Sprintf("--%s=%d", arg, v)
 		case string:
-			arg = fmt.Sprintf("--%s=%s", arg, val.(string))
+			arg = fmt.Sprintf("--%s=%s", arg, v)
 		default:
 			arg = fmt.Sprintf("--%s", arg)
 		}
diff --git a/pkg/compiler/visitor.go b/pkg/compiler/visitor.go
index 8b87e9a2..e412e6f9 100644
--- a/pkg/compiler/visitor.go
+++ b/pkg/compiler/visitor.go
@@ -1430,37 +1430,37 @@ func (v *visitor) visit(node antlr.Tree, scope *scope) (core.Expression, error)
 	var out core.Expression
 	var err error
 
-	switch node.(type) {
+	switch ctx := node.(type) {
 	case *fql.BodyContext:
-		out, err = v.doVisitBody(node.(*fql.BodyContext), scope)
+		out, err = v.doVisitBody(ctx, scope)
 	case *fql.ExpressionContext:
-		out, err = v.doVisitExpression(node.(*fql.ExpressionContext), scope)
+		out, err = v.doVisitExpression(ctx, scope)
 	case *fql.ForExpressionContext:
-		out, err = v.doVisitForExpression(node.(*fql.ForExpressionContext), scope)
+		out, err = v.doVisitForExpression(ctx, scope)
 	case *fql.ReturnExpressionContext:
-		out, err = v.doVisitReturnExpression(node.(*fql.ReturnExpressionContext), scope)
+		out, err = v.doVisitReturnExpression(ctx, scope)
 	case *fql.ArrayLiteralContext:
-		out, err = v.doVisitArrayLiteral(node.(*fql.ArrayLiteralContext), scope)
+		out, err = v.doVisitArrayLiteral(ctx, scope)
 	case *fql.ObjectLiteralContext:
-		out, err = v.doVisitObjectLiteral(node.(*fql.ObjectLiteralContext), scope)
+		out, err = v.doVisitObjectLiteral(ctx, scope)
 	case *fql.StringLiteralContext:
-		out, err = v.doVisitStringLiteral(node.(*fql.StringLiteralContext))
+		out, err = v.doVisitStringLiteral(ctx)
 	case *fql.IntegerLiteralContext:
-		out, err = v.doVisitIntegerLiteral(node.(*fql.IntegerLiteralContext))
+		out, err = v.doVisitIntegerLiteral(ctx)
 	case *fql.FloatLiteralContext:
-		out, err = v.doVisitFloatLiteral(node.(*fql.FloatLiteralContext))
+		out, err = v.doVisitFloatLiteral(ctx)
 	case *fql.BooleanLiteralContext:
-		out, err = v.doVisitBooleanLiteral(node.(*fql.BooleanLiteralContext))
+		out, err = v.doVisitBooleanLiteral(ctx)
 	case *fql.NoneLiteralContext:
-		out, err = v.doVisitNoneLiteral(node.(*fql.NoneLiteralContext))
+		out, err = v.doVisitNoneLiteral(ctx)
 	case *fql.VariableContext:
-		out, err = v.doVisitVariable(node.(*fql.VariableContext), scope)
+		out, err = v.doVisitVariable(ctx, scope)
 	case *fql.VariableDeclarationContext:
-		out, err = v.doVisitVariableDeclaration(node.(*fql.VariableDeclarationContext), scope)
+		out, err = v.doVisitVariableDeclaration(ctx, scope)
 	case *fql.FunctionCallExpressionContext:
-		out, err = v.doVisitFunctionCallExpression(node.(*fql.FunctionCallExpressionContext), scope)
+		out, err = v.doVisitFunctionCallExpression(ctx, scope)
 	case *fql.ParamContext:
-		out, err = v.doVisitParamContext(node.(*fql.ParamContext), scope)
+		out, err = v.doVisitParamContext(ctx, scope)
 	default:
 		err = v.unexpectedToken(node)
 	}
diff --git a/pkg/runtime/collections/collection.go b/pkg/runtime/collections/collection.go
index ca48a061..84e9d6b0 100644
--- a/pkg/runtime/collections/collection.go
+++ b/pkg/runtime/collections/collection.go
@@ -7,49 +7,42 @@ import (
 )
 
 type (
-	Collection interface {
-		core.Value
+	// Measurable represents an interface of a value that can has length.
+	Measurable interface {
 		Length() values.Int
 	}
 
 	IndexedCollection interface {
-		Collection
+		core.Value
+		Measurable
 		Get(idx values.Int) core.Value
 		Set(idx values.Int, value core.Value) error
 	}
 
 	KeyedCollection interface {
-		Collection
+		core.Value
+		Measurable
 		Keys() []string
 		Get(key values.String) (core.Value, values.Boolean)
 		Set(key values.String, value core.Value)
 	}
 
-	IterableCollection interface {
-		core.Value
-		Iterate(ctx context.Context) (CollectionIterator, error)
-	}
-
-	CollectionIterator interface {
-		Next(ctx context.Context) (value core.Value, key core.Value, err error)
-	}
-
-	collectionIteratorWrapper struct {
+	coreIterator struct {
 		valVar string
 		keyVar string
-		values CollectionIterator
+		values core.Iterator
 	}
 )
 
-func NewCollectionIterator(
+func NewCoreIterator(
 	valVar,
 	keyVar string,
-	values CollectionIterator,
+	values core.Iterator,
 ) (Iterator, error) {
-	return &collectionIteratorWrapper{valVar, keyVar, values}, nil
+	return &coreIterator{valVar, keyVar, values}, nil
 }
 
-func (iterator *collectionIteratorWrapper) Next(ctx context.Context, scope *core.Scope) (*core.Scope, error) {
+func (iterator *coreIterator) Next(ctx context.Context, scope *core.Scope) (*core.Scope, error) {
 	val, key, err := iterator.values.Next(ctx)
 
 	if err != nil {
diff --git a/pkg/runtime/core/value.go b/pkg/runtime/core/value.go
index fa581889..3c4853c6 100644
--- a/pkg/runtime/core/value.go
+++ b/pkg/runtime/core/value.go
@@ -18,6 +18,17 @@ type (
 		Copy() Value
 	}
 
+	// Iterable represents an interface of a value that can be iterated by using an iterator.
+	Iterable interface {
+		Iterate(ctx context.Context) (Iterator, error)
+	}
+
+	// Iterator represents an interface of a value iterator.
+	// When iterator is exhausted it must return None as a value.
+	Iterator interface {
+		Next(ctx context.Context) (value Value, key Value, err error)
+	}
+
 	// Getter represents an interface of
 	// complex types that needs to be used to read values by path.
 	// The interface is created to let user-defined types be used in dot notation data access.
diff --git a/pkg/runtime/expressions/data_source.go b/pkg/runtime/expressions/data_source.go
index eb8f6795..81f352a7 100644
--- a/pkg/runtime/expressions/data_source.go
+++ b/pkg/runtime/expressions/data_source.go
@@ -54,20 +54,19 @@ func (ds *DataSource) Iterate(ctx context.Context, scope *core.Scope) (collectio
 			return collections.NewHTMLNodeIterator(ds.valVariable, ds.keyVariable, data.(values.HTMLNode))
 		default:
 			// fallback to user defined types
-			switch data.(type) {
-			case collections.IterableCollection:
-				collection := data.(collections.IterableCollection)
+			switch collection := data.(type) {
+			case core.Iterable:
 				iterator, err := collection.Iterate(ctx)
 
 				if err != nil {
 					return nil, err
 				}
 
-				return collections.NewCollectionIterator(ds.valVariable, ds.keyVariable, iterator)
+				return collections.NewCoreIterator(ds.valVariable, ds.keyVariable, iterator)
 			case collections.KeyedCollection:
-				return collections.NewIndexedIterator(ds.valVariable, ds.keyVariable, data.(collections.IndexedCollection))
+				return collections.NewKeyedIterator(ds.valVariable, ds.keyVariable, collection)
 			case collections.IndexedCollection:
-				return collections.NewKeyedIterator(ds.valVariable, ds.keyVariable, data.(collections.KeyedCollection))
+				return collections.NewIndexedIterator(ds.valVariable, ds.keyVariable, collection)
 			default:
 				return nil, core.TypeError(
 					data.Type(),
@@ -75,6 +74,7 @@ func (ds *DataSource) Iterate(ctx context.Context, scope *core.Scope) (collectio
 					types.Object,
 					types.HTMLDocument,
 					types.HTMLElement,
+					core.NewType("Iterable"),
 				)
 			}
 		}
diff --git a/pkg/runtime/expressions/data_source_test.go b/pkg/runtime/expressions/data_source_test.go
index caebfb22..1e01e419 100644
--- a/pkg/runtime/expressions/data_source_test.go
+++ b/pkg/runtime/expressions/data_source_test.go
@@ -2,12 +2,12 @@ package expressions_test
 
 import (
 	"context"
-	"github.com/MontFerret/ferret/pkg/runtime/expressions"
-	"github.com/MontFerret/ferret/pkg/runtime/values"
 	"testing"
 
 	"github.com/MontFerret/ferret/pkg/runtime/collections"
 	"github.com/MontFerret/ferret/pkg/runtime/core"
+	"github.com/MontFerret/ferret/pkg/runtime/expressions"
+	"github.com/MontFerret/ferret/pkg/runtime/values"
 	. "github.com/smartystreets/goconvey/convey"
 )
 
@@ -51,7 +51,7 @@ func (c *testIterableCollection) Hash() uint64 {
 func (c *testIterableCollection) Copy() core.Value {
 	return c
 }
-func (c *testIterableCollection) Iterate(ctx context.Context) (collections.CollectionIterator, error) {
+func (c *testIterableCollection) Iterate(ctx context.Context) (core.Iterator, error) {
 	return &testCollectionIterator{c.values, -1}, nil
 }
 
diff --git a/pkg/runtime/values/date_time.go b/pkg/runtime/values/date_time.go
index d6bec709..584e83ee 100644
--- a/pkg/runtime/values/date_time.go
+++ b/pkg/runtime/values/date_time.go
@@ -31,9 +31,9 @@ func ParseDateTime(input interface{}) (DateTime, error) {
 }
 
 func ParseDateTimeWith(input interface{}, layout string) (DateTime, error) {
-	switch input.(type) {
+	switch value := input.(type) {
 	case string:
-		t, err := time.Parse(layout, input.(string))
+		t, err := time.Parse(layout, value)
 
 		if err != nil {
 			return DateTime{time.Now()}, err
diff --git a/pkg/runtime/values/helpers.go b/pkg/runtime/values/helpers.go
index a66afed3..bd7fe722 100644
--- a/pkg/runtime/values/helpers.go
+++ b/pkg/runtime/values/helpers.go
@@ -206,44 +206,42 @@ func SetIn(ctx context.Context, to core.Value, byPath []core.Value, value core.V
 }
 
 func Parse(input interface{}) core.Value {
-	switch input.(type) {
+	switch value := input.(type) {
 	case bool:
-		return NewBoolean(input.(bool))
+		return NewBoolean(value)
 	case string:
-		return NewString(input.(string))
+		return NewString(value)
 	case int:
-		return NewInt(input.(int))
+		return NewInt(value)
 	case float64:
-		return NewFloat(input.(float64))
+		return NewFloat(value)
 	case float32:
-		return NewFloat(float64(input.(float32)))
+		return NewFloat(float64(value))
 	case time.Time:
-		return NewDateTime(input.(time.Time))
+		return NewDateTime(value)
 	case []interface{}:
-		input := input.([]interface{})
-		arr := NewArray(len(input))
+		arr := NewArray(len(value))
 
-		for _, el := range input {
+		for _, el := range value {
 			arr.Push(Parse(el))
 		}
 
 		return arr
 	case map[string]interface{}:
-		input := input.(map[string]interface{})
 		obj := NewObject()
 
-		for key, el := range input {
+		for key, el := range value {
 			obj.Set(NewString(key), Parse(el))
 		}
 
 		return obj
 	case []byte:
-		return NewBinary(input.([]byte))
+		return NewBinary(value)
 	case nil:
 		return None
 	default:
-		v := reflect.ValueOf(input)
-		t := reflect.TypeOf(input)
+		v := reflect.ValueOf(value)
+		t := reflect.TypeOf(value)
 		kind := t.Kind()
 
 		if kind == reflect.Slice || kind == reflect.Array {
@@ -251,8 +249,8 @@ func Parse(input interface{}) core.Value {
 			arr := NewArray(size)
 
 			for i := 0; i < size; i++ {
-				value := v.Index(i)
-				arr.Push(Parse(value.Interface()))
+				curVal := v.Index(i)
+				arr.Push(Parse(curVal.Interface()))
 			}
 
 			return arr
@@ -264,9 +262,9 @@ func Parse(input interface{}) core.Value {
 
 			for _, k := range keys {
 				key := Parse(k.Interface())
-				value := v.MapIndex(k)
+				curVal := v.MapIndex(k)
 
-				obj.Set(NewString(key.String()), Parse(value.Interface()))
+				obj.Set(NewString(key.String()), Parse(curVal.Interface()))
 			}
 
 			return obj
@@ -278,9 +276,9 @@ func Parse(input interface{}) core.Value {
 
 			for i := 0; i < size; i++ {
 				field := t.Field(i)
-				value := v.Field(i)
+				fieldValue := v.Field(i)
 
-				obj.Set(NewString(field.Name), Parse(value.Interface()))
+				obj.Set(NewString(field.Name), Parse(fieldValue.Interface()))
 			}
 
 			return obj
diff --git a/pkg/stdlib/arrays/append_test.go b/pkg/stdlib/arrays/append_test.go
index 38053c29..8445be78 100644
--- a/pkg/stdlib/arrays/append_test.go
+++ b/pkg/stdlib/arrays/append_test.go
@@ -23,7 +23,7 @@ func TestAppend(t *testing.T) {
 
 		So(err, ShouldBeNil)
 		So(out, ShouldNotEqual, arr)
-		So(out.(collections.Collection).Length(), ShouldBeGreaterThan, arr.Length())
+		So(out.(collections.Measurable).Length(), ShouldBeGreaterThan, arr.Length())
 	})
 
 	Convey("Should ignore non-unique items", t, func() {
@@ -39,12 +39,12 @@ func TestAppend(t *testing.T) {
 
 		So(err, ShouldBeNil)
 		So(out, ShouldNotEqual, arr)
-		So(out.(collections.Collection).Length(), ShouldEqual, arr.Length())
+		So(out.(collections.Measurable).Length(), ShouldEqual, arr.Length())
 
 		out2, err := arrays.Append(context.Background(), arr, values.NewInt(6), values.True)
 
 		So(err, ShouldBeNil)
 		So(out2, ShouldNotEqual, arr)
-		So(out2.(collections.Collection).Length(), ShouldBeGreaterThan, arr.Length())
+		So(out2.(collections.Measurable).Length(), ShouldBeGreaterThan, arr.Length())
 	})
 }
diff --git a/pkg/stdlib/collections/length.go b/pkg/stdlib/collections/length.go
index 1d8eb22a..8afe053d 100644
--- a/pkg/stdlib/collections/length.go
+++ b/pkg/stdlib/collections/length.go
@@ -18,7 +18,7 @@ func Length(_ context.Context, inputs ...core.Value) (core.Value, error) {
 
 	value := inputs[0]
 
-	c, ok := value.(collections.Collection)
+	c, ok := value.(collections.Measurable)
 
 	if !ok {
 		return values.None, core.TypeError(value.Type(),
@@ -28,7 +28,7 @@ func Length(_ context.Context, inputs ...core.Value) (core.Value, error) {
 			types.HTMLElement,
 			types.HTMLDocument,
 			types.Binary,
-			core.NewType("Collection"),
+			core.NewType("Measurable"),
 		)
 	}
 
diff --git a/pkg/stdlib/html/pagination.go b/pkg/stdlib/html/pagination.go
index 77f60527..edb8492a 100644
--- a/pkg/stdlib/html/pagination.go
+++ b/pkg/stdlib/html/pagination.go
@@ -3,7 +3,6 @@ package html
 import (
 	"context"
 
-	"github.com/MontFerret/ferret/pkg/runtime/collections"
 	"github.com/MontFerret/ferret/pkg/runtime/core"
 	"github.com/MontFerret/ferret/pkg/runtime/values"
 	"github.com/MontFerret/ferret/pkg/runtime/values/types"
@@ -81,7 +80,7 @@ func (p *Paging) Copy() core.Value {
 	return values.None
 }
 
-func (p *Paging) Iterate(_ context.Context) (collections.CollectionIterator, error) {
+func (p *Paging) Iterate(_ context.Context) (core.Iterator, error) {
 	return &PagingIterator{p.document, p.selector, -1}, nil
 }
 
diff --git a/pkg/stdlib/html/wait_class.go b/pkg/stdlib/html/wait_class.go
index f2640091..27b8cfa2 100644
--- a/pkg/stdlib/html/wait_class.go
+++ b/pkg/stdlib/html/wait_class.go
@@ -41,7 +41,7 @@ func WaitClass(_ context.Context, args ...core.Value) (core.Value, error) {
 	timeout := values.NewInt(defaultTimeout)
 
 	// lets figure out what is passed as 1st argument
-	switch args[0].(type) {
+	switch obj := args[0].(type) {
 	case values.DHTMLDocument:
 		// revalidate args with more accurate amount
 		err := core.ValidateArgs(args, 3, 4)
@@ -57,12 +57,6 @@ func WaitClass(_ context.Context, args ...core.Value) (core.Value, error) {
 			return values.None, err
 		}
 
-		doc, ok := args[0].(values.DHTMLDocument)
-
-		if !ok {
-			return values.None, core.Errors(core.ErrInvalidType, ErrNotDynamic)
-		}
-
 		selector := args[1].(values.String)
 		class := args[2].(values.String)
 
@@ -76,14 +70,8 @@ func WaitClass(_ context.Context, args ...core.Value) (core.Value, error) {
 			timeout = args[3].(values.Int)
 		}
 
-		return values.None, doc.WaitForClass(selector, class, timeout)
+		return values.None, obj.WaitForClass(selector, class, timeout)
 	case values.DHTMLNode:
-		el, ok := args[0].(values.DHTMLNode)
-
-		if !ok {
-			return values.None, core.Errors(core.ErrInvalidType, ErrNotDynamic)
-		}
-
 		class := args[1].(values.String)
 
 		if len(args) == 3 {
@@ -96,7 +84,7 @@ func WaitClass(_ context.Context, args ...core.Value) (core.Value, error) {
 			timeout = args[2].(values.Int)
 		}
 
-		return values.None, el.WaitForClass(class, timeout)
+		return values.None, obj.WaitForClass(class, timeout)
 	default:
 		return values.None, core.Errors(core.ErrInvalidType, ErrNotDynamic)
 	}