type checking and error handling for builtins improved.

.drone.yml

@@ -5,7 +5,7 @@ steps:
   - name: build
     image: golang:latest
     commands:
-      - make test
+      - go test -v -short -cover -coverprofile=coverage.txt ./...
 
   - name: coverage
     image: plugins/codecov

builtins/abs.go

@@ -1,20 +1,24 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 // Abs  ...
 func Abs(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"abs", args,
-			len(args))
+		typing.ExactArgs(1),
+		typing.WithTypes(object.INTEGER_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if i, ok := args[0].(*object.Integer); ok {
+	i := args[0].(*object.Integer)
 	value := i.Value
 	if value < 0 {
 		value = value * -1
 	}
 	return &object.Integer{Value: value}
-	}
-	return newError("argument to `abs` not supported, got %s", args[0].Type())
 }

builtins/args.go

@@ -1,9 +1,19 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 // Args ...
 func Args(args ...object.Object) object.Object {
+	if err := typing.Check(
+		"args", args,
+		typing.ExactArgs(0),
+	); err != nil {
+		return newError(err.Error())
+	}
+
 	elements := make([]object.Object, len(object.Arguments))
 	for i, arg := range object.Arguments {
 		elements[i] = &object.String{Value: arg}

builtins/assert.go

@@ -1,6 +1,9 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 import (
 	"fmt"
@@ -9,17 +12,12 @@ import (
 
 // Assert ...
 func Assert(args ...object.Object) object.Object {
-	if len(args) != 2 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=2",
+		"assert", args,
-			len(args))
+		typing.ExactArgs(2),
-	}
+		typing.WithTypes(object.BOOLEAN_OBJ, object.STRING_OBJ),
-	if args[0].Type() != object.BOOLEAN_OBJ {
+	); err != nil {
-		return newError("argument #1 to `assert` must be BOOLEAN, got %s",
+		return newError(err.Error())
-			args[0].Type())
-	}
-	if args[1].Type() != object.STRING_OBJ {
-		return newError("argument #2 to `assert` must be STRING, got %s",
-			args[0].Type())
 	}
 
 	if !args[0].(*object.Boolean).Value {

builtins/bin.go

@@ -3,18 +3,20 @@ package builtins
 import (
 	"fmt"
 	"monkey/object"
+	"monkey/typing"
 	"strconv"
 )
 
 // Bin ...
 func Bin(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"bin", args,
-			len(args))
+		typing.ExactArgs(1),
+		typing.WithTypes(object.INTEGER_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if i, ok := args[0].(*object.Integer); ok {
+	i := args[0].(*object.Integer)
 	return &object.String{Value: fmt.Sprintf("0b%s", strconv.FormatInt(i.Value, 2))}
-	}
-	return newError("argument to `bin` not supported, got %s", args[0].Type())
 }

builtins/bool.go

@@ -1,40 +1,18 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 // Bool ...
 func Bool(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1", len(args))
+		"bool", args,
+		typing.ExactArgs(1),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	switch arg := args[0].(type) {
+	return &object.Boolean{Value: args[0].Bool()}
-	case *object.Null:
-		return &object.Boolean{Value: false}
-	case *object.Boolean:
-		return arg
-	case *object.Integer:
-		if arg.Value == 0 {
-			return &object.Boolean{Value: false}
-		}
-		return &object.Boolean{Value: true}
-	case *object.String:
-		if len(arg.Value) > 0 {
-			return &object.Boolean{Value: true}
-		}
-		return &object.Boolean{Value: false}
-	case *object.Array:
-		if len(arg.Elements) > 0 {
-			return &object.Boolean{Value: true}
-		}
-		return &object.Boolean{Value: false}
-	case *object.Hash:
-		if len(arg.Pairs) > 0 {
-			return &object.Boolean{Value: true}
-		}
-		return &object.Boolean{Value: false}
-
-	default:
-		return newError("argument to `bool` not supported, got=%s", args[0].Type())
-	}
 }

builtins/chr.go

@@ -3,17 +3,19 @@ package builtins
 import (
 	"fmt"
 	"monkey/object"
+	"monkey/typing"
 )
 
 // Chr ...
 func Chr(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"chr", args,
-			len(args))
+		typing.ExactArgs(1),
+		typing.WithTypes(object.INTEGER_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if i, ok := args[0].(*object.Integer); ok {
+	i := args[0].(*object.Integer)
 	return &object.String{Value: fmt.Sprintf("%c", rune(i.Value))}
-	}
-	return newError("argument to `chr` not supported, got %s", args[0].Type())
 }

builtins/divmod.go

@@ -1,24 +1,24 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 // Divmod ...
 func Divmod(args ...object.Object) object.Object {
-	if len(args) != 2 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=2",
+		"divmod", args,
-			len(args))
+		typing.ExactArgs(2),
+		typing.WithTypes(object.INTEGER_OBJ, object.INTEGER_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if a, ok := args[0].(*object.Integer); ok {
+	a := args[0].(*object.Integer)
-		if b, ok := args[1].(*object.Integer); ok {
+	b := args[1].(*object.Integer)
 	elements := make([]object.Object, 2)
 	elements[0] = &object.Integer{Value: a.Value / b.Value}
 	elements[1] = &object.Integer{Value: a.Value % b.Value}
 	return &object.Array{Elements: elements}
-		} else {
-			return newError("expected argument #2 to `divmod` to be `int` got=%s", args[1].Type())
-		}
-	} else {
-		return newError("expected argument #1 to `divmod` to be `int` got=%s", args[0].Type())
-	}
 }

builtins/exit.go

@@ -1,15 +1,22 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 // Exit ...
 func Exit(args ...object.Object) object.Object {
+	if err := typing.Check(
+		"exit", args,
+		typing.RangeOfArgs(0, 1),
+		typing.WithTypes(object.INTEGER_OBJ),
+	); err != nil {
+		return newError(err.Error())
+	}
+
 	var status int
 	if len(args) == 1 {
-		if args[0].Type() != object.INTEGER_OBJ {
-			return newError("argument to `exit` must be INTEGER, got %s",
-				args[0].Type())
-		}
 		status = int(args[0].(*object.Integer).Value)
 	}
 

builtins/ffi.go

@@ -1,6 +1,9 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 import (
 	"fmt"
@@ -9,22 +12,16 @@ import (
 
 // FFI ...
 func FFI(args ...object.Object) object.Object {
-	if len(args) != 2 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=2",
+		"ffi", args,
-			len(args))
+		typing.ExactArgs(2),
+		typing.WithTypes(object.STRING_OBJ, object.STRING_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	arg, ok := args[0].(*object.String)
+	name := args[0].(*object.String).Value
-	if !ok {
+	symbol := args[1].(*object.String).Value
-		return newError("argument #1 to `ffi` expected to be `str` got=%T", args[0].Type())
-	}
-	name := arg.Value
-
-	arg, ok = args[1].(*object.String)
-	if !ok {
-		return newError("argument #2 to `ffi` expected to be `str` got=%T", args[0].Type())
-	}
-	symbol := arg.Value
 
 	p, err := plugin.Open(fmt.Sprintf("%s.so", name))
 	if err != nil {
@@ -36,5 +33,5 @@ func FFI(args ...object.Object) object.Object {
 		return newError("error finding symbol: %s", err)
 	}
 
-	return &object.Builtin{Name: symbol, Fn: v.(func(...object.Object) object.Object)}
+	return &object.Builtin{Name: symbol, Fn: v.(object.BuiltinFunction)}
 }

builtins/find.go

@@ -2,6 +2,7 @@ package builtins
 
 import (
 	"monkey/object"
+	"monkey/typing"
 	"sort"
 )
 
@@ -11,19 +12,29 @@ import (
 
 // Find ...
 func Find(args ...object.Object) object.Object {
-	if len(args) != 2 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=2",
+		"find", args,
-			len(args))
+		typing.ExactArgs(2),
+	); err != nil {
+		return newError(err.Error())
 	}
 
+	// find substring in string
 	if haystack, ok := args[0].(*object.String); ok {
-		if needle, ok := args[1].(*object.String); ok {
+		if err := typing.Check(
+			"find", args,
+			typing.WithTypes(object.STRING_OBJ, object.STRING_OBJ),
+		); err != nil {
+			return newError(err.Error())
+		}
+
+		needle := args[1].(*object.String)
 		index := strings.Index(haystack.Value, needle.Value)
 		return &object.Integer{Value: int64(index)}
-		} else {
-			return newError("expected arg #2 to be `str` got got=%T", args[1])
 	}
-	} else if haystack, ok := args[0].(*object.Array); ok {
+
+	// find in array
+	if haystack, ok := args[0].(*object.Array); ok {
 		needle := args[1].(object.Comparable)
 		i := sort.Search(len(haystack.Elements), func(i int) bool {
 			return needle.Compare(haystack.Elements[i]) == 0
@@ -32,7 +43,10 @@ func Find(args ...object.Object) object.Object {
 			return &object.Integer{Value: int64(i)}
 		}
 		return &object.Integer{Value: -1}
-	} else {
-		return newError("expected arg #1 to be `str` or `array` got got=%T", args[0])
 	}
+
+	return newError(
+		"TypeError: find() expected argument #1 to be `array` or `str` got `%s`",
+		args[0].Type(),
+	)
 }

builtins/first.go

@@ -1,16 +1,18 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 // First ...
 func First(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"first", args,
-			len(args))
+		typing.ExactArgs(1),
-	}
+		typing.WithTypes(object.ARRAY_OBJ),
-	if args[0].Type() != object.ARRAY_OBJ {
+	); err != nil {
-		return newError("argument to `first` must be array, got %s",
+		return newError(err.Error())
-			args[0].Type())
 	}
 
 	arr := args[0].(*object.Array)

builtins/hash.go

@@ -1,16 +1,22 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 // HashOf ...
 func HashOf(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"hash", args,
-			len(args))
+		typing.ExactArgs(1),
+	); err != nil {
+		return newError(err.Error())
 	}
 
 	if hash, ok := args[0].(object.Hashable); ok {
 		return &object.Integer{Value: int64(hash.HashKey().Value)}
 	}
-	return newError("argument #1 to `hash()` is not hashable: %s", args[0].Inspect())
+
+	return newError("TypeError: hash() expected argument #1 to be hashable")
 }

builtins/hex.go

@@ -3,18 +3,20 @@ package builtins
 import (
 	"fmt"
 	"monkey/object"
+	"monkey/typing"
 	"strconv"
 )
 
 // Hex ...
 func Hex(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"hex", args,
-			len(args))
+		typing.ExactArgs(1),
+		typing.WithTypes(object.INTEGER_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if i, ok := args[0].(*object.Integer); ok {
+	i := args[0].(*object.Integer)
 	return &object.String{Value: fmt.Sprintf("0x%s", strconv.FormatInt(i.Value, 16))}
-	}
-	return newError("argument to `hex` not supported, got %s", args[0].Type())
 }

builtins/id.go

@@ -3,13 +3,16 @@ package builtins
 import (
 	"fmt"
 	"monkey/object"
+	"monkey/typing"
 )
 
 // IdOf ...
 func IdOf(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"id", args,
-			len(args))
+		typing.ExactArgs(1),
+	); err != nil {
+		return newError(err.Error())
 	}
 
 	arg := args[0]
@@ -32,7 +35,7 @@ func IdOf(args ...object.Object) object.Object {
 		return &object.String{Value: fmt.Sprintf("%p", c)}
 	} else if b, ok := arg.(*object.Builtin); ok {
 		return &object.String{Value: fmt.Sprintf("%p", b)}
-	} else {
-		return newError("argument 31 to `id()` unsupported got=%T", arg.Type())
 	}
+
+	return nil
 }

builtins/input.go

@@ -1,6 +1,9 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 import (
 	"bufio"
@@ -11,15 +14,17 @@ import (
 
 // Input ...
 func Input(args ...object.Object) object.Object {
-	if len(args) > 0 {
+	if err := typing.Check(
-		obj, ok := args[0].(*object.String)
+		"input", args,
-		if !ok {
+		typing.RangeOfArgs(0, 1),
-			return newError(
+		typing.WithTypes(object.STRING_OBJ),
-				"argument to `input` not supported, got %s",
+	); err != nil {
-				args[0].Type(),
+		return newError(err.Error())
-			)
 	}
-		fmt.Fprintf(os.Stdout, obj.Value)
+
+	if len(args) == 1 {
+		prompt := args[0].(*object.String).Value
+		fmt.Fprintf(os.Stdout, prompt)
 	}
 
 	buffer := bufio.NewReader(os.Stdin)

builtins/int.go

@@ -1,13 +1,19 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 import "strconv"
 
 // Int ...
 func Int(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1", len(args))
+		"int", args,
+		typing.ExactArgs(1),
+	); err != nil {
+		return newError(err.Error())
 	}
 
 	switch arg := args[0].(type) {
@@ -23,9 +29,8 @@ func Int(args ...object.Object) object.Object {
 		if err != nil {
 			return newError("could not parse string to int: %s", err)
 		}
-
 		return &object.Integer{Value: n}
 	default:
-		return newError("argument to `int` not supported, got=%s", args[0].Type())
+		return &object.Integer{}
 	}
 }

builtins/join.go

@@ -1,6 +1,9 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 import (
 	"strings"
@@ -8,22 +11,19 @@ import (
 
 // Join ...
 func Join(args ...object.Object) object.Object {
-	if len(args) != 2 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"join", args,
-			len(args))
+		typing.ExactArgs(2),
+		typing.WithTypes(object.ARRAY_OBJ, object.STRING_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if arr, ok := args[0].(*object.Array); ok {
+	arr := args[0].(*object.Array)
-		if sep, ok := args[1].(*object.String); ok {
+	sep := args[1].(*object.String)
 	a := make([]string, len(arr.Elements))
 	for i, el := range arr.Elements {
 		a[i] = el.String()
 	}
 	return &object.String{Value: strings.Join(a, sep.Value)}
-		} else {
-			return newError("expected arg #2 to be `str` got got=%T", args[1])
-		}
-	} else {
-		return newError("expected arg #1 to be `array` got got=%T", args[0])
-	}
 }

builtins/last.go

@@ -1,16 +1,18 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 // Last ...
 func Last(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"last", args,
-			len(args))
+		typing.ExactArgs(1),
-	}
+		typing.WithTypes(object.ARRAY_OBJ),
-	if args[0].Type() != object.ARRAY_OBJ {
+	); err != nil {
-		return newError("argument to `last` must be array, got %s",
+		return newError(err.Error())
-			args[0].Type())
 	}
 
 	arr := args[0].(*object.Array)

builtins/len.go

@@ -1,23 +1,21 @@
 package builtins
 
-import "monkey/object"
+import (
-
+	"monkey/object"
-import "unicode/utf8"
+	"monkey/typing"
+)
 
 // Len ...
 func Len(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"len", args,
-			len(args))
+		typing.ExactArgs(1),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	switch arg := args[0].(type) {
+	if size, ok := args[0].(object.Sizeable); ok {
-	case *object.Array:
+		return &object.Integer{Value: int64(size.Len())}
-		return &object.Integer{Value: int64(len(arg.Elements))}
-	case *object.String:
-		return &object.Integer{Value: int64(utf8.RuneCountInString(arg.Value))}
-	default:
-		return newError("argument to `len` not supported, got %s",
-			args[0].Type())
 	}
+	return newError("TypeError: object of type '%s' has no len()", args[0].Type())
 }

builtins/lower.go

@@ -1,18 +1,22 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 import "strings"
 
 // Lower ...
 func Lower(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"lower", args,
-			len(args))
+		typing.ExactArgs(1),
+		typing.WithTypes(object.STRING_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if str, ok := args[0].(*object.String); ok {
+	str := args[0].(*object.String)
 	return &object.String{Value: strings.ToLower(str.Value)}
-	}
-	return newError("expected `str` argument to `lower` got=%T", args[0])
 }

builtins/max.go

@@ -2,17 +2,21 @@ package builtins
 
 import (
 	"monkey/object"
+	"monkey/typing"
 	"sort"
 )
 
 // Max ...
 func Max(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"max", args,
-			len(args))
+		typing.ExactArgs(1),
+		typing.WithTypes(object.ARRAY_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if a, ok := args[0].(*object.Array); ok {
+	a := args[0].(*object.Array)
 	// TODO: Make this more generic
 	xs := make([]int, len(a.Elements))
 	for n, e := range a.Elements {
@@ -24,6 +28,4 @@ func Max(args ...object.Object) object.Object {
 	}
 	sort.Ints(xs)
 	return &object.Integer{Value: int64(xs[len(xs)-1])}
-	}
-	return newError("argument #1 to `max` expected to be `array` got=%T", args[0].Type())
 }

builtins/min.go

@@ -2,17 +2,21 @@ package builtins
 
 import (
 	"monkey/object"
+	"monkey/typing"
 	"sort"
 )
 
 // Min ...
 func Min(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"min", args,
-			len(args))
+		typing.ExactArgs(1),
+		typing.WithTypes(object.ARRAY_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if a, ok := args[0].(*object.Array); ok {
+	a := args[0].(*object.Array)
 	// TODO: Make this more generic
 	xs := make([]int, len(a.Elements))
 	for n, e := range a.Elements {
@@ -24,6 +28,4 @@ func Min(args ...object.Object) object.Object {
 	}
 	sort.Ints(xs)
 	return &object.Integer{Value: int64(xs[0])}
-	}
-	return newError("argument #1 to `min` expected to be `array` got=%T", args[0].Type())
 }

builtins/oct.go

@@ -3,18 +3,20 @@ package builtins
 import (
 	"fmt"
 	"monkey/object"
+	"monkey/typing"
 	"strconv"
 )
 
 // Oct ...
 func Oct(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"oct", args,
-			len(args))
+		typing.ExactArgs(1),
+		typing.WithTypes(object.INTEGER_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if i, ok := args[0].(*object.Integer); ok {
+	i := args[0].(*object.Integer)
 	return &object.String{Value: fmt.Sprintf("0%s", strconv.FormatInt(i.Value, 8))}
-	}
-	return newError("argument to `oct` not supported, got %s", args[0].Type())
 }

builtins/ord.go

@@ -1,19 +1,26 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 // Ord ...
 func Ord(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"ord", args,
-			len(args))
+		typing.ExactArgs(1),
+		typing.WithTypes(object.STRING_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if s, ok := args[0].(*object.String); ok {
+	s := args[0].(*object.String)
 	if len(s.Value) == 1 {
 		return &object.Integer{Value: int64(s.Value[0])}
 	}
-		return newError("`ord()` expected a character but got string of length %d", len(s.Value))
+	return newError(
-	}
+		"TypeError: ord() expected a single character `str` got=%s",
-	return newError("argument to `ord` not supported, got %s", args[0].Type())
+		s.Inspect(),
+	)
 }

builtins/pop.go

@@ -1,23 +1,25 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 // Pop ...
 func Pop(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"pop", args,
-			len(args))
+		typing.ExactArgs(1),
-	}
+		typing.WithTypes(object.ARRAY_OBJ),
-	if args[0].Type() != object.ARRAY_OBJ {
+	); err != nil {
-		return newError("argument to `pop` must be array, got %s",
+		return newError(err.Error())
-			args[0].Type())
 	}
 
 	arr := args[0].(*object.Array)
 	length := len(arr.Elements)
 
 	if length == 0 {
-		return newError("cannot pop from an empty array")
+		return newError("IndexError: pop from an empty array")
 	}
 
 	element := arr.Elements[length-1]

builtins/pow.go

@@ -1,6 +1,9 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 func pow(x, y int64) int64 {
 	p := int64(1)
@@ -16,19 +19,16 @@ func pow(x, y int64) int64 {
 
 // Pow ...
 func Pow(args ...object.Object) object.Object {
-	if len(args) != 2 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=2",
+		"pow", args,
-			len(args))
+		typing.ExactArgs(2),
+		typing.WithTypes(object.INTEGER_OBJ, object.INTEGER_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if x, ok := args[0].(*object.Integer); ok {
+	x := args[0].(*object.Integer)
-		if y, ok := args[1].(*object.Integer); ok {
+	y := args[1].(*object.Integer)
 	value := pow(x.Value, y.Value)
 	return &object.Integer{Value: value}
-		} else {
-			return newError("expected argument #2 to `pow` to be `int` got=%s", args[1].Type())
-		}
-	} else {
-		return newError("expected argument #1 to `pow` to be `int` got=%s", args[0].Type())
-	}
 }

builtins/print.go

@@ -1,14 +1,23 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 import "fmt"
 
 // Print ...
 func Print(args ...object.Object) object.Object {
-	for _, arg := range args {
+	if err := typing.Check(
-		fmt.Println(arg.String())
+		"print", args,
+		typing.MinimumArgs(1),
+		typing.WithTypes(object.STRING_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
+	fmt.Println(args[0].String())
+
 	return nil
 }

builtins/push.go

@@ -1,28 +1,22 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 // Push ...
 func Push(args ...object.Object) object.Object {
-	if len(args) != 2 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=2",
+		"push", args,
-			len(args))
+		typing.ExactArgs(2),
-	}
+		typing.WithTypes(object.ARRAY_OBJ),
-	if args[0].Type() != object.ARRAY_OBJ {
+	); err != nil {
-		return newError("argument to `push` must be array, got %s",
+		return newError(err.Error())
-			args[0].Type())
 	}
 
 	arr := args[0].(*object.Array)
-	length := len(arr.Elements)
+	newArray := arr.Copy()
-
+	newArray.Append(args[1])
-	newElements := make([]object.Object, length+1)
+	return newArray
-	copy(newElements, arr.Elements)
-	if immutable, ok := args[1].(object.Immutable); ok {
-		newElements[length] = immutable.Clone()
-	} else {
-		newElements[length] = args[1]
-	}
-
-	return &object.Array{Elements: newElements}
 }

builtins/read.go

@@ -1,27 +1,25 @@
 package builtins
 
-import "monkey/object"
-
 import (
-	"os"
+	"io/ioutil"
+	"monkey/object"
+	"monkey/typing"
 )
 
 // Read ...
 func Read(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"read", args,
-			len(args))
+		typing.ExactArgs(1),
+		typing.WithTypes(object.STRING_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	arg, ok := args[0].(*object.String)
+	filename := args[0].(*object.String).Value
-	if !ok {
+	data, err := ioutil.ReadFile(filename)
-		return newError("argument to `read` expected to be `str` got=%T", args[0].Type())
-	}
-
-	filename := arg.Value
-	data, err := os.ReadFile(filename)
 	if err != nil {
-		return newError("error reading file: %s", err)
+		return newError("IOError: error reading from file %s: %s", filename, err)
 	}
 
 	return &object.String{Value: string(data)}

builtins/rest.go

@@ -1,25 +1,22 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 // Rest ...
 func Rest(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"rest", args,
-			len(args))
+		typing.ExactArgs(1),
-	}
+		typing.WithTypes(object.ARRAY_OBJ),
-	if args[0].Type() != object.ARRAY_OBJ {
+	); err != nil {
-		return newError("argument to `rest` must be array, got %s",
+		return newError(err.Error())
-			args[0].Type())
 	}
 
 	arr := args[0].(*object.Array)
-	length := len(arr.Elements)
+	newArray := arr.Copy()
-	if length > 0 {
+	newArray.PopLeft()
-		newElements := make([]object.Object, length-1, length-1)
+	return newArray
-		copy(newElements, arr.Elements[1:length])
-		return &object.Array{Elements: newElements}
-	}
-
-	return nil
 }

builtins/reversed.go

@@ -2,19 +2,21 @@ package builtins
 
 import (
 	"monkey/object"
+	"monkey/typing"
 )
 
 // Reversed ...
 func Reversed(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"reversed", args,
-			len(args))
+		typing.ExactArgs(1),
+		typing.WithTypes(object.ARRAY_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if a, ok := args[0].(*object.Array); ok {
+	arr := args[0].(*object.Array)
-		newArray := a.Copy()
+	newArray := arr.Copy()
 	newArray.Reverse()
 	return newArray
-	}
-	return newError("argument #1 to `reversed` expected to be `array` got=%T", args[0].Type())
 }

builtins/sorted.go

@@ -2,20 +2,22 @@ package builtins
 
 import (
 	"monkey/object"
+	"monkey/typing"
 	"sort"
 )
 
 // Sorted ...
 func Sorted(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"sort", args,
-			len(args))
+		typing.ExactArgs(1),
+		typing.WithTypes(object.ARRAY_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if a, ok := args[0].(*object.Array); ok {
+	arr := args[0].(*object.Array)
-		newArray := a.Copy()
+	newArray := arr.Copy()
 	sort.Sort(newArray)
 	return newArray
-	}
-	return newError("argument #1 to `sorted` expected to be `array` got=%T", args[0].Type())
 }

builtins/split.go

@@ -1,6 +1,9 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 import (
 	"strings"
@@ -8,22 +11,19 @@ import (
 
 // Split ...
 func Split(args ...object.Object) object.Object {
-	if len(args) < 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"split", args,
-			len(args))
+		typing.RangeOfArgs(1, 2),
+		typing.WithTypes(object.STRING_OBJ, object.STRING_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if obj, ok := args[0].(*object.String); ok {
 	var sep string
-
+	s := args[0].(*object.String).Value
-		s := obj.Value
 
 	if len(args) == 2 {
-			if obj, ok := args[1].(*object.String); ok {
+		sep = args[1].(*object.String).Value
-				sep = obj.Value
-			} else {
-				return newError("expected arg #2 to be `str` got=%T", args[1])
-			}
 	}
 
 	tokens := strings.Split(s, sep)
@@ -32,7 +32,4 @@ func Split(args ...object.Object) object.Object {
 		elements[i] = &object.String{Value: token}
 	}
 	return &object.Array{Elements: elements}
-	} else {
-		return newError("expected arg #1 to be `str` got got=%T", args[0])
-	}
 }

builtins/str.go

@@ -1,21 +1,18 @@
 package builtins
 
-import "monkey/object"
-
 import (
-	"fmt"
+	"monkey/object"
+	"monkey/typing"
 )
 
 // Str ...
 func Str(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1", len(args))
+		"str", args,
+		typing.ExactArgs(1),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	arg, ok := args[0].(fmt.Stringer)
+	return &object.String{Value: args[0].String()}
-	if !ok {
-		return newError("argument to `str` not supported, got %s", args[0].Type())
-	}
-
-	return &object.String{Value: arg.String()}
 }

builtins/typeof.go

@@ -1,11 +1,17 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 // TypeOf ...
 func TypeOf(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1", len(args))
+		"type", args,
+		typing.ExactArgs(1),
+	); err != nil {
+		return newError(err.Error())
 	}
 
 	return &object.String{Value: string(args[0].Type())}

builtins/upper.go

@@ -1,18 +1,20 @@
 package builtins
 
-import "monkey/object"
+import (
-
+	"monkey/object"
-import "strings"
+	"monkey/typing"
+	"strings"
+)
 
 // Upper ...
 func Upper(args ...object.Object) object.Object {
-	if len(args) != 1 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=1",
+		"upper", args,
-			len(args))
+		typing.ExactArgs(1),
+		typing.WithTypes(object.STRING_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if str, ok := args[0].(*object.String); ok {
+	return &object.String{Value: strings.ToUpper(args[0].(*object.String).Value)}
-		return &object.String{Value: strings.ToUpper(str.Value)}
-	}
-	return newError("expected `str` argument to `upper` got=%T", args[0])
 }

builtins/write.go

@@ -1,33 +1,27 @@
 package builtins
 
-import "monkey/object"
-
 import (
-	"os"
+	"io/ioutil"
+	"monkey/object"
+	"monkey/typing"
 )
 
 // Write ...
 func Write(args ...object.Object) object.Object {
-	if len(args) != 2 {
+	if err := typing.Check(
-		return newError("wrong number of arguments. got=%d, want=2",
+		"write", args,
-			len(args))
+		typing.ExactArgs(2),
+		typing.WithTypes(object.STRING_OBJ, object.STRING_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	arg, ok := args[0].(*object.String)
+	filename := args[0].(*object.String).Value
-	if !ok {
+	data := []byte(args[1].(*object.String).Value)
-		return newError("argument #1 to `write` expected to be `str` got=%T", args[0].Type())
-	}
-	filename := arg.Value
 
-	arg, ok = args[1].(*object.String)
+	err := ioutil.WriteFile(filename, data, 0755)
-	if !ok {
-		return newError("argument #2 to `write` expected to be `str` got=%T", args[1].Type())
-	}
-	data := []byte(arg.Value)
-
-	err := os.WriteFile(filename, data, 0755)
 	if err != nil {
-		return newError("error writing file: %s", err)
+		return newError("IOError: error writing file %s: %s", filename, err)
 	}
 
 	return &object.Null{}

evaluator/evaluator_test.go

@@ -398,24 +398,24 @@ func TestBuiltinFunctions(t *testing.T) {
 		{`len("")`, 0},
 		{`len("four")`, 4},
 		{`len("hello world")`, 11},
-		{`len(1)`, errors.New("argument to `len` not supported, got int")},
+		{`len(1)`, errors.New("TypeError: object of type 'int' has no len()")},
-		{`len("one", "two")`, errors.New("wrong number of arguments. got=2, want=1")},
+		{`len("one", "two")`, errors.New("TypeError: len() takes exactly 1 argument (2 given)")},
 		{`len("∑")`, 1},
 		{`len([1, 2, 3])`, 3},
 		{`len([])`, 0},
 		{`first([1, 2, 3])`, 1},
 		{`first([])`, nil},
-		{`first(1)`, errors.New("argument to `first` must be array, got int")},
+		{`first(1)`, errors.New("TypeError: first() expected argument #1 to be `array` got `int`")},
 		{`last([1, 2, 3])`, 3},
 		{`last([])`, nil},
-		{`last(1)`, errors.New("argument to `last` must be array, got int")},
+		{`last(1)`, errors.New("TypeError: last() expected argument #1 to be `array` got `int`")},
 		{`rest([1, 2, 3])`, []int{2, 3}},
 		{`rest([])`, nil},
 		{`push([], 1)`, []int{1}},
-		{`push(1, 1)`, errors.New("argument to `push` must be array, got int")},
+		{`push(1, 1)`, errors.New("TypeError: push() expected argument #1 to be `array` got `int`")},
 		{`print("Hello World")`, nil},
 		{`input()`, ""},
-		{`pop([])`, errors.New("cannot pop from an empty array")},
+		{`pop([])`, errors.New("IndexError: pop from an empty array")},
 		{`pop([1])`, 1},
 		{`bool(1)`, true},
 		{`bool(0)`, false},

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 {
+func (ao *Array) Less(i, j int) bool {
-	if cmp, ok := a.Elements[i].(Comparable); ok {
+	if cmp, ok := ao.Elements[i].(Comparable); ok {
-		return cmp.Compare(a.Elements[j]) == -1
+		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
 }

typing/typing.go

@@ -0,0 +1,56 @@
+package typing
+
+import (
+	"fmt"
+	"monkey/object"
+)
+
+type CheckFunc func(name string, args []object.Object) error
+
+func Check(name string, args []object.Object, checks ...CheckFunc) error {
+	for _, check := range checks {
+		if err := check(name, args); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func ExactArgs(n int) CheckFunc {
+	return func(name string, args []object.Object) error {
+		if len(args) != n {
+			return fmt.Errorf("TypeError: %s() takes exactly %d argument (%d given)", name, n, len(args))
+		}
+		return nil
+	}
+}
+
+func MinimumArgs(n int) CheckFunc {
+	return func(name string, args []object.Object) error {
+		if len(args) < n {
+			return fmt.Errorf("TypeError: %s() takes a minimum %d arguments (%d given)", name, n, len(args))
+		}
+		return nil
+	}
+}
+
+func RangeOfArgs(n, m int) CheckFunc {
+	return func(name string, args []object.Object) error {
+		if len(args) < n || len(args) > m {
+			return fmt.Errorf("TypeError: %s() takes at least %d arguments at most %d (%d given)", name, n, m, len(args))
+		}
+		return nil
+	}
+}
+
+func WithTypes(types ...object.ObjectType) CheckFunc {
+	return func(name string, args []object.Object) error {
+		for i, t := range types {
+			if i < len(args) && args[i].Type() != t {
+				return fmt.Errorf("TypeError: %s() expected argument #%d to be `%s` got `%s`", name, i+1, t, args[i].Type())
+			}
+		}
+		return nil
+	}
+}

vm/vm_test.go

@@ -709,12 +709,12 @@ func TestBuiltinFunctions(t *testing.T) {
 		{
 			`len(1)`,
 			&object.Error{
-				Message: "argument to `len` not supported, got int",
+				Message: "TypeError: object of type 'int' has no len()",
 			},
 		},
 		{`len("one", "two")`,
 			&object.Error{
-				Message: "wrong number of arguments. got=2, want=1",
+				Message: "TypeError: len() takes exactly 1 argument (2 given)",
 			},
 		},
 		{`len([1, 2, 3])`, 3},
@@ -724,27 +724,27 @@ func TestBuiltinFunctions(t *testing.T) {
 		{`first([])`, Null},
 		{`first(1)`,
 			&object.Error{
-				Message: "argument to `first` must be array, got int",
+				Message: "TypeError: first() expected argument #1 to be `array` got `int`",
 			},
 		},
 		{`last([1, 2, 3])`, 3},
 		{`last([])`, Null},
 		{`last(1)`,
 			&object.Error{
-				Message: "argument to `last` must be array, got int",
+				Message: "TypeError: last() expected argument #1 to be `array` got `int`",
 			},
 		},
 		{`rest([1, 2, 3])`, []int{2, 3}},
-		{`rest([])`, Null},
+		{`rest([])`, []int{}},
 		{`push([], 1)`, []int{1}},
 		{`push(1, 1)`,
 			&object.Error{
-				Message: "argument to `push` must be array, got int",
+				Message: "TypeError: push() expected argument #1 to be `array` got `int`",
 			},
 		},
 		{`input()`, ""},
 		{`pop([])`, &object.Error{
-			Message: "cannot pop from an empty array",
+			Message: "IndexError: pop from an empty array",
 		},
 		},
 		{`pop([1])`, 1},