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Posted to github@beam.apache.org by GitBox <gi...@apache.org> on 2020/09/12 00:49:54 UTC

[GitHub] [beam] youngoli commented on a change in pull request #12588: [BEAM-7009] Add Go SDK Standard Coders yaml tests.

youngoli commented on a change in pull request #12588:
URL: https://github.com/apache/beam/pull/12588#discussion_r486767704



##########
File path: sdks/go/pkg/beam/core/runtime/exec/coder.go
##########
@@ -81,24 +82,82 @@ func MakeElementEncoder(c *coder.Coder) ElementEncoder {
 		return &stringEncoder{}
 
 	case coder.Custom:
-		return &customEncoder{
+		enc := &customEncoder{
 			t:   c.Custom.Type,
 			enc: makeEncoder(c.Custom.Enc.Fn),
 		}
+		if c.Custom.Name != "schema" {
+			return enc
+		}
+		// Custom schema coding is shorthand for using beam infrastructure
+		// wrapped in a custom coder.

Review comment:
       I don't really understand this comment, or how it relates to the check below.

##########
File path: sdks/go/pkg/beam/core/runtime/exec/coder.go
##########
@@ -117,17 +176,88 @@ func MakeElementDecoder(c *coder.Coder) ElementDecoder {
 		return &stringDecoder{}
 
 	case coder.Custom:
-		return &customDecoder{
+		dec := &customDecoder{
 			t:   c.Custom.Type,
 			dec: makeDecoder(c.Custom.Dec.Fn),
 		}
 
+		fmt.Println("getting decoder", c.Custom)

Review comment:
       Looks like another debug println.

##########
File path: sdks/go/pkg/beam/core/runtime/exec/coder.go
##########
@@ -534,26 +1038,34 @@ func (*intervalWindowEncoder) EncodeSingle(elm typex.Window, w io.Writer) error
 
 type intervalWindowDecoder struct{}
 
-func (*intervalWindowDecoder) Decode(r io.Reader) ([]typex.Window, error) {
+func (d *intervalWindowDecoder) Decode(r io.Reader) ([]typex.Window, error) {
 	// Encoding: upper bound and duration
 
 	n, err := coder.DecodeInt32(r) // #windows
 
 	ret := make([]typex.Window, n, n)
 	for i := int32(0); i < n; i++ {
-		end, err := coder.DecodeEventTime(r)
-		if err != nil {
-			return nil, err
-		}
-		duration, err := coder.DecodeVarUint64(r)
+		w, err := d.DecodeSingle(r)
 		if err != nil {
 			return nil, err
 		}
-		ret[i] = window.IntervalWindow{Start: mtime.FromMilliseconds(end.Milliseconds() - int64(duration)), End: end}
+		ret[i] = w
 	}
 	return ret, err
 }
 
+func (*intervalWindowDecoder) DecodeSingle(r io.Reader) (typex.Window, error) {
+	end, err := coder.DecodeEventTime(r)
+	if err != nil {
+		return nil, err
+	}
+	duration, err := coder.DecodeVarInt(r)

Review comment:
       What's the reason to switch from `DecodeVarUint64` to `DecodeVarInt` here?

##########
File path: sdks/go/test/regression/coders/fromyaml/fromyaml.go
##########
@@ -0,0 +1,415 @@
+// Licensed to the Apache Software Foundation (ASF) under one or more
+// contributor license agreements.  See the NOTICE file distributed with
+// this work for additional information regarding copyright ownership.
+// The ASF licenses this file to You under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance with
+// the License.  You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// fromyaml generates a resource file from the standard_coders.yaml
+// file for use in these coder regression tests.
+//
+// It expects to be run in it's test directory, or via it's go test.
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"math"
+	"reflect"
+	"runtime/debug"
+	"strconv"
+	"strings"
+
+	"github.com/apache/beam/sdks/go/pkg/beam/core/graph/mtime"
+	"github.com/apache/beam/sdks/go/pkg/beam/core/graph/window"
+	"github.com/apache/beam/sdks/go/pkg/beam/core/runtime/exec"
+	"github.com/apache/beam/sdks/go/pkg/beam/core/runtime/graphx"
+	"github.com/apache/beam/sdks/go/pkg/beam/core/util/reflectx"
+	pipepb "github.com/apache/beam/sdks/go/pkg/beam/model/pipeline_v1"
+	"github.com/google/go-cmp/cmp"
+	"golang.org/x/text/encoding/charmap"
+	yaml "gopkg.in/yaml.v2"
+)
+
+var unimplementedCoders = map[string]bool{
+	"beam:coder:param_windowed_value:v1": true,
+	"beam:coder:timer:v1":                true,
+}
+
+// Coder is a representation a serialized beam coder.
+type Coder struct {
+	Urn              string  `yaml:"urn,omitempty"`
+	Payload          string  `yaml:"payload,omitempty"`
+	Components       []Coder `yaml:"components,omitempty"`
+	NonDeterministic bool    `yaml:"non_deterministic,omitempty"`
+}
+
+type logger interface {
+	Errorf(string, ...interface{})
+	Logf(string, ...interface{})
+}
+
+// Spec is a set of conditions that a coder must pass.
+type Spec struct {
+	Coder    Coder         `yaml:"coder,omitempty"`
+	Nested   *bool         `yaml:"nested,omitempty"`
+	Examples yaml.MapSlice `yaml:"examples,omitempty"`
+	Log      logger
+
+	id       int // for generating coder ids.
+	coderPBs map[string]*pipepb.Coder
+}
+
+func (s *Spec) nextID() string {
+	ret := fmt.Sprintf("%d", s.id)
+	s.id++
+	return ret
+}
+
+func (s *Spec) testStandardCoder() (err error) {
+	if unimplementedCoders[s.Coder.Urn] {
+		log.Printf("skipping unimplemented coder urn: %v", s.Coder.Urn)
+		return nil
+	}
+	// Construct the coder proto equivalents.
+
+	// Only nested tests need to be run, since nestedness is a pre-portability
+	// concept.
+	// For legacy Java reasons, the row coder examples are all marked nested: false
+	// so we need to check that before skipping unnested tests.
+	if s.Coder.Urn != "beam:coder:row:v1" && s.Nested != nil && !*s.Nested {
+		log.Printf("skipping unnested coder spec: %v\n", s.Coder)
+		return nil
+	}
+
+	s.coderPBs = make(map[string]*pipepb.Coder)
+	id := s.parseCoder(s.Coder)
+	b := graphx.NewCoderUnmarshaller(s.coderPBs)
+	underTest, err := b.Coder(id)
+	if err != nil {
+		return fmt.Errorf("unable to create coder: %v", err)
+	}
+
+	defer func() {
+		if e := recover(); e != nil {
+			err = fmt.Errorf("panicked on coder %v || %v:\n\t%v :\n%s", underTest, s.Coder, e, debug.Stack())
+		}
+	}()
+
+	var decFails, encFails int
+	for _, eg := range s.Examples {
+
+		// Test Decoding
+		// Ideally we'd use the beam package coders, but KVs make that complicated.
+		// This can be cleaned up once a type parametered beam.KV type exists.
+		dec := exec.MakeElementDecoder(underTest)
+		encoded := eg.Key.(string)
+		var elem exec.FullValue
+
+		// What I would have expected.
+		//		r := charmap.ISO8859_1.NewDecoder().Reader(strings.NewReader(encoded))
+		recoded, err := charmap.ISO8859_1.NewEncoder().String(encoded)
+		if err != nil {
+			return err
+		}
+		r := strings.NewReader(recoded)
+		if err := dec.DecodeTo(r, &elem); err != nil {
+			return fmt.Errorf("err decoding %q: %v", encoded, err)
+		}
+		if !diff(s.Coder, &elem, eg) {
+			decFails++
+			continue
+		}
+
+		// Test Encoding
+		if s.Coder.NonDeterministic {
+			// Skip verifying nondeterministic encodings.
+			continue
+		}
+		enc := exec.MakeElementEncoder(underTest)
+		var out bytes.Buffer
+		if err := enc.Encode(&elem, &out); err != nil {
+			return err
+		}
+		if d := cmp.Diff(recoded, string(out.Bytes())); d != "" {
+			log.Printf("Encoding error: diff(-want,+got): %v\n", d)
+		}
+	}
+	if decFails+encFails > 0 {
+		return fmt.Errorf("failed to decode %v times, and encode %v times", decFails, encFails)
+	}
+
+	return nil
+}
+
+var cmpOpts = []cmp.Option{
+	cmp.Transformer("bytes2string", func(in []byte) (out string) {
+		return string(in)
+	}),
+}
+
+func diff(c Coder, elem *exec.FullValue, eg yaml.MapItem) bool {
+	var got, want interface{}
+	switch c.Urn {
+	case "beam:coder:bytes:v1":
+		got = string(elem.Elm.([]byte))
+		switch egv := eg.Value.(type) {
+		case string:
+			want = egv
+		case []byte:
+			want = string(egv)
+		}
+	case "beam:coder:varint:v1":
+		got, want = elem.Elm.(int64), int64(eg.Value.(int))
+	case "beam:coder:double:v1":
+		got = elem.Elm.(float64)
+		switch v := eg.Value.(string); v {
+		case "NaN":
+			// Do the NaN comparison here since NaN by definition != NaN.
+			if math.IsNaN(got.(float64)) {
+				want, got = 1, 1
+			} else {
+				want = math.NaN()
+			}
+		case "-Infinity":
+			want = math.Inf(-1)
+		case "Infinity":
+			want = math.Inf(1)
+		default:
+			want, _ = strconv.ParseFloat(v, 64)
+		}
+
+	case "beam:coder:kv:v1":
+		v := eg.Value.(yaml.MapSlice)
+		pass := true
+		if !diff(c.Components[0], &exec.FullValue{Elm: elem.Elm}, v[0]) {
+			pass = false
+		}
+		if !diff(c.Components[1], &exec.FullValue{Elm: elem.Elm2}, v[1]) {
+			pass = false
+		}
+		return pass
+
+	case "beam:coder:iterable:v1":
+		pass := true
+		gotrv := reflect.ValueOf(elem.Elm)
+		wantrv := reflect.ValueOf(eg.Value)
+		if gotrv.Len() != wantrv.Len() {
+			log.Printf("Lengths don't match. got %v, want %v;  %v, %v", gotrv.Len(), wantrv.Len(), gotrv, wantrv)
+			return false
+		}
+		for i := 0; i < wantrv.Len(); i++ {
+			if !diff(c.Components[0],
+				&exec.FullValue{Elm: gotrv.Index(i).Interface()},
+				yaml.MapItem{Value: wantrv.Index(i).Interface()}) {
+				pass = false
+			}
+
+		}
+		return pass
+	case "beam:coder:interval_window:v1":
+		var a, b int
+		val := eg.Value
+		if is, ok := eg.Value.([]interface{}); ok {
+			val = is[0]
+		}
+		v := val.(yaml.MapSlice)
+
+		a = v[0].Value.(int)
+		b = v[1].Value.(int)
+		end := mtime.FromMilliseconds(int64(a))
+		start := end - mtime.Time(int64(b))
+		want = window.IntervalWindow{Start: start, End: end}
+		// If this is nested in an iterable, windows won't be populated.
+		if len(elem.Windows) == 0 {
+			got = elem.Elm
+		} else {
+			got = elem.Windows[0]
+		}
+
+	case "beam:coder:global_window:v1":
+		want = window.GlobalWindow{}
+		// If this is nested in an iterable, windows won't be populated.
+		if len(elem.Windows) == 0 {
+			got = window.GlobalWindow(elem.Elm.(struct{}))
+		} else {
+			got = elem.Windows[0]
+		}
+	case "beam:coder:windowed_value:v1", "beam:coder:param_windowed_value:v1":
+		// elem contains all the information, but we need to compare the element+timestamp
+		// separately from the windows, to avoid repeated expected value parsing logic.
+		pass := true
+		vs := eg.Value.(yaml.MapSlice)
+		if !diff(c.Components[0], elem, vs[0]) {
+			pass = false
+		}
+		if d := cmp.Diff(
+			mtime.FromMilliseconds(int64(vs[1].Value.(int))),
+			elem.Timestamp, cmpOpts...); d != "" {
+
+			pass = false
+		}
+		if !diff(c.Components[1], elem, vs[3]) {
+			pass = false
+		}
+		// TODO compare pane information.
+		return pass
+	case "beam:coder:row:v1":
+		fs := eg.Value.(yaml.MapSlice)
+		var rfs []reflect.StructField
+		// There are only 2 pointer examples, but they reuse field names,
+		// so we key off the proto hash to know which example we're handling.
+		ptrEg := strings.Contains(c.Payload, "51ace21c7393")
+		for _, rf := range fs {
+			name := rf.Key.(string)
+			t := nameToType[name]
+			if ptrEg {
+				t = reflect.PtrTo(t)
+			}
+			rfs = append(rfs, reflect.StructField{
+				Name: strings.ToUpper(name[:1]) + name[1:],
+				Type: t,
+				Tag:  reflect.StructTag(fmt.Sprintf("beam:\"%v\"", name)),
+			})
+		}
+		rv := reflect.New(reflect.StructOf(rfs)).Elem()
+		for i, rf := range fs {
+			setField(rv, i, rf.Value)
+		}
+
+		got, want = elem.Elm, rv.Interface()
+	default:
+		got, want = elem.Elm, eg.Value
+	}
+	if d := cmp.Diff(want, got, cmpOpts...); d != "" {
+		log.Printf("Decoding error: diff(-want,+got): %v\n", d)
+		return false
+	}
+	return true
+}
+
+// standard_coders.yaml uses the name for type indication, except for nullability.
+var nameToType = map[string]reflect.Type{
+	"str":     reflectx.String,
+	"i32":     reflectx.Int32,
+	"f64":     reflectx.Float64,
+	"arr":     reflect.SliceOf(reflectx.String),
+	"f_bool":  reflectx.Bool,
+	"f_bytes": reflect.PtrTo(reflectx.ByteSlice),
+	"f_map":   reflect.MapOf(reflectx.String, reflect.PtrTo(reflectx.Int64)),
+}
+
+func setField(rv reflect.Value, i int, v interface{}) {
+	if v == nil {
+		return
+	}
+	rf := rv.Field(i)
+	if rf.Kind() == reflect.Ptr {
+		// Ensure it's initialized.
+		rf.Set(reflect.New(rf.Type().Elem()))
+		rf = rf.Elem()
+	}
+	switch rf.Kind() {
+	case reflect.String:
+		rf.SetString(v.(string))
+	case reflect.Int32:
+		rf.SetInt(int64(v.(int)))
+	case reflect.Float64:
+		c, err := strconv.ParseFloat(v.(string), 64)
+		if err != nil {
+			panic(err)
+		}
+		rf.SetFloat(c)
+	case reflect.Slice:
+		if rf.Type() == reflectx.ByteSlice {
+			rf.Set(reflect.ValueOf([]byte(v.(string))))
+			break
+		}
+		// Value is a []interface{} with string values.

Review comment:
       Is this assumption made specifically based on the yaml file this is meant to be used with? Or is this based on something in the schema spec I'm forgetting?

##########
File path: sdks/go/pkg/beam/core/runtime/exec/coder.go
##########
@@ -117,17 +176,88 @@ func MakeElementDecoder(c *coder.Coder) ElementDecoder {
 		return &stringDecoder{}
 
 	case coder.Custom:
-		return &customDecoder{
+		dec := &customDecoder{
 			t:   c.Custom.Type,
 			dec: makeDecoder(c.Custom.Dec.Fn),
 		}
 
+		fmt.Println("getting decoder", c.Custom)
+		if c.Custom.Name != "schema" {
+			return dec
+		}
+		// Custom schema coding is shorthand for using beam infrastructure
+		// wrapped in a custom coder.
+		switch c.T.Type().Kind() {
+		case reflect.Slice:
+			return &lpDecoder{
+				dec: &iterableDecoder{
+					t:   c.Custom.Type,
+					dec: dec,
+				},
+			}
+		case reflect.Array:
+			return &lpDecoder{
+				dec: &arrayDecoder{

Review comment:
       Just to confirm, using an `arrayDecoder` without an `arrayEncoder` specifically works because the format that `iterableEncoder` encodes to is also decodable as an array, right? That might be worth mentioning as a comment, because I was wondering why you can decode arrays with an `arrayDecoder` if they were encoded with an `iterableEncoder`.

##########
File path: sdks/go/pkg/beam/core/runtime/graphx/coder.go
##########
@@ -249,31 +256,35 @@ func (b *CoderUnmarshaller) makeCoder(c *pipepb.Coder) (*coder.Coder, error) {
 			return nil, err
 		}
 
-		// No payload means this coder was length prefixed by the runner
-		// but is likely self describing - AKA a beam coder.
-		if len(sub.GetSpec().GetPayload()) == 0 {
-			return b.makeCoder(sub)
-		}
 		// TODO(lostluck) 2018/10/17: Make this strict again, once dataflow can use
 		// the portable pipeline model directly (BEAM-2885)
-		if sub.GetSpec().GetUrn() != "" && sub.GetSpec().GetUrn() != urnCustomCoder {
-			// TODO(herohde) 11/17/2017: revisit this restriction
-			return nil, errors.Errorf("could not unmarshal length prefix coder from %v, want a custom coder as a sub component but got %v", c, sub)
-		}
-
-		var ref v1pb.CustomCoder
-		if err := protox.DecodeBase64(string(sub.GetSpec().GetPayload()), &ref); err != nil {
-			return nil, err
-		}
-		custom, err := decodeCustomCoder(&ref)
-		if err != nil {
-			return nil, err
+		switch u := sub.GetSpec().GetUrn(); u {
+		case "", urnCustomCoder:
+			var ref v1pb.CustomCoder
+			if err := protox.DecodeBase64(string(sub.GetSpec().GetPayload()), &ref); err != nil {
+				return nil, err
+			}
+			custom, err := decodeCustomCoder(&ref)
+			if err != nil {
+				return nil, err
+			}
+			custom.ID = components[0]
+			t := typex.New(custom.Type)
+			cc := &coder.Coder{Kind: coder.Custom, T: t, Custom: custom}
+			fmt.Println("decoded customcoder", cc)

Review comment:
       Looks like a debug Println left in.

##########
File path: sdks/go/pkg/beam/core/runtime/exec/coder.go
##########
@@ -434,6 +600,267 @@ func convertIfNeeded(v interface{}, allocated *FullValue) *FullValue {
 	return allocated
 }
 
+type iterableEncoder struct {
+	t   reflect.Type
+	enc ElementEncoder
+}
+
+func (c *iterableEncoder) Encode(val *FullValue, w io.Writer) error {
+	// Do a reflect, get the length.
+	rv := reflect.ValueOf(val.Elm)
+	size := rv.Len()
+	if err := coder.EncodeInt32((int32)(size), w); err != nil {
+		return err
+	}
+	var e FullValue
+	for i := 0; i < size; i++ {
+		e.Elm = rv.Index(i).Interface()
+		err := c.enc.Encode(&e, w)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+type iterableDecoder struct {
+	t   reflect.Type
+	dec ElementDecoder
+}
+
+func (c *iterableDecoder) DecodeTo(r io.Reader, fv *FullValue) error {
+	// (1) Read count prefixed encoded data
+
+	size, err := coder.DecodeInt32(r)
+	if err != nil {
+		return err
+	}
+	n := int(size)
+	switch {
+	case n >= 0:
+		rv, err := c.decodeToSlice(int(n), r)

Review comment:
       Is it necessary to cast n to an int here? Isn't it already an int from line 638?

##########
File path: sdks/go/pkg/beam/core/runtime/exec/coder.go
##########
@@ -434,6 +600,267 @@ func convertIfNeeded(v interface{}, allocated *FullValue) *FullValue {
 	return allocated
 }
 
+type iterableEncoder struct {
+	t   reflect.Type
+	enc ElementEncoder
+}
+
+func (c *iterableEncoder) Encode(val *FullValue, w io.Writer) error {
+	// Do a reflect, get the length.
+	rv := reflect.ValueOf(val.Elm)
+	size := rv.Len()
+	if err := coder.EncodeInt32((int32)(size), w); err != nil {
+		return err
+	}
+	var e FullValue
+	for i := 0; i < size; i++ {
+		e.Elm = rv.Index(i).Interface()
+		err := c.enc.Encode(&e, w)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+type iterableDecoder struct {
+	t   reflect.Type
+	dec ElementDecoder
+}
+
+func (c *iterableDecoder) DecodeTo(r io.Reader, fv *FullValue) error {
+	// (1) Read count prefixed encoded data
+
+	size, err := coder.DecodeInt32(r)
+	if err != nil {
+		return err
+	}
+	n := int(size)
+	switch {
+	case n >= 0:
+		rv, err := c.decodeToSlice(int(n), r)
+		if err != nil {
+			return err
+		}
+		*fv = FullValue{Elm: rv.Interface()}
+		return nil
+	case n == -1:
+		rv := reflect.MakeSlice(c.t, 0, 0)
+		chunk, err := coder.DecodeVarInt(r)
+		if err != nil {
+			return err
+		}
+		for chunk != 0 {
+			rvi, err := c.decodeToSlice(int(chunk), r)
+			if err != nil {
+				return err
+			}
+			rv = reflect.AppendSlice(rv, rvi)
+			chunk, err = coder.DecodeVarInt(r)
+			if err != nil {
+				return err
+			}
+		}
+		*fv = FullValue{Elm: rv.Interface()}
+	}
+
+	return nil
+}
+
+func (c *iterableDecoder) decodeToSlice(n int, r io.Reader) (reflect.Value, error) {
+	var e FullValue
+	rv := reflect.MakeSlice(c.t, n, n)
+	for i := 0; i < int(n); i++ {
+		err := c.dec.DecodeTo(r, &e)
+		if err != nil {
+			return reflect.Value{}, err
+		}
+		if e.Elm != nil {
+			rv.Index(i).Set(reflect.ValueOf(e.Elm))
+		} else {
+			rv.Index(i).Set(reflect.ValueOf(e.Windows[0]))

Review comment:
       I don't get why it's setting the value of the element to `e.Windows[0]`. Is it a way to pass in nil?

##########
File path: sdks/go/pkg/beam/core/runtime/exec/coder.go
##########
@@ -434,6 +600,267 @@ func convertIfNeeded(v interface{}, allocated *FullValue) *FullValue {
 	return allocated
 }
 
+type iterableEncoder struct {
+	t   reflect.Type
+	enc ElementEncoder
+}
+
+func (c *iterableEncoder) Encode(val *FullValue, w io.Writer) error {
+	// Do a reflect, get the length.
+	rv := reflect.ValueOf(val.Elm)
+	size := rv.Len()
+	if err := coder.EncodeInt32((int32)(size), w); err != nil {
+		return err
+	}
+	var e FullValue
+	for i := 0; i < size; i++ {
+		e.Elm = rv.Index(i).Interface()
+		err := c.enc.Encode(&e, w)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+type iterableDecoder struct {
+	t   reflect.Type
+	dec ElementDecoder
+}
+
+func (c *iterableDecoder) DecodeTo(r io.Reader, fv *FullValue) error {
+	// (1) Read count prefixed encoded data
+
+	size, err := coder.DecodeInt32(r)
+	if err != nil {
+		return err
+	}
+	n := int(size)
+	switch {
+	case n >= 0:
+		rv, err := c.decodeToSlice(int(n), r)
+		if err != nil {
+			return err
+		}
+		*fv = FullValue{Elm: rv.Interface()}
+		return nil
+	case n == -1:
+		rv := reflect.MakeSlice(c.t, 0, 0)
+		chunk, err := coder.DecodeVarInt(r)
+		if err != nil {
+			return err
+		}
+		for chunk != 0 {
+			rvi, err := c.decodeToSlice(int(chunk), r)
+			if err != nil {
+				return err
+			}
+			rv = reflect.AppendSlice(rv, rvi)
+			chunk, err = coder.DecodeVarInt(r)
+			if err != nil {
+				return err
+			}
+		}
+		*fv = FullValue{Elm: rv.Interface()}
+	}
+
+	return nil
+}
+
+func (c *iterableDecoder) decodeToSlice(n int, r io.Reader) (reflect.Value, error) {
+	var e FullValue
+	rv := reflect.MakeSlice(c.t, n, n)
+	for i := 0; i < int(n); i++ {

Review comment:
       Likewise, is it necessary to cast n here too?




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