IndexGet of Array, Index, ImmutableIndex, Bytes, String, Undefined

docs/objects.md

@@ -84,7 +84,7 @@ If `IndexGet` returns an error (`err`), the VM will treat it as a run-time error
 
 Array and Map implementation forces the type of index Object to be Int and String respectively, but, it's not a required behavior of the VM. It is completely okay to take various index types as long as it is consistent. 
 
-By convention, Array or Array-like types return `ErrIndexOutOfBounds` error (as a runtime error) when the index is invalid (out of the bounds), and, Map or Map-like types return `Undefined` value (instead of a run-time error) when the key does not exist. But, again, this is not a required behavior.
+By convention, Array or Array-like types and Map or Map-like types return `Undefined` value when the key does not exist. But, again, this is not a required behavior.
 
 ### Index-Assignable Interface
 
@@ -98,8 +98,6 @@ type IndexAssignable interface {
 
 Array and Map implementation forces the type of index Object to be Int and String respectively, but, it's not a required behavior of the VM. It is completely okay to take various index types as long as it is consistent. 
 
-By convention, Array or Array-like types return `ErrIndexOutOfBounds` error (as a runtime error) when the index is invalid (out of the bounds).
-
 ### Iterable Interface
 
 If the type implements [Iterable](https://godoc.org/github.com/d5/tengo/objects#Iterable) interface, its values can be used in `for-in` statements (`for key, value in object { ... }`).

docs/tutorial.md

@@ -68,6 +68,8 @@ _See [Runtime Types](https://github.com/d5/tengo/blob/master/docs/runtime-types.
 
 You can use the dot selector (`.`) and indexer (`[]`) operator to read or write elements of arrays, strings, or maps.
 
+Reading a nonexistent index returns `Undefined` value.
+
 ```golang
 ["one", "two", "three"][1]	// == "two"
 
@@ -80,6 +82,8 @@ m.a				// == 1
 m["b"][1]			// == 3
 m.c()				// == 10
 m.x = 5				// add 'x' to map 'm'
+m["b"][5]                       // == undefined
+m["b"][5].d                     // == undefined
 //m.b[5] = 0			// but this is an error: index out of bounds
 ```
 > [Run in Playground](https://tengolang.com/?s=d510c75ed8f06ef1e22c1aaf8a7d4565c793514c)
@@ -91,6 +95,7 @@ a := [1, 2, 3, 4, 5][1:3]	// == [2, 3]
 b := [1, 2, 3, 4, 5][3:]	// == [4, 5]
 c := [1, 2, 3, 4, 5][:3]	// == [1, 2, 3]
 d := "hello world"[2:10]	// == "llo worl"
+c := [1, 2, 3, 4, 5][-1:10]    // == [1, 2, 3, 4, 5]
 ```
 > [Run in Playground](https://tengolang.com/?s=214ab490bb24549578770984985f6b161aed915d)
 

objects/array.go

@@ -94,7 +94,7 @@ func (o *Array) IndexGet(index Object) (res Object, err error) {
 	idxVal := int(intIdx.Value)
 
 	if idxVal < 0 || idxVal >= len(o.Value) {
-		err = ErrIndexOutOfBounds
+		res = UndefinedValue
 		return
 	}
 

objects/bytes.go

@@ -66,7 +66,7 @@ func (o *Bytes) IndexGet(index Object) (res Object, err error) {
 	idxVal := int(intIdx.Value)
 
 	if idxVal < 0 || idxVal >= len(o.Value) {
-		err = ErrIndexOutOfBounds
+		res = UndefinedValue
 		return
 	}
 

objects/immautable_array.go

@@ -91,7 +91,7 @@ func (o *ImmutableArray) IndexGet(index Object) (res Object, err error) {
 	idxVal := int(intIdx.Value)
 
 	if idxVal < 0 || idxVal >= len(o.Value) {
-		err = ErrIndexOutOfBounds
+		res = UndefinedValue
 		return
 	}
 

objects/string.go

@@ -73,7 +73,7 @@ func (o *String) IndexGet(index Object) (res Object, err error) {
 	}
 
 	if idxVal < 0 || idxVal >= len(o.runeStr) {
-		err = ErrIndexOutOfBounds
+		res = UndefinedValue
 		return
 	}
 

objects/undefined.go

@@ -35,3 +35,8 @@ func (o *Undefined) IsFalsy() bool {
 func (o *Undefined) Equals(x Object) bool {
 	return o == x
 }
+
+// IndexGet returns an element at a given index.
+func (o *Undefined) IndexGet(index Object) (Object, error) {
+	return UndefinedValue, nil
+}

runtime/vm.go

@@ -631,57 +631,69 @@ func (v *VM) Run() error {
 				}
 			}
 
-			var highIdx int64
-			if *high == objects.UndefinedValue {
-				highIdx = -1
-			} else if high, ok := (*high).(*objects.Int); ok {
-				highIdx = high.Value
-			} else {
-				return fmt.Errorf("non-integer slice index: %s", high.TypeName())
-			}
-
 			switch left := (*left).(type) {
 			case *objects.Array:
 				numElements := int64(len(left.Value))
-
-				if lowIdx < 0 || lowIdx > numElements {
-					return fmt.Errorf("index out of bounds: %d", lowIdx)
-				}
-				if highIdx < 0 {
+				var highIdx int64
+				if *high == objects.UndefinedValue {
 					highIdx = numElements
-				} else if highIdx < 0 || highIdx > numElements {
-					return fmt.Errorf("index out of bounds: %d", highIdx)
+				} else if high, ok := (*high).(*objects.Int); ok {
+					highIdx = high.Value
+				} else {
+					return fmt.Errorf("non-integer slice index: %s", high.TypeName())
 				}
 
 				if lowIdx > highIdx {
 					return fmt.Errorf("invalid slice index: %d > %d", lowIdx, highIdx)
 				}
 
+				if lowIdx < 0 {
+					lowIdx = 0
+				} else if lowIdx > numElements {
+					lowIdx = numElements
+				}
+
+				if highIdx < 0 {
+					highIdx = 0
+				} else if highIdx > numElements {
+					highIdx = numElements
+				}
+
 				if v.sp >= StackSize {
 					return ErrStackOverflow
 				}
 
 				var val objects.Object = &objects.Array{Value: left.Value[lowIdx:highIdx]}
-
 				v.stack[v.sp] = &val
 				v.sp++
 
 			case *objects.ImmutableArray:
 				numElements := int64(len(left.Value))
-
-				if lowIdx < 0 || lowIdx > numElements {
-					return fmt.Errorf("index out of bounds: %d", lowIdx)
-				}
-				if highIdx < 0 {
+				var highIdx int64
+				if *high == objects.UndefinedValue {
 					highIdx = numElements
-				} else if highIdx < 0 || highIdx > numElements {
-					return fmt.Errorf("index out of bounds: %d", highIdx)
+				} else if high, ok := (*high).(*objects.Int); ok {
+					highIdx = high.Value
+				} else {
+					return fmt.Errorf("non-integer slice index: %s", high.TypeName())
 				}
 
 				if lowIdx > highIdx {
 					return fmt.Errorf("invalid slice index: %d > %d", lowIdx, highIdx)
 				}
 
+				if lowIdx < 0 {
+					lowIdx = 0
+				} else if lowIdx > numElements {
+					lowIdx = numElements
+				}
+
+				if highIdx < 0 {
+					highIdx = 0
+				} else if highIdx > numElements {
+					highIdx = numElements
+				}
+
 				if v.sp >= StackSize {
 					return ErrStackOverflow
 				}
@@ -693,20 +705,31 @@ func (v *VM) Run() error {
 
 			case *objects.String:
 				numElements := int64(len(left.Value))
-
-				if lowIdx < 0 || lowIdx > numElements {
-					return fmt.Errorf("index out of bounds: %d", lowIdx)
-				}
-				if highIdx < 0 {
+				var highIdx int64
+				if *high == objects.UndefinedValue {
 					highIdx = numElements
-				} else if highIdx < 0 || highIdx > numElements {
-					return fmt.Errorf("index out of bounds: %d", highIdx)
+				} else if high, ok := (*high).(*objects.Int); ok {
+					highIdx = high.Value
+				} else {
+					return fmt.Errorf("non-integer slice index: %s", high.TypeName())
 				}
 
 				if lowIdx > highIdx {
 					return fmt.Errorf("invalid slice index: %d > %d", lowIdx, highIdx)
 				}
 
+				if lowIdx < 0 {
+					lowIdx = 0
+				} else if lowIdx > numElements {
+					lowIdx = numElements
+				}
+
+				if highIdx < 0 {
+					highIdx = 0
+				} else if highIdx > numElements {
+					highIdx = numElements
+				}
+
 				if v.sp >= StackSize {
 					return ErrStackOverflow
 				}
@@ -718,20 +741,31 @@ func (v *VM) Run() error {
 
 			case *objects.Bytes:
 				numElements := int64(len(left.Value))
-
-				if lowIdx < 0 || lowIdx >= numElements {
-					return fmt.Errorf("index out of bounds: %d", lowIdx)
-				}
-				if highIdx < 0 {
+				var highIdx int64
+				if *high == objects.UndefinedValue {
 					highIdx = numElements
-				} else if highIdx < 0 || highIdx > numElements {
-					return fmt.Errorf("index out of bounds: %d", highIdx)
+				} else if high, ok := (*high).(*objects.Int); ok {
+					highIdx = high.Value
+				} else {
+					return fmt.Errorf("non-integer slice index: %s", high.TypeName())
 				}
 
 				if lowIdx > highIdx {
 					return fmt.Errorf("invalid slice index: %d > %d", lowIdx, highIdx)
 				}
 
+				if lowIdx < 0 {
+					lowIdx = 0
+				} else if lowIdx > numElements {
+					lowIdx = numElements
+				}
+
+				if highIdx < 0 {
+					highIdx = 0
+				} else if highIdx > numElements {
+					highIdx = numElements
+				}
+
 				if v.sp >= StackSize {
 					return ErrStackOverflow
 				}
@@ -740,9 +774,6 @@ func (v *VM) Run() error {
 
 				v.stack[v.sp] = &val
 				v.sp++
-
-			default:
-				return fmt.Errorf("cannot slice %s", left.TypeName())
 			}
 
 		case compiler.OpCall:

runtime/vm_array_test.go

@@ -3,6 +3,8 @@ package runtime_test
 import (
 	"fmt"
 	"testing"
+
+	"github.com/d5/tengo/objects"
 )
 
 func TestArray(t *testing.T) {
@@ -12,6 +14,9 @@ func TestArray(t *testing.T) {
 	expect(t, `a1 := [1, 2, 3]; a2 := a1; a1[0] = 5; out = a2`, ARR{5, 2, 3})
 	expect(t, `func () { a1 := [1, 2, 3]; a2 := a1; a1[0] = 5; out = a2 }()`, ARR{5, 2, 3})
 
+	// array index set
+	expectError(t, `a1 := [1, 2, 3]; a1[3] = 5`)
+
 	// index operator
 	arr := ARR{1, 2, 3, 4, 5, 6}
 	arrStr := `[1, 2, 3, 4, 5, 6]`
@@ -22,21 +27,29 @@ func TestArray(t *testing.T) {
 		expect(t, fmt.Sprintf("out = %s[1 + %d - 1]", arrStr, idx), arr[idx])
 		expect(t, fmt.Sprintf("idx := %d; out = %s[idx]", idx, arrStr), arr[idx])
 	}
-	expectError(t, fmt.Sprintf("%s[%d]", arrStr, -1))
-	expectError(t, fmt.Sprintf("%s[%d]", arrStr, arrLen))
+
+	expect(t, fmt.Sprintf("%s[%d]", arrStr, -1), objects.UndefinedValue)
+	expect(t, fmt.Sprintf("%s[%d]", arrStr, arrLen), objects.UndefinedValue)
 
 	// slice operator
 	for low := 0; low < arrLen; low++ {
+		expect(t, fmt.Sprintf("out = %s[%d:%d]", arrStr, low, low), ARR{})
 		for high := low; high <= arrLen; high++ {
 			expect(t, fmt.Sprintf("out = %s[%d:%d]", arrStr, low, high), arr[low:high])
 			expect(t, fmt.Sprintf("out = %s[0 + %d : 0 + %d]", arrStr, low, high), arr[low:high])
 			expect(t, fmt.Sprintf("out = %s[1 + %d - 1 : 1 + %d - 1]", arrStr, low, high), arr[low:high])
 			expect(t, fmt.Sprintf("out = %s[:%d]", arrStr, high), arr[:high])
 			expect(t, fmt.Sprintf("out = %s[%d:]", arrStr, low), arr[low:])
-			expect(t, fmt.Sprintf("out = %s[:]", arrStr), arr[:])
 		}
 	}
-	expectError(t, fmt.Sprintf("%s[%d:]", arrStr, -1))
-	expectError(t, fmt.Sprintf("%s[:%d]", arrStr, arrLen+1))
+
+	expect(t, fmt.Sprintf("out = %s[:]", arrStr), arr)
+	expect(t, fmt.Sprintf("out = %s[%d:]", arrStr, -1), arr)
+	expect(t, fmt.Sprintf("out = %s[:%d]", arrStr, arrLen+1), arr)
+	expect(t, fmt.Sprintf("out = %s[%d:%d]", arrStr, 2, 2), ARR{})
+
+	expectError(t, fmt.Sprintf("out = %s[:%d]", arrStr, -1))
+	expectError(t, fmt.Sprintf("out = %s[%d:]", arrStr, arrLen+1))
+	expectError(t, fmt.Sprintf("out = %s[%d:%d]", arrStr, 0, -1))
 	expectError(t, fmt.Sprintf("%s[%d:%d]", arrStr, 2, 1))
 }

runtime/vm_bytes_test.go

@@ -2,6 +2,8 @@ package runtime_test
 
 import (
 	"testing"
+
+	"github.com/d5/tengo/objects"
 )
 
 func TestBytes(t *testing.T) {
@@ -12,4 +14,5 @@ func TestBytes(t *testing.T) {
 	expect(t, `out = bytes("abcde")[0]`, 97)
 	expect(t, `out = bytes("abcde")[1]`, 98)
 	expect(t, `out = bytes("abcde")[4]`, 101)
+	expect(t, `out = bytes("abcde")[10]`, objects.UndefinedValue)
 }

runtime/vm_immutable_test.go

@@ -1,6 +1,10 @@
 package runtime_test
 
-import "testing"
+import (
+	"testing"
+
+	"github.com/d5/tengo/objects"
+)
 
 func TestImmutable(t *testing.T) {
 	// primitive types are already immutable values
@@ -26,6 +30,7 @@ func TestImmutable(t *testing.T) {
 	expect(t, `out = immutable([1, 2, 3, 4])[1]`, 2)
 	expect(t, `out = immutable([1, 2, 3, 4])[1:3]`, ARR{2, 3})
 	expect(t, `a := immutable([1,2,3]); a = 5; out = a`, 5)
+	expect(t, `a := immutable([1, 2, 3]); out = a[5]`, objects.UndefinedValue)
 
 	// map
 	expectError(t, `a := immutable({b: 1, c: 2}); a.b = 5`)
@@ -42,6 +47,7 @@ func TestImmutable(t *testing.T) {
 	expect(t, `out = immutable({a:1,b:2}).b`, 2)
 	expect(t, `out = immutable({a:1,b:2})["b"]`, 2)
 	expect(t, `a := immutable({a:1,b:2}); a = 5; out = 5`, 5)
+	expect(t, `a := immutable({a:1,b:2}); out = a.c`, objects.UndefinedValue)
 
 	expect(t, `a := immutable({b: 5, c: "foo"}); out = a.b`, 5)
 	expectError(t, `a := immutable({b: 5, c: "foo"}); a.b = 10`)