type checking and error handling for builtins improved.

object/array.go

@@ -14,6 +14,36 @@ func (ao *Array) Type() ObjectType {
 	return ARRAY_OBJ
 }
 
+func (ao *Array) Bool() bool {
+	return len(ao.Elements) > 0
+}
+
+func (a *Array) PopLeft() Object {
+	if len(a.Elements) > 0 {
+		e := a.Elements[0]
+		a.Elements = a.Elements[1:]
+		return e
+	}
+	return &Null{}
+}
+
+func (a *Array) PopRight() Object {
+	if len(a.Elements) > 0 {
+		e := a.Elements[(len(a.Elements) - 1)]
+		a.Elements = a.Elements[:(len(a.Elements) - 1)]
+		return e
+	}
+	return &Null{}
+}
+
+func (a *Array) Prepend(obj Object) {
+	a.Elements = append([]Object{obj}, a.Elements...)
+}
+
+func (a *Array) Append(obj Object) {
+	a.Elements = append(a.Elements, obj)
+}
+
 func (ao *Array) Inspect() string {
 	var out bytes.Buffer
 
@@ -32,9 +62,9 @@ func (ao *Array) String() string {
 	return ao.Inspect()
 }
 
-func (a *Array) Less(i, j int) bool {
-	if cmp, ok := a.Elements[i].(Comparable); ok {
-		return cmp.Compare(a.Elements[j]) == -1
+func (ao *Array) Less(i, j int) bool {
+	if cmp, ok := ao.Elements[i].(Comparable); ok {
+		return cmp.Compare(ao.Elements[j]) == -1
 	}
 	return false
 }

object/bool.go

@@ -8,6 +8,10 @@ type Boolean struct {
 	Value bool
 }
 
+func (b *Boolean) Bool() bool {
+	return b.Value
+}
+
 func (b *Boolean) Type() ObjectType {
 	return BOOLEAN_OBJ
 }

object/builtin.go

@@ -7,12 +7,18 @@ type Builtin struct {
 	Fn   BuiltinFunction
 }
 
+func (b *Builtin) Bool() bool {
+	return true
+}
+
 func (b *Builtin) Type() ObjectType {
 	return BUILTIN_OBJ
 }
+
 func (b *Builtin) Inspect() string {
 	return fmt.Sprintf("<built-in function %s>", b.Name)
 }
+
 func (b *Builtin) String() string {
 	return b.Inspect()
 }

object/closure.go

@@ -11,12 +11,18 @@ type CompiledFunction struct {
 	NumParameters int
 }
 
+func (cf *CompiledFunction) Bool() bool {
+	return true
+}
+
 func (cf *CompiledFunction) Type() ObjectType {
 	return COMPILED_FUNCTION_OBJ
 }
+
 func (cf *CompiledFunction) Inspect() string {
 	return fmt.Sprintf("CompiledFunction[%p]", cf)
 }
+
 func (cf *CompiledFunction) String() string {
 	return cf.Inspect()
 }
@@ -26,12 +32,18 @@ type Closure struct {
 	Free []Object
 }
 
+func (c *Closure) Bool() bool {
+	return true
+}
+
 func (c *Closure) Type() ObjectType {
 	return CLOSURE_OBJ
 }
+
 func (c *Closure) Inspect() string {
 	return fmt.Sprintf("Closure[%p]", c)
 }
+
 func (c *Closure) String() string {
 	return c.Inspect()
 }

object/error.go

@@ -4,15 +4,22 @@ type Error struct {
 	Message string
 }
 
+func (e *Error) Bool() bool {
+	return false
+}
+
 func (e *Error) Type() ObjectType {
 	return ERROR_OBJ
 }
+
 func (e *Error) Inspect() string {
 	return "Error: " + e.Message
 }
+
 func (e *Error) Clone() Object {
 	return &Error{Message: e.Message}
 }
+
 func (e *Error) String() string {
 	return e.Message
 }

object/function.go

@@ -12,9 +12,14 @@ type Function struct {
 	Env        *Environment
 }
 
+func (f *Function) Bool() bool {
+	return false
+}
+
 func (f *Function) Type() ObjectType {
 	return FUNCTION_OBJ
 }
+
 func (f *Function) Inspect() string {
 	var out bytes.Buffer
 
@@ -32,6 +37,7 @@ func (f *Function) Inspect() string {
 
 	return out.String()
 }
+
 func (f *Function) String() string {
 	return f.Inspect()
 }
@@ -40,12 +46,18 @@ type ReturnValue struct {
 	Value Object
 }
 
+func (rv *ReturnValue) Bool() bool {
+	return true
+}
+
 func (rv *ReturnValue) Type() ObjectType {
 	return RETURN_VALUE_OBJ
 }
+
 func (rv *ReturnValue) Inspect() string {
 	return rv.Value.Inspect()
 }
+
 func (rv *ReturnValue) String() string {
 	return rv.Inspect()
 }

object/hash.go

@@ -44,6 +44,14 @@ type Hash struct {
 	Pairs map[HashKey]HashPair
 }
 
+func (h *Hash) Len() int {
+	return len(h.Pairs)
+}
+
+func (h *Hash) Bool() bool {
+	return len(h.Pairs) > 0
+}
+
 func (h *Hash) Type() ObjectType {
 	return HASH_OBJ
 }
@@ -62,6 +70,7 @@ func (h *Hash) Inspect() string {
 
 	return out.String()
 }
+
 func (h *Hash) String() string {
 	return h.Inspect()
 }

object/int.go

@@ -6,6 +6,10 @@ type Integer struct {
 	Value int64
 }
 
+func (i *Integer) Bool() bool {
+	return i.Value != 0
+}
+
 func (i *Integer) Type() ObjectType {
 	return INTEGER_OBJ
 }

object/null.go

@@ -2,6 +2,10 @@ package object
 
 type Null struct{}
 
+func (n *Null) Bool() bool {
+	return false
+}
+
 func (n *Null) Type() ObjectType {
 	return NULL_OBJ
 }

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
 }
 

object/str.go

@@ -1,11 +1,22 @@
 package object
 
-import "fmt"
+import (
+	"fmt"
+	"unicode/utf8"
+)
 
 type String struct {
 	Value string
 }
 
+func (s *String) Len() int {
+	return utf8.RuneCountInString(s.Value)
+}
+
+func (s *String) Bool() bool {
+	return s.Value != ""
+}
+
 func (s *String) Type() ObjectType {
 	return STRING_OBJ
 }