diff --git a/compiler/stdmods/errors.go b/compiler/stdmods/errors.go
new file mode 100644
index 0000000..3c265eb
--- /dev/null
+++ b/compiler/stdmods/errors.go
@@ -0,0 +1,11 @@
+package stdmods
+
+import "github.com/d5/tengo/objects"
+
+func wrapError(err error) objects.Object {
+	if err == nil {
+		return objects.TrueValue
+	}
+
+	return &objects.Error{Value: &objects.String{Value: err.Error()}}
+}
diff --git a/compiler/stdmods/func_typedefs.go b/compiler/stdmods/func_typedefs.go
index cf71edd..7db07f8 100644
--- a/compiler/stdmods/func_typedefs.go
+++ b/compiler/stdmods/func_typedefs.go
@@ -1,28 +1,21 @@
 package stdmods
 
 import (
-	"fmt"
-
 	"github.com/d5/tengo/objects"
 )
 
-// FuncAFRF transform a function of 'func(float64) float64' signature
+// FuncAR transform a function of 'func()' signature
 // into a user function object.
-func FuncAFRF(fn func(float64) float64) *objects.UserFunction {
+func FuncAR(fn func()) *objects.UserFunction {
 	return &objects.UserFunction{
 		Value: func(args ...objects.Object) (ret objects.Object, err error) {
-			if len(args) != 1 {
+			if len(args) != 0 {
 				return nil, objects.ErrWrongNumArguments
 			}
 
-			switch arg := args[0].(type) {
-			case *objects.Int:
-				return &objects.Float{Value: fn(float64(arg.Value))}, nil
-			case *objects.Float:
-				return &objects.Float{Value: fn(arg.Value)}, nil
-			default:
-				return nil, fmt.Errorf("invalid argument type: %s", arg.TypeName())
-			}
+			fn()
+
+			return objects.UndefinedValue, nil
 		},
 	}
 }
@@ -41,6 +34,44 @@ func FuncARF(fn func() float64) *objects.UserFunction {
 	}
 }
 
+// FuncARSs transform a function of 'func() []string' signature
+// into a user function object.
+func FuncARSs(fn func() []string) *objects.UserFunction {
+	return &objects.UserFunction{
+		Value: func(args ...objects.Object) (ret objects.Object, err error) {
+			if len(args) != 0 {
+				return nil, objects.ErrWrongNumArguments
+			}
+
+			arr := &objects.Array{}
+			for _, osArg := range fn() {
+				arr.Value = append(arr.Value, &objects.String{Value: osArg})
+			}
+
+			return arr, nil
+		},
+	}
+}
+
+// FuncAFRF transform a function of 'func(float64) float64' signature
+// into a user function object.
+func FuncAFRF(fn func(float64) float64) *objects.UserFunction {
+	return &objects.UserFunction{
+		Value: func(args ...objects.Object) (ret objects.Object, err error) {
+			if len(args) != 1 {
+				return nil, objects.ErrWrongNumArguments
+			}
+
+			f1, ok := objects.ToFloat64(args[0])
+			if !ok {
+				return nil, objects.ErrInvalidTypeConversion
+			}
+
+			return &objects.Float{Value: fn(f1)}, nil
+		},
+	}
+}
+
 // FuncAIRF transform a function of 'func(int) float64' signature
 // into a user function object.
 func FuncAIRF(fn func(int) float64) *objects.UserFunction {
@@ -50,14 +81,12 @@ func FuncAIRF(fn func(int) float64) *objects.UserFunction {
 				return nil, objects.ErrWrongNumArguments
 			}
 
-			switch arg := args[0].(type) {
-			case *objects.Int:
-				return &objects.Float{Value: fn(int(arg.Value))}, nil
-			case *objects.Float:
-				return &objects.Float{Value: fn(int(arg.Value))}, nil
-			default:
-				return nil, fmt.Errorf("invalid argument type: %s", arg.TypeName())
+			i1, ok := objects.ToInt(args[0])
+			if !ok {
+				return nil, objects.ErrInvalidTypeConversion
 			}
+
+			return &objects.Float{Value: fn(i1)}, nil
 		},
 	}
 }
@@ -71,14 +100,12 @@ func FuncAFRI(fn func(float64) int) *objects.UserFunction {
 				return nil, objects.ErrWrongNumArguments
 			}
 
-			switch arg := args[0].(type) {
-			case *objects.Int:
-				return &objects.Int{Value: int64(fn(float64(arg.Value)))}, nil
-			case *objects.Float:
-				return &objects.Int{Value: int64(fn(arg.Value))}, nil
-			default:
-				return nil, fmt.Errorf("invalid argument type: %s", arg.TypeName())
+			f1, ok := objects.ToFloat64(args[0])
+			if !ok {
+				return nil, objects.ErrInvalidTypeConversion
 			}
+
+			return &objects.Int{Value: int64(fn(f1))}, nil
 		},
 	}
 }
@@ -92,26 +119,17 @@ func FuncAFFRF(fn func(float64, float64) float64) *objects.UserFunction {
 				return nil, objects.ErrWrongNumArguments
 			}
 
-			var arg0, arg1 float64
-
-			switch arg := args[0].(type) {
-			case *objects.Int:
-				arg0 = float64(arg.Value)
-			case *objects.Float:
-				arg0 = arg.Value
-			default:
-				return nil, fmt.Errorf("invalid argument type: %s", arg.TypeName())
-			}
-			switch arg := args[1].(type) {
-			case *objects.Int:
-				arg1 = float64(arg.Value)
-			case *objects.Float:
-				arg1 = arg.Value
-			default:
-				return nil, fmt.Errorf("invalid argument type: %s", arg.TypeName())
+			f1, ok := objects.ToFloat64(args[0])
+			if !ok {
+				return nil, objects.ErrInvalidTypeConversion
 			}
 
-			return &objects.Float{Value: fn(arg0, arg1)}, nil
+			f2, ok := objects.ToFloat64(args[1])
+			if !ok {
+				return nil, objects.ErrInvalidTypeConversion
+			}
+
+			return &objects.Float{Value: fn(f1, f2)}, nil
 		},
 	}
 }
@@ -125,27 +143,17 @@ func FuncAIFRF(fn func(int, float64) float64) *objects.UserFunction {
 				return nil, objects.ErrWrongNumArguments
 			}
 
-			var arg0 int
-			var arg1 float64
-
-			switch arg := args[0].(type) {
-			case *objects.Int:
-				arg0 = int(arg.Value)
-			case *objects.Float:
-				arg0 = int(arg.Value)
-			default:
-				return nil, fmt.Errorf("invalid argument type: %s", arg.TypeName())
-			}
-			switch arg := args[1].(type) {
-			case *objects.Int:
-				arg1 = float64(arg.Value)
-			case *objects.Float:
-				arg1 = arg.Value
-			default:
-				return nil, fmt.Errorf("invalid argument type: %s", arg.TypeName())
+			i1, ok := objects.ToInt(args[0])
+			if !ok {
+				return nil, objects.ErrInvalidTypeConversion
 			}
 
-			return &objects.Float{Value: fn(arg0, arg1)}, nil
+			f2, ok := objects.ToFloat64(args[1])
+			if !ok {
+				return nil, objects.ErrInvalidTypeConversion
+			}
+
+			return &objects.Float{Value: fn(i1, f2)}, nil
 		},
 	}
 }
@@ -159,27 +167,17 @@ func FuncAFIRF(fn func(float64, int) float64) *objects.UserFunction {
 				return nil, objects.ErrWrongNumArguments
 			}
 
-			var arg0 float64
-			var arg1 int
-
-			switch arg := args[0].(type) {
-			case *objects.Int:
-				arg0 = float64(arg.Value)
-			case *objects.Float:
-				arg0 = float64(arg.Value)
-			default:
-				return nil, fmt.Errorf("invalid argument type: %s", arg.TypeName())
-			}
-			switch arg := args[1].(type) {
-			case *objects.Int:
-				arg1 = int(arg.Value)
-			case *objects.Float:
-				arg1 = int(arg.Value)
-			default:
-				return nil, fmt.Errorf("invalid argument type: %s", arg.TypeName())
+			f1, ok := objects.ToFloat64(args[0])
+			if !ok {
+				return nil, objects.ErrInvalidTypeConversion
 			}
 
-			return &objects.Float{Value: fn(arg0, arg1)}, nil
+			i2, ok := objects.ToInt(args[1])
+			if !ok {
+				return nil, objects.ErrInvalidTypeConversion
+			}
+
+			return &objects.Float{Value: fn(f1, i2)}, nil
 		},
 	}
 }
@@ -193,27 +191,17 @@ func FuncAFIRB(fn func(float64, int) bool) *objects.UserFunction {
 				return nil, objects.ErrWrongNumArguments
 			}
 
-			var arg0 float64
-			var arg1 int
-
-			switch arg := args[0].(type) {
-			case *objects.Int:
-				arg0 = float64(arg.Value)
-			case *objects.Float:
-				arg0 = arg.Value
-			default:
-				return nil, fmt.Errorf("invalid argument type: %s", arg.TypeName())
-			}
-			switch arg := args[1].(type) {
-			case *objects.Int:
-				arg1 = int(arg.Value)
-			case *objects.Float:
-				arg1 = int(arg.Value)
-			default:
-				return nil, fmt.Errorf("invalid argument type: %s", arg.TypeName())
+			f1, ok := objects.ToFloat64(args[0])
+			if !ok {
+				return nil, objects.ErrInvalidTypeConversion
 			}
 
-			return &objects.Bool{Value: fn(arg0, arg1)}, nil
+			i2, ok := objects.ToInt(args[1])
+			if !ok {
+				return nil, objects.ErrInvalidTypeConversion
+			}
+
+			return &objects.Bool{Value: fn(f1, i2)}, nil
 		},
 	}
 }
@@ -227,17 +215,31 @@ func FuncAFRB(fn func(float64) bool) *objects.UserFunction {
 				return nil, objects.ErrWrongNumArguments
 			}
 
-			var arg0 float64
-			switch arg := args[0].(type) {
-			case *objects.Int:
-				arg0 = float64(arg.Value)
-			case *objects.Float:
-				arg0 = arg.Value
-			default:
-				return nil, fmt.Errorf("invalid argument type: %s", arg.TypeName())
+			f1, ok := objects.ToFloat64(args[0])
+			if !ok {
+				return nil, objects.ErrInvalidTypeConversion
 			}
 
-			return &objects.Bool{Value: fn(arg0)}, nil
+			return &objects.Bool{Value: fn(f1)}, nil
+		},
+	}
+}
+
+// FuncASRE transform a function of 'func(string) error' signature into a user function object.
+// User function will return 'true' if underlying native function returns nil.
+func FuncASRE(fn func(string) error) *objects.UserFunction {
+	return &objects.UserFunction{
+		Value: func(args ...objects.Object) (objects.Object, error) {
+			if len(args) != 1 {
+				return nil, objects.ErrWrongNumArguments
+			}
+
+			s1, ok := objects.ToString(args[0])
+			if !ok {
+				return nil, objects.ErrInvalidTypeConversion
+			}
+
+			return wrapError(fn(s1)), nil
 		},
 	}
 }
diff --git a/compiler/stdmods/os.go b/compiler/stdmods/os.go
new file mode 100644
index 0000000..c04fa94
--- /dev/null
+++ b/compiler/stdmods/os.go
@@ -0,0 +1,103 @@
+package stdmods
+
+import (
+	"os"
+
+	"github.com/d5/tengo/objects"
+)
+
+var osModule = map[string]objects.Object{
+	"o_rdonly": &objects.Int{Value: int64(os.O_RDONLY)},
+	"o_wronly": &objects.Int{Value: int64(os.O_WRONLY)},
+	"o_rdwr":   &objects.Int{Value: int64(os.O_RDWR)},
+	"o_append": &objects.Int{Value: int64(os.O_APPEND)},
+	"o_create": &objects.Int{Value: int64(os.O_CREATE)},
+	"o_excl":   &objects.Int{Value: int64(os.O_EXCL)},
+	"o_sync":   &objects.Int{Value: int64(os.O_SYNC)},
+	"o_trunc":  &objects.Int{Value: int64(os.O_TRUNC)},
+
+	"mode_dir":         &objects.Int{Value: int64(os.ModeDir)},
+	"mode_append":      &objects.Int{Value: int64(os.ModeAppend)},
+	"mode_exclusive":   &objects.Int{Value: int64(os.ModeExclusive)},
+	"mode_temporary":   &objects.Int{Value: int64(os.ModeTemporary)},
+	"mode_symlink":     &objects.Int{Value: int64(os.ModeSymlink)},
+	"mode_device":      &objects.Int{Value: int64(os.ModeDevice)},
+	"mode_named_pipe":  &objects.Int{Value: int64(os.ModeNamedPipe)},
+	"mode_socket":      &objects.Int{Value: int64(os.ModeSocket)},
+	"mode_setuid":      &objects.Int{Value: int64(os.ModeSetuid)},
+	"mode_setgui":      &objects.Int{Value: int64(os.ModeSetgid)},
+	"mode_char_device": &objects.Int{Value: int64(os.ModeCharDevice)},
+	"mode_sticky":      &objects.Int{Value: int64(os.ModeSticky)},
+	"mode_irregular":   &objects.Int{Value: int64(os.ModeIrregular)},
+	"mode_type":        &objects.Int{Value: int64(os.ModeType)},
+	"mode_perm":        &objects.Int{Value: int64(os.ModePerm)},
+
+	"path_separator":      &objects.Char{Value: os.PathSeparator},
+	"path_list_separator": &objects.Char{Value: os.PathListSeparator},
+	"dev_null":            &objects.String{Value: os.DevNull},
+
+	"args":     &objects.UserFunction{Value: osArgs},
+	"chdir":    FuncASRE(os.Chdir),
+	"chmod":    &objects.UserFunction{Value: osChmod},
+	"chown":    &objects.UserFunction{Value: osChown},
+	"clearenv": FuncAR(os.Clearenv),
+	"environ":  FuncARSs(os.Environ),
+
+	// TODO: system errors
+	//"err_invalid":         &objects.Error{Value: os.ErrInvalid.Error()},
+	// TODO: STDIN, STDOUT, STDERR
+	// "stdin": nil,
+	// "stdout": nil,
+	// "stderr": nil,
+}
+
+func osArgs(args ...objects.Object) (objects.Object, error) {
+	if len(args) != 0 {
+		return nil, objects.ErrWrongNumArguments
+	}
+
+	arr := &objects.Array{}
+	for _, osArg := range os.Args {
+		arr.Value = append(arr.Value, &objects.String{Value: osArg})
+	}
+
+	return arr, nil
+}
+
+func osChmod(args ...objects.Object) (objects.Object, error) {
+	if len(args) != 2 {
+		return nil, objects.ErrWrongNumArguments
+	}
+
+	s1, ok := objects.ToString(args[0])
+	if !ok {
+		return nil, objects.ErrInvalidTypeConversion
+	}
+	i2, ok := objects.ToInt(args[1])
+	if !ok {
+		return nil, objects.ErrInvalidTypeConversion
+	}
+
+	return wrapError(os.Chmod(s1, os.FileMode(i2))), nil
+}
+
+func osChown(args ...objects.Object) (objects.Object, error) {
+	if len(args) != 3 {
+		return nil, objects.ErrWrongNumArguments
+	}
+
+	s1, ok := objects.ToString(args[0])
+	if !ok {
+		return nil, objects.ErrInvalidTypeConversion
+	}
+	i2, ok := objects.ToInt(args[1])
+	if !ok {
+		return nil, objects.ErrInvalidTypeConversion
+	}
+	i3, ok := objects.ToInt(args[2])
+	if !ok {
+		return nil, objects.ErrInvalidTypeConversion
+	}
+
+	return wrapError(os.Chown(s1, i2, i3)), nil
+}
diff --git a/compiler/stdmods/stdmods.go b/compiler/stdmods/stdmods.go
index 9cf5963..c32f02a 100644
--- a/compiler/stdmods/stdmods.go
+++ b/compiler/stdmods/stdmods.go
@@ -5,4 +5,5 @@ import "github.com/d5/tengo/objects"
 // Modules contain the standard modules.
 var Modules = map[string]*objects.ModuleMap{
 	"math": {Value: mathModule},
+	"os":   {Value: osModule},
 }