type checking and error handling for builtins improved.

object/object.go

@@ -1,5 +1,7 @@
 package object
 
+import "fmt"
+
 // Type represents the type of an object
 type ObjectType string
 
@@ -25,6 +27,13 @@ type Comparable interface {
 	Compare(other Object) int
 }
 
+// Sizeable is the interface for returning the size of an Object.
+// Object(s) that have a valid size must implement  this interface and the
+// Len() method.
+type Sizeable interface {
+	Len() int
+}
+
 // Immutable is the interface for all immutable objects which must implement
 // the Clone() method used by binding names to values.
 type Immutable interface {
@@ -34,8 +43,9 @@ type Immutable interface {
 // Object represents a value and implementations are expected to implement
 // `Type()` and `Inspect()` functions
 type Object interface {
+	fmt.Stringer
 	Type() ObjectType
-	String() string
+	Bool() bool
 	Inspect() string
 }