Big refactor to separate operation-traversal from code-generation (#51)

## Summary:
I've felt for a while that types.go is way too confusing, and as I
started to implement some of the more complex cases of generating
interface-types, the cracks were really starting to show.  Luckily, I
also finally realized how to fix it: we need to separate the process of
traversing the GraphQL operation and schema to decide what types to
generate from the process of actually generating those types.  This
requires an extra set of intermediate data structures, but I think it
makes things quite a lot easier to understand -- and, importantly, it
means that the code-generation doesn't need to go in the order we
traverse the query/schema.

So in this commit, I did that huge refactor.  It's probably best to just
review types.go and traverse.go as if they were new; the old code was
quite hard to understand and the new code will hopefully make a lot more
sense.  (And to that end, review comments about what could be organized
better or needs more documentation are very much in order, even for code
that is mostly unchanged.)

This does introduce one bug, sort of, which is that rather than
generating broken code for list-of-interface fields, we generate no code
at all.  (A TODO in unmarshal.go describes why.)  I'll fix this when I
add support for those fields.  (It's all behind the AllowBrokenFeatures
flag, anyway.)  Otherwise, the only changes to generated code are that a
few methods are ordered differently, because we now generate the
implements-interface methods with the interface, rather than the
implementations, as it's much simpler that way.  (In GraphQL, unlike Go,
we know the list of all possible implementations of each interface, so
this is possible.)

## Test plan:
golangci-lint run ./... && go test ./...

Author: benjaminjkraft

Reviewers: dnerdy, benjaminjkraft, aberkan, csilvers, MiguelCastillo

Required Reviewers: 

Approved by: dnerdy

Checks: ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Test (1.13), ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Test (1.13), ✅ Lint

Pull request URL: https://github.com/Khan/genqlient/pull/51

generate/convert.go

@@ -0,0 +1,356 @@
+package generate
+
+// This file implements the core type-generation logic of genqlient, whereby we
+// traverse an operation-definition (and the schema against which it will be
+// executed), and convert that into Go types.  It returns data structures
+// representing the types to be generated; these are defined, and converted
+// into code, in types.go.
+//
+// The entrypoints are convertOperation, which builds the response-type for a
+// query, and convertInputType, which builds the argument-types.
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/vektah/gqlparser/v2/ast"
+)
+
+// baseTypeForOperation returns the definition of the GraphQL type to which the
+// root of the operation corresponds, e.g. the "Query" or "Mutation" type.
+func (g *generator) baseTypeForOperation(operation ast.Operation) (*ast.Definition, error) {
+	switch operation {
+	case ast.Query:
+		return g.schema.Query, nil
+	case ast.Mutation:
+		return g.schema.Mutation, nil
+	case ast.Subscription:
+		if !allowBrokenFeatures {
+			return nil, errorf(nil, "genqlient does not yet support subscriptions")
+		}
+		return g.schema.Subscription, nil
+	default:
+		return nil, errorf(nil, "unexpected operation: %v", operation)
+	}
+}
+
+// convertOperation builds the response-type into which the given operation's
+// result will be unmarshaled.
+func (g *generator) convertOperation(
+	operation *ast.OperationDefinition,
+	queryOptions *GenqlientDirective,
+) (goType, error) {
+	name := operation.Name + "Response"
+
+	if def, ok := g.typeMap[name]; ok {
+		return nil, errorf(operation.Position, "%s defined twice:\n%s", name, def)
+	}
+
+	baseType, err := g.baseTypeForOperation(operation.Operation)
+	if err != nil {
+		return nil, errorf(operation.Position, "%v", err)
+	}
+
+	goTyp, err := g.convertDefinition(
+		name, operation.Name, baseType, operation.Position,
+		operation.SelectionSet, queryOptions)
+
+	if structType, ok := goTyp.(*goStructType); ok {
+		// Override the ordinary description; the GraphQL documentation for
+		// Query/Mutation is unlikely to be of value.
+		// TODO(benkraft): This is a bit awkward/fragile.
+		structType.Description =
+			fmt.Sprintf("%v is returned by %v on success.", name, operation.Name)
+		structType.Incomplete = false
+	}
+
+	return goTyp, err
+}
+
+var builtinTypes = map[string]string{
+	// GraphQL guarantees int32 is enough, but using int seems more idiomatic
+	"Int":     "int",
+	"Float":   "float64",
+	"String":  "string",
+	"Boolean": "bool",
+	"ID":      "string",
+}
+
+// typeName computes the name, in Go, that we should use for the given
+// GraphQL type definition.  This is dependent on its location within the query
+// (see DESIGN.md for more on why we generate type-names this way), which is
+// determined by the prefix argument; the nextPrefix result should be passed to
+// calls to typeName on any child types.
+func (g *generator) typeName(prefix string, typ *ast.Definition) (name, nextPrefix string) {
+	typeGoName := upperFirst(typ.Name)
+	if typ.Kind == ast.Enum || typ.Kind == ast.InputObject {
+		// If we're an enum or an input-object, there is only one type we
+		// will ever possibly generate for this type, so we don't need any
+		// of the qualifiers.  This is especially helpful because the
+		// caller is very likely to need to reference these types in their
+		// code.
+		return typeGoName, typeGoName
+	}
+
+	name = prefix
+	if !strings.HasSuffix(prefix, typeGoName) {
+		// If the field and type names are the same, we can avoid the
+		// duplication.  (We include the field name in case there are
+		// multiple fields with the same type, and the type name because
+		// that's the actual name (the rest are really qualifiers); but if
+		// they are the same then including it once suffices for both
+		// purposes.)
+		name += typeGoName
+	}
+
+	if typ.Kind == ast.Interface || typ.Kind == ast.Union {
+		// for interface/union types, we do not add the type name to the
+		// name prefix; we want to have QueryFieldType rather than
+		// QueryFieldInterfaceType.  So we just use the input prefix.
+		return name, prefix
+	}
+
+	// Otherwise, the name will also be the prefix for the next type.
+	return name, name
+}
+
+// convertInputType decides the Go type we will generate corresponding to an
+// argument to a GraphQL operation.
+func (g *generator) convertInputType(
+	opName string,
+	typ *ast.Type,
+	options, queryOptions *GenqlientDirective,
+) (goType, error) {
+	// Sort of a hack: case the input type name to match the op-name.
+	// TODO(benkraft): this is another thing that breaks the assumption that we
+	// only need one of an input type, albeit in a relatively safe way.
+	name := matchFirst(typ.Name(), opName)
+	// note prefix is ignored here (see generator.typeName), as is selectionSet
+	// (for input types we use the whole thing)).
+	return g.convertType(name, "", typ, nil, options, queryOptions)
+}
+
+// convertType decides the Go type we will generate corresponding to a
+// particular GraphQL type.  In this context, "type" represents the type of a
+// field, and may be a list or a reference to a named type, with or without the
+// "non-null" annotation.
+func (g *generator) convertType(
+	name, namePrefix string,
+	typ *ast.Type,
+	selectionSet ast.SelectionSet,
+	options, queryOptions *GenqlientDirective,
+) (goType, error) {
+	if typ.Elem != nil {
+		// Type is a list.
+		elem, err := g.convertType(
+			name, namePrefix, typ.Elem, selectionSet, options, queryOptions)
+		return &goSliceType{elem}, err
+	}
+
+	// If this is a builtin type or custom scalar, just refer to it.
+	def := g.schema.Types[typ.Name()]
+	goTyp, err := g.convertDefinition(
+		name, namePrefix, def, typ.Position, selectionSet, queryOptions)
+
+	if options.GetPointer() {
+		// Whatever we get, wrap it in a pointer.  (Because of the way the
+		// options work, recursing here isn't as connvenient.)
+		// Note this does []*T or [][]*T, not e.g. *[][]T.  See #16.
+		goTyp = &goPointerType{goTyp}
+	}
+	return goTyp, err
+}
+
+// convertDefinition decides the Go type we will generate corresponding to a
+// particular GraphQL named type.
+//
+// In this context, "definition" (and "named type") refer to an
+// *ast.Definition, which represents the definition of a type in the GraphQL
+// schema, which may be referenced by a field-type (see convertType).
+func (g *generator) convertDefinition(
+	name, namePrefix string,
+	def *ast.Definition,
+	pos *ast.Position,
+	selectionSet ast.SelectionSet,
+	queryOptions *GenqlientDirective,
+) (goType, error) {
+	qualifiedGoName, ok := g.Config.Scalars[def.Name]
+	if ok {
+		goRef, err := g.addRef(qualifiedGoName)
+		return &goOpaqueType{goRef}, err
+	}
+	goBuiltinName, ok := builtinTypes[def.Name]
+	if ok {
+		return &goOpaqueType{goBuiltinName}, nil
+	}
+
+	switch def.Kind {
+	case ast.Object:
+		goType := &goStructType{
+			GoName:      name,
+			Description: def.Description,
+			GraphQLName: def.Name,
+			Fields:      make([]*goStructField, len(selectionSet)),
+			Incomplete:  true,
+		}
+		g.typeMap[name] = goType
+
+		for i, selection := range selectionSet {
+			_, selectionDirective, err := g.parsePrecedingComment(
+				selection, selection.GetPosition())
+			if err != nil {
+				return nil, err
+			}
+			selectionOptions := queryOptions.merge(selectionDirective)
+
+			switch selection := selection.(type) {
+			case *ast.Field:
+				goType.Fields[i], err = g.convertField(
+					namePrefix, selection, selectionOptions, queryOptions)
+				if err != nil {
+					return nil, err
+				}
+			case *ast.FragmentSpread:
+				return nil, errorf(selection.Position, "not implemented: %T", selection)
+			case *ast.InlineFragment:
+				return nil, errorf(selection.Position, "not implemented: %T", selection)
+			default:
+				return nil, errorf(nil, "invalid selection type: %T", selection)
+			}
+		}
+		return goType, nil
+
+	case ast.InputObject:
+		goType := &goStructType{
+			GoName:      name,
+			Description: def.Description,
+			GraphQLName: def.Name,
+			Fields:      make([]*goStructField, len(def.Fields)),
+		}
+		g.typeMap[name] = goType
+
+		for i, field := range def.Fields {
+			goName := upperFirst(field.Name)
+			// Several of the arguments don't really make sense here:
+			// - no field-specific options can apply, because this is
+			//   a field in the type, not in the query (see also #14).
+			// - namePrefix is ignored for input types; see note in
+			//   generator.typeName.
+			// TODO(benkraft): Can we refactor to avoid passing the values that
+			// will be ignored?  We know field.Type is a scalar, enum, or input
+			// type.  But plumbing that is a bit tricky in practice.
+			fieldGoType, err := g.convertType(
+				field.Type.Name(), "", field.Type, nil, queryOptions, queryOptions)
+			if err != nil {
+				return nil, err
+			}
+
+			goType.Fields[i] = &goStructField{
+				GoName:      goName,
+				GoType:      fieldGoType,
+				JSONName:    field.Name,
+				Description: field.Description,
+			}
+		}
+		return goType, nil
+
+	case ast.Interface, ast.Union:
+		if !allowBrokenFeatures {
+			return nil, errorf(pos, "not implemented: %v", def.Kind)
+		}
+
+		implementationTypes := g.schema.GetPossibleTypes(def)
+		goType := &goInterfaceType{
+			GoName:          name,
+			Description:     def.Description,
+			GraphQLName:     def.Name,
+			Implementations: make([]*goStructType, len(implementationTypes)),
+		}
+		g.typeMap[name] = goType
+
+		for i, implDef := range implementationTypes {
+			implName, implNamePrefix := g.typeName(namePrefix, implDef)
+			implTyp, err := g.convertDefinition(
+				implName, implNamePrefix, implDef, pos, selectionSet, queryOptions)
+			if err != nil {
+				return nil, err
+			}
+
+			implStructTyp, ok := implTyp.(*goStructType)
+			if !ok { // (should never happen on a valid schema)
+				return nil, errorf(
+					pos, "interface %s had non-object implementation %s",
+					def.Name, implDef.Name)
+			}
+			goType.Implementations[i] = implStructTyp
+		}
+		return goType, nil
+
+	case ast.Enum:
+		goType := &goEnumType{
+			GoName:      name,
+			Description: def.Description,
+			Values:      make([]goEnumValue, len(def.EnumValues)),
+		}
+		g.typeMap[name] = goType
+
+		for i, val := range def.EnumValues {
+			goType.Values[i] = goEnumValue{Name: val.Name, Description: val.Description}
+		}
+		return goType, nil
+
+	case ast.Scalar:
+		return nil, errorf(
+			pos, "unknown scalar %v: please add it to genqlient.yaml", def.Name)
+	default:
+		return nil, errorf(pos, "unexpected kind: %v", def.Kind)
+	}
+}
+
+// convertField converts a single GraphQL operation-field into a GraphQL type.
+//
+// Note that input-type fields are handled separately (inline in
+// convertDefinition), because they come from the type-definition, not the
+// operation.
+func (g *generator) convertField(
+	namePrefix string,
+	field *ast.Field,
+	fieldOptions, queryOptions *GenqlientDirective,
+) (*goStructField, error) {
+	if field.Definition == nil {
+		// Unclear why gqlparser hasn't already rejected this,
+		// but empirically it might not.
+		return nil, errorf(
+			field.Position, "undefined field %v", field.Alias)
+	}
+
+	// Needs to be exported for JSON-marshaling
+	goName := upperFirst(field.Alias)
+
+	typ := field.Definition.Type
+	fieldTypedef := g.schema.Types[typ.Name()]
+
+	// Note we don't deduplicate suffixes here -- if our prefix is GetUser and
+	// the field name is User, we do GetUserUser.  This is important because if
+	// you have a field called user on a type called User we need
+	// `query q { user { user { id } } }` to generate two types, QUser and
+	// QUserUser.  Note also this is named based on the GraphQL alias (Go
+	// name), not the field-name, because if we have
+	// `query q { a: f { b }, c: f { d } }` we need separate types for a and c,
+	// even though they are the same type in GraphQL, because they have
+	// different fields.
+	name, namePrefix := g.typeName(namePrefix+goName, fieldTypedef)
+	fieldGoType, err := g.convertType(
+		name, namePrefix, typ, field.SelectionSet,
+		fieldOptions, queryOptions)
+	if err != nil {
+		return nil, err
+	}
+
+	return &goStructField{
+		GoName:      goName,
+		GoType:      fieldGoType,
+		JSONName:    field.Alias,
+		Description: field.Definition.Description,
+	}, nil
+}

generate/generate.go

@@ -1,5 +1,9 @@
 package generate
 
+// This file implements the main entrypoint and framework for the genqlient
+// code-generation process.  See comments in Generate for the high-level
+// overview.
+
 import (
 	"bytes"
 	"encoding/json"
@@ -24,7 +28,7 @@ type generator struct {
 	// The list of operations for which to generate code.
 	Operations []operation
 	// The types needed for these operations.
-	typeMap map[string]string
+	typeMap map[string]goType
 	// Imports needed for these operations, path -> alias and alias -> true
 	imports     map[string]string
 	usedAliases map[string]bool
@@ -67,7 +71,7 @@ type argument struct {
 func newGenerator(config *Config, schema *ast.Schema) *generator {
 	g := generator{
 		Config:        config,
-		typeMap:       map[string]string{},
+		typeMap:       map[string]goType{},
 		imports:       map[string]string{},
 		usedAliases:   map[string]bool{},
 		templateCache: map[string]*template.Template{},
@@ -90,7 +94,7 @@ func newGenerator(config *Config, schema *ast.Schema) *generator {
 	return &g
 }
 
-func (g *generator) Types() string {
+func (g *generator) Types() (string, error) {
 	names := make([]string, 0, len(g.typeMap))
 	for name := range g.typeMap {
 		names = append(names, name)
@@ -102,10 +106,16 @@ func (g *generator) Types() string {
 	sort.Strings(names)
 
 	defs := make([]string, 0, len(g.typeMap))
+	var builder strings.Builder
 	for _, name := range names {
-		defs = append(defs, g.typeMap[name])
+		builder.Reset()
+		err := g.typeMap[name].WriteDefinition(&builder, g)
+		if err != nil {
+			return "", err
+		}
+		defs = append(defs, builder.String())
 	}
-	return strings.Join(defs, "\n\n")
+	return strings.Join(defs, "\n\n"), nil
 }
 
 func (g *generator) getArgument(
@@ -119,7 +129,7 @@ func (g *generator) getArgument(
 	}
 
 	graphQLName := arg.Variable
-	goType, err := g.getTypeForInputType(
+	goTyp, err := g.convertInputType(
 		opName, arg.Type, directive, operationDirective)
 	if err != nil {
 		return argument{}, err
@@ -127,7 +137,7 @@ func (g *generator) getArgument(
 	return argument{
 		GraphQLName: graphQLName,
 		GoName:      lowerFirst(graphQLName),
-		GoType:      goType,
+		GoType:      goTyp.Reference(),
 		IsSlice:     arg.Type.Elem != nil,
 		Options:     operationDirective.merge(directive),
 	}, nil
@@ -158,7 +168,7 @@ func (g *generator) addOperation(op *ast.OperationDefinition) error {
 		}
 	}
 
-	responseName, err := g.getTypeForOperation(op, directive)
+	responseType, err := g.convertOperation(op, directive)
 	if err != nil {
 		return err
 	}
@@ -185,7 +195,7 @@ func (g *generator) addOperation(op *ast.OperationDefinition) error {
 		// The newline just makes it format a little nicer.
 		Body:           "\n" + builder.String(),
 		Args:           args,
-		ResponseName:   responseName,
+		ResponseName:   responseType.Reference(),
 		SourceFilename: sourceFilename,
 	})
 
@@ -193,7 +203,14 @@ func (g *generator) addOperation(op *ast.OperationDefinition) error {
 }
 
 // Generate returns a map from absolute-path filename to generated content.
+//
+// This is the main entrypoint to the code-generation process for callers who
+// wish to manage the config-reading (ReadAndValidateConfig) and file-writing
+// on their own.  (Those are wired in by Main.)
 func Generate(config *Config) (map[string][]byte, error) {
+	// Step 1: Read in the schema and operations from the files defined by the
+	// config (and validate the operations against the schema).  This is all
+	// defined in parse.go.
 	schema, err := getSchema(config.Schema)
 	if err != nil {
 		return nil, err
@@ -204,9 +221,9 @@ func Generate(config *Config) (map[string][]byte, error) {
 		return nil, err
 	}
 
-	// TODO: we could also allow this, and generate an empty file with just the
-	// package-name, if it turns out to be more convenient that way.  (As-is,
-	// we generate a broken file, with just (unused) imports.)
+	// TODO(benkraft): we could also allow this, and generate an empty file
+	// with just the package-name, if it turns out to be more convenient that
+	// way.  (As-is, we generate a broken file, with just (unused) imports.)
 	if len(document.Operations) == 0 {
 		// Hard to have a position when there are no operations :(
 		return nil, errorf(nil, "no queries found, looked in: %v",
@@ -218,6 +235,9 @@ func Generate(config *Config) (map[string][]byte, error) {
 			"genqlient does not yet support fragments")
 	}
 
+	// Step 2: For each operation, convert it into data structures representing
+	// Go types (defined in types.go).  The bulk of this logic is in
+	// convert.go.
 	g := newGenerator(config, schema)
 	for _, op := range document.Operations {
 		if err = g.addOperation(op); err != nil {
@@ -225,6 +245,9 @@ func Generate(config *Config) (map[string][]byte, error) {
 		}
 	}
 
+	// Step 3: Glue it all together!  Most of this is done inline in the
+	// template, but the call to g.Types() in the template calls out to
+	// types.go to actually generate the code for each type.
 	var buf bytes.Buffer
 	err = g.execute("operation.go.tmpl", &buf, g)
 	if err != nil {

generate/testdata/snapshots/TestGenerate-InterfaceListField.graphql-InterfaceListField.graphql.go

@@ -0,0 +1,87 @@
+package test
+
+// Code generated by github.com/Khan/genqlient, DO NOT EDIT.
+
+import (
+	"github.com/Khan/genqlient/graphql"
+	"github.com/Khan/genqlient/internal/testutil"
+)
+
+// InterfaceNoFragmentsQueryResponse is returned by InterfaceNoFragmentsQuery on success.
+type InterfaceNoFragmentsQueryResponse struct {
+	Root InterfaceNoFragmentsQueryRootTopic `json:"root"`
+}
+
+// InterfaceNoFragmentsQueryRootTopic includes the requested fields of the GraphQL type Topic.
+type InterfaceNoFragmentsQueryRootTopic struct {
+	// ID is documented in the Content interface.
+	Id       testutil.ID                                         `json:"id"`
+	Name     string                                              `json:"name"`
+	Children []InterfaceNoFragmentsQueryRootTopicChildrenContent `json:"children"`
+}
+
+// InterfaceNoFragmentsQueryRootTopicChildrenArticle includes the requested fields of the GraphQL type Article.
+type InterfaceNoFragmentsQueryRootTopicChildrenArticle struct {
+	Typename string `json:"__typename"`
+	// ID is the identifier of the content.
+	Id   testutil.ID `json:"id"`
+	Name string      `json:"name"`
+}
+
+// InterfaceNoFragmentsQueryRootTopicChildrenContent includes the requested fields of the GraphQL type Content.
+// The GraphQL type's documentation follows.
+//
+// Content is implemented by various types like Article, Video, and Topic.
+type InterfaceNoFragmentsQueryRootTopicChildrenContent interface {
+	implementsGraphQLInterfaceInterfaceNoFragmentsQueryRootTopicChildrenContent()
+}
+
+func (v *InterfaceNoFragmentsQueryRootTopicChildrenArticle) implementsGraphQLInterfaceInterfaceNoFragmentsQueryRootTopicChildrenContent() {
+}
+func (v *InterfaceNoFragmentsQueryRootTopicChildrenVideo) implementsGraphQLInterfaceInterfaceNoFragmentsQueryRootTopicChildrenContent() {
+}
+func (v *InterfaceNoFragmentsQueryRootTopicChildrenTopic) implementsGraphQLInterfaceInterfaceNoFragmentsQueryRootTopicChildrenContent() {
+}
+
+// InterfaceNoFragmentsQueryRootTopicChildrenTopic includes the requested fields of the GraphQL type Topic.
+type InterfaceNoFragmentsQueryRootTopicChildrenTopic struct {
+	Typename string `json:"__typename"`
+	// ID is the identifier of the content.
+	Id   testutil.ID `json:"id"`
+	Name string      `json:"name"`
+}
+
+// InterfaceNoFragmentsQueryRootTopicChildrenVideo includes the requested fields of the GraphQL type Video.
+type InterfaceNoFragmentsQueryRootTopicChildrenVideo struct {
+	Typename string `json:"__typename"`
+	// ID is the identifier of the content.
+	Id   testutil.ID `json:"id"`
+	Name string      `json:"name"`
+}
+
+func InterfaceNoFragmentsQuery(
+	client graphql.Client,
+) (*InterfaceNoFragmentsQueryResponse, error) {
+	var retval InterfaceNoFragmentsQueryResponse
+	err := client.MakeRequest(
+		nil,
+		"InterfaceNoFragmentsQuery",
+		`
+query InterfaceNoFragmentsQuery {
+	root {
+		id
+		name
+		children {
+			__typename
+			id
+			name
+		}
+	}
+}
+`,
+		&retval,
+		nil,
+	)
+	return &retval, err
+}
+

generate/testdata/snapshots/TestGenerate-InterfaceNesting.graphql-InterfaceNesting.graphql.go

@@ -3,8 +3,6 @@ package test
 // Code generated by github.com/Khan/genqlient, DO NOT EDIT.
 
 import (
-	"encoding/json"
-
 	"github.com/Khan/genqlient/graphql"
 	"github.com/Khan/genqlient/internal/testutil"
 )
@@ -18,54 +16,7 @@ type InterfaceNestingResponse struct {
 type InterfaceNestingRootTopic struct {
 	// ID is documented in the Content interface.
 	Id       testutil.ID                                `json:"id"`
-	Children []InterfaceNestingRootTopicChildrenContent `json:"-"`
-}
-
-func (v *InterfaceNestingRootTopic) UnmarshalJSON(b []byte) error {
-	var firstPass struct {
-		*InterfaceNestingRootTopic
-		Children json.RawMessage `json:"children"`
-	}
-	firstPass.InterfaceNestingRootTopic = v
-
-	err := json.Unmarshal(b, &firstPass)
-	if err != nil {
-		return err
-	}
-
-	var tn struct {
-		TypeName string `json:"__typename"`
-	}
-	err = json.Unmarshal(firstPass.Children, &tn)
-	if err != nil {
-		return err
-	}
-	switch tn.TypeName {
-
-	case "Article":
-
-		v.Children = InterfaceNestingRootTopicChildrenArticle{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	case "Video":
-
-		v.Children = InterfaceNestingRootTopicChildrenVideo{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	case "Topic":
-
-		v.Children = InterfaceNestingRootTopicChildrenTopic{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	}
-	if err != nil {
-		return err
-	}
-
-	return nil
+	Children []InterfaceNestingRootTopicChildrenContent `json:"children"`
 }
 
 // InterfaceNestingRootTopicChildrenArticle includes the requested fields of the GraphQL type Article.
@@ -75,61 +26,11 @@ type InterfaceNestingRootTopicChildrenArticle struct {
 	Parent InterfaceNestingRootTopicChildrenArticleParentTopic `json:"parent"`
 }
 
-func (v InterfaceNestingRootTopicChildrenArticle) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenContent() {
-}
-
 // InterfaceNestingRootTopicChildrenArticleParentTopic includes the requested fields of the GraphQL type Topic.
 type InterfaceNestingRootTopicChildrenArticleParentTopic struct {
 	// ID is documented in the Content interface.
 	Id       testutil.ID                                                          `json:"id"`
-	Children []InterfaceNestingRootTopicChildrenArticleParentTopicChildrenContent `json:"-"`
-}
-
-func (v *InterfaceNestingRootTopicChildrenArticleParentTopic) UnmarshalJSON(b []byte) error {
-	var firstPass struct {
-		*InterfaceNestingRootTopicChildrenArticleParentTopic
-		Children json.RawMessage `json:"children"`
-	}
-	firstPass.InterfaceNestingRootTopicChildrenArticleParentTopic = v
-
-	err := json.Unmarshal(b, &firstPass)
-	if err != nil {
-		return err
-	}
-
-	var tn struct {
-		TypeName string `json:"__typename"`
-	}
-	err = json.Unmarshal(firstPass.Children, &tn)
-	if err != nil {
-		return err
-	}
-	switch tn.TypeName {
-
-	case "Article":
-
-		v.Children = InterfaceNestingRootTopicChildrenArticleParentTopicChildrenArticle{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	case "Video":
-
-		v.Children = InterfaceNestingRootTopicChildrenArticleParentTopicChildrenVideo{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	case "Topic":
-
-		v.Children = InterfaceNestingRootTopicChildrenArticleParentTopicChildrenTopic{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	}
-	if err != nil {
-		return err
-	}
-
-	return nil
+	Children []InterfaceNestingRootTopicChildrenArticleParentTopicChildrenContent `json:"children"`
 }
 
 // InterfaceNestingRootTopicChildrenArticleParentTopicChildrenArticle includes the requested fields of the GraphQL type Article.
@@ -138,9 +39,6 @@ type InterfaceNestingRootTopicChildrenArticleParentTopicChildrenArticle struct {
 	Id testutil.ID `json:"id"`
 }
 
-func (v InterfaceNestingRootTopicChildrenArticleParentTopicChildrenArticle) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenArticleParentTopicChildrenContent() {
-}
-
 // InterfaceNestingRootTopicChildrenArticleParentTopicChildrenContent includes the requested fields of the GraphQL type Content.
 // The GraphQL type's documentation follows.
 //
@@ -149,24 +47,25 @@ type InterfaceNestingRootTopicChildrenArticleParentTopicChildrenContent interfac
 	implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenArticleParentTopicChildrenContent()
 }
 
+func (v InterfaceNestingRootTopicChildrenArticleParentTopicChildrenArticle) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenArticleParentTopicChildrenContent() {
+}
+func (v InterfaceNestingRootTopicChildrenArticleParentTopicChildrenVideo) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenArticleParentTopicChildrenContent() {
+}
+func (v InterfaceNestingRootTopicChildrenArticleParentTopicChildrenTopic) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenArticleParentTopicChildrenContent() {
+}
+
 // InterfaceNestingRootTopicChildrenArticleParentTopicChildrenTopic includes the requested fields of the GraphQL type Topic.
 type InterfaceNestingRootTopicChildrenArticleParentTopicChildrenTopic struct {
 	// ID is the identifier of the content.
 	Id testutil.ID `json:"id"`
 }
 
-func (v InterfaceNestingRootTopicChildrenArticleParentTopicChildrenTopic) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenArticleParentTopicChildrenContent() {
-}
-
 // InterfaceNestingRootTopicChildrenArticleParentTopicChildrenVideo includes the requested fields of the GraphQL type Video.
 type InterfaceNestingRootTopicChildrenArticleParentTopicChildrenVideo struct {
 	// ID is the identifier of the content.
 	Id testutil.ID `json:"id"`
 }
 
-func (v InterfaceNestingRootTopicChildrenArticleParentTopicChildrenVideo) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenArticleParentTopicChildrenContent() {
-}
-
 // InterfaceNestingRootTopicChildrenContent includes the requested fields of the GraphQL type Content.
 // The GraphQL type's documentation follows.
 //
@@ -175,6 +74,13 @@ type InterfaceNestingRootTopicChildrenContent interface {
 	implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenContent()
 }
 
+func (v InterfaceNestingRootTopicChildrenArticle) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenContent() {
+}
+func (v InterfaceNestingRootTopicChildrenVideo) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenContent() {
+}
+func (v InterfaceNestingRootTopicChildrenTopic) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenContent() {
+}
+
 // InterfaceNestingRootTopicChildrenTopic includes the requested fields of the GraphQL type Topic.
 type InterfaceNestingRootTopicChildrenTopic struct {
 	// ID is the identifier of the content.
@@ -182,61 +88,11 @@ type InterfaceNestingRootTopicChildrenTopic struct {
 	Parent InterfaceNestingRootTopicChildrenTopicParentTopic `json:"parent"`
 }
 
-func (v InterfaceNestingRootTopicChildrenTopic) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenContent() {
-}
-
 // InterfaceNestingRootTopicChildrenTopicParentTopic includes the requested fields of the GraphQL type Topic.
 type InterfaceNestingRootTopicChildrenTopicParentTopic struct {
 	// ID is documented in the Content interface.
 	Id       testutil.ID                                                        `json:"id"`
-	Children []InterfaceNestingRootTopicChildrenTopicParentTopicChildrenContent `json:"-"`
-}
-
-func (v *InterfaceNestingRootTopicChildrenTopicParentTopic) UnmarshalJSON(b []byte) error {
-	var firstPass struct {
-		*InterfaceNestingRootTopicChildrenTopicParentTopic
-		Children json.RawMessage `json:"children"`
-	}
-	firstPass.InterfaceNestingRootTopicChildrenTopicParentTopic = v
-
-	err := json.Unmarshal(b, &firstPass)
-	if err != nil {
-		return err
-	}
-
-	var tn struct {
-		TypeName string `json:"__typename"`
-	}
-	err = json.Unmarshal(firstPass.Children, &tn)
-	if err != nil {
-		return err
-	}
-	switch tn.TypeName {
-
-	case "Article":
-
-		v.Children = InterfaceNestingRootTopicChildrenTopicParentTopicChildrenArticle{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	case "Video":
-
-		v.Children = InterfaceNestingRootTopicChildrenTopicParentTopicChildrenVideo{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	case "Topic":
-
-		v.Children = InterfaceNestingRootTopicChildrenTopicParentTopicChildrenTopic{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	}
-	if err != nil {
-		return err
-	}
-
-	return nil
+	Children []InterfaceNestingRootTopicChildrenTopicParentTopicChildrenContent `json:"children"`
 }
 
 // InterfaceNestingRootTopicChildrenTopicParentTopicChildrenArticle includes the requested fields of the GraphQL type Article.
@@ -245,9 +101,6 @@ type InterfaceNestingRootTopicChildrenTopicParentTopicChildrenArticle struct {
 	Id testutil.ID `json:"id"`
 }
 
-func (v InterfaceNestingRootTopicChildrenTopicParentTopicChildrenArticle) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenTopicParentTopicChildrenContent() {
-}
-
 // InterfaceNestingRootTopicChildrenTopicParentTopicChildrenContent includes the requested fields of the GraphQL type Content.
 // The GraphQL type's documentation follows.
 //
@@ -256,24 +109,25 @@ type InterfaceNestingRootTopicChildrenTopicParentTopicChildrenContent interface
 	implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenTopicParentTopicChildrenContent()
 }
 
+func (v InterfaceNestingRootTopicChildrenTopicParentTopicChildrenArticle) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenTopicParentTopicChildrenContent() {
+}
+func (v InterfaceNestingRootTopicChildrenTopicParentTopicChildrenVideo) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenTopicParentTopicChildrenContent() {
+}
+func (v InterfaceNestingRootTopicChildrenTopicParentTopicChildrenTopic) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenTopicParentTopicChildrenContent() {
+}
+
 // InterfaceNestingRootTopicChildrenTopicParentTopicChildrenTopic includes the requested fields of the GraphQL type Topic.
 type InterfaceNestingRootTopicChildrenTopicParentTopicChildrenTopic struct {
 	// ID is the identifier of the content.
 	Id testutil.ID `json:"id"`
 }
 
-func (v InterfaceNestingRootTopicChildrenTopicParentTopicChildrenTopic) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenTopicParentTopicChildrenContent() {
-}
-
 // InterfaceNestingRootTopicChildrenTopicParentTopicChildrenVideo includes the requested fields of the GraphQL type Video.
 type InterfaceNestingRootTopicChildrenTopicParentTopicChildrenVideo struct {
 	// ID is the identifier of the content.
 	Id testutil.ID `json:"id"`
 }
 
-func (v InterfaceNestingRootTopicChildrenTopicParentTopicChildrenVideo) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenTopicParentTopicChildrenContent() {
-}
-
 // InterfaceNestingRootTopicChildrenVideo includes the requested fields of the GraphQL type Video.
 type InterfaceNestingRootTopicChildrenVideo struct {
 	// ID is the identifier of the content.
@@ -281,61 +135,11 @@ type InterfaceNestingRootTopicChildrenVideo struct {
 	Parent InterfaceNestingRootTopicChildrenVideoParentTopic `json:"parent"`
 }
 
-func (v InterfaceNestingRootTopicChildrenVideo) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenContent() {
-}
-
 // InterfaceNestingRootTopicChildrenVideoParentTopic includes the requested fields of the GraphQL type Topic.
 type InterfaceNestingRootTopicChildrenVideoParentTopic struct {
 	// ID is documented in the Content interface.
 	Id       testutil.ID                                                        `json:"id"`
-	Children []InterfaceNestingRootTopicChildrenVideoParentTopicChildrenContent `json:"-"`
-}
-
-func (v *InterfaceNestingRootTopicChildrenVideoParentTopic) UnmarshalJSON(b []byte) error {
-	var firstPass struct {
-		*InterfaceNestingRootTopicChildrenVideoParentTopic
-		Children json.RawMessage `json:"children"`
-	}
-	firstPass.InterfaceNestingRootTopicChildrenVideoParentTopic = v
-
-	err := json.Unmarshal(b, &firstPass)
-	if err != nil {
-		return err
-	}
-
-	var tn struct {
-		TypeName string `json:"__typename"`
-	}
-	err = json.Unmarshal(firstPass.Children, &tn)
-	if err != nil {
-		return err
-	}
-	switch tn.TypeName {
-
-	case "Article":
-
-		v.Children = InterfaceNestingRootTopicChildrenVideoParentTopicChildrenArticle{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	case "Video":
-
-		v.Children = InterfaceNestingRootTopicChildrenVideoParentTopicChildrenVideo{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	case "Topic":
-
-		v.Children = InterfaceNestingRootTopicChildrenVideoParentTopicChildrenTopic{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	}
-	if err != nil {
-		return err
-	}
-
-	return nil
+	Children []InterfaceNestingRootTopicChildrenVideoParentTopicChildrenContent `json:"children"`
 }
 
 // InterfaceNestingRootTopicChildrenVideoParentTopicChildrenArticle includes the requested fields of the GraphQL type Article.
@@ -344,9 +148,6 @@ type InterfaceNestingRootTopicChildrenVideoParentTopicChildrenArticle struct {
 	Id testutil.ID `json:"id"`
 }
 
-func (v InterfaceNestingRootTopicChildrenVideoParentTopicChildrenArticle) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenVideoParentTopicChildrenContent() {
-}
-
 // InterfaceNestingRootTopicChildrenVideoParentTopicChildrenContent includes the requested fields of the GraphQL type Content.
 // The GraphQL type's documentation follows.
 //
@@ -355,24 +156,25 @@ type InterfaceNestingRootTopicChildrenVideoParentTopicChildrenContent interface
 	implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenVideoParentTopicChildrenContent()
 }
 
+func (v InterfaceNestingRootTopicChildrenVideoParentTopicChildrenArticle) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenVideoParentTopicChildrenContent() {
+}
+func (v InterfaceNestingRootTopicChildrenVideoParentTopicChildrenVideo) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenVideoParentTopicChildrenContent() {
+}
+func (v InterfaceNestingRootTopicChildrenVideoParentTopicChildrenTopic) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenVideoParentTopicChildrenContent() {
+}
+
 // InterfaceNestingRootTopicChildrenVideoParentTopicChildrenTopic includes the requested fields of the GraphQL type Topic.
 type InterfaceNestingRootTopicChildrenVideoParentTopicChildrenTopic struct {
 	// ID is the identifier of the content.
 	Id testutil.ID `json:"id"`
 }
 
-func (v InterfaceNestingRootTopicChildrenVideoParentTopicChildrenTopic) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenVideoParentTopicChildrenContent() {
-}
-
 // InterfaceNestingRootTopicChildrenVideoParentTopicChildrenVideo includes the requested fields of the GraphQL type Video.
 type InterfaceNestingRootTopicChildrenVideoParentTopicChildrenVideo struct {
 	// ID is the identifier of the content.
 	Id testutil.ID `json:"id"`
 }
 
-func (v InterfaceNestingRootTopicChildrenVideoParentTopicChildrenVideo) implementsGraphQLInterfaceInterfaceNestingRootTopicChildrenVideoParentTopicChildrenContent() {
-}
-
 func InterfaceNesting(
 	client graphql.Client,
 ) (*InterfaceNestingResponse, error) {

generate/testdata/snapshots/TestGenerate-InterfaceNoFragments.graphql-InterfaceNoFragments.graphql.go

@@ -3,8 +3,6 @@ package test
 // Code generated by github.com/Khan/genqlient, DO NOT EDIT.
 
 import (
-	"encoding/json"
-
 	"github.com/Khan/genqlient/graphql"
 	"github.com/Khan/genqlient/internal/testutil"
 )
@@ -19,54 +17,7 @@ type InterfaceNoFragmentsQueryRootTopic struct {
 	// ID is documented in the Content interface.
 	Id       testutil.ID                                         `json:"id"`
 	Name     string                                              `json:"name"`
-	Children []InterfaceNoFragmentsQueryRootTopicChildrenContent `json:"-"`
-}
-
-func (v *InterfaceNoFragmentsQueryRootTopic) UnmarshalJSON(b []byte) error {
-	var firstPass struct {
-		*InterfaceNoFragmentsQueryRootTopic
-		Children json.RawMessage `json:"children"`
-	}
-	firstPass.InterfaceNoFragmentsQueryRootTopic = v
-
-	err := json.Unmarshal(b, &firstPass)
-	if err != nil {
-		return err
-	}
-
-	var tn struct {
-		TypeName string `json:"__typename"`
-	}
-	err = json.Unmarshal(firstPass.Children, &tn)
-	if err != nil {
-		return err
-	}
-	switch tn.TypeName {
-
-	case "Article":
-
-		v.Children = InterfaceNoFragmentsQueryRootTopicChildrenArticle{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	case "Video":
-
-		v.Children = InterfaceNoFragmentsQueryRootTopicChildrenVideo{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	case "Topic":
-
-		v.Children = InterfaceNoFragmentsQueryRootTopicChildrenTopic{}
-		err = json.Unmarshal(
-			firstPass.Children, &v.Children)
-
-	}
-	if err != nil {
-		return err
-	}
-
-	return nil
+	Children []InterfaceNoFragmentsQueryRootTopicChildrenContent `json:"children"`
 }
 
 // InterfaceNoFragmentsQueryRootTopicChildrenArticle includes the requested fields of the GraphQL type Article.
@@ -76,9 +27,6 @@ type InterfaceNoFragmentsQueryRootTopicChildrenArticle struct {
 	Name string      `json:"name"`
 }
 
-func (v InterfaceNoFragmentsQueryRootTopicChildrenArticle) implementsGraphQLInterfaceInterfaceNoFragmentsQueryRootTopicChildrenContent() {
-}
-
 // InterfaceNoFragmentsQueryRootTopicChildrenContent includes the requested fields of the GraphQL type Content.
 // The GraphQL type's documentation follows.
 //
@@ -87,6 +35,13 @@ type InterfaceNoFragmentsQueryRootTopicChildrenContent interface {
 	implementsGraphQLInterfaceInterfaceNoFragmentsQueryRootTopicChildrenContent()
 }
 
+func (v InterfaceNoFragmentsQueryRootTopicChildrenArticle) implementsGraphQLInterfaceInterfaceNoFragmentsQueryRootTopicChildrenContent() {
+}
+func (v InterfaceNoFragmentsQueryRootTopicChildrenVideo) implementsGraphQLInterfaceInterfaceNoFragmentsQueryRootTopicChildrenContent() {
+}
+func (v InterfaceNoFragmentsQueryRootTopicChildrenTopic) implementsGraphQLInterfaceInterfaceNoFragmentsQueryRootTopicChildrenContent() {
+}
+
 // InterfaceNoFragmentsQueryRootTopicChildrenTopic includes the requested fields of the GraphQL type Topic.
 type InterfaceNoFragmentsQueryRootTopicChildrenTopic struct {
 	// ID is the identifier of the content.
@@ -94,9 +49,6 @@ type InterfaceNoFragmentsQueryRootTopicChildrenTopic struct {
 	Name string      `json:"name"`
 }
 
-func (v InterfaceNoFragmentsQueryRootTopicChildrenTopic) implementsGraphQLInterfaceInterfaceNoFragmentsQueryRootTopicChildrenContent() {
-}
-
 // InterfaceNoFragmentsQueryRootTopicChildrenVideo includes the requested fields of the GraphQL type Video.
 type InterfaceNoFragmentsQueryRootTopicChildrenVideo struct {
 	// ID is the identifier of the content.
@@ -104,9 +56,6 @@ type InterfaceNoFragmentsQueryRootTopicChildrenVideo struct {
 	Name string      `json:"name"`
 }
 
-func (v InterfaceNoFragmentsQueryRootTopicChildrenVideo) implementsGraphQLInterfaceInterfaceNoFragmentsQueryRootTopicChildrenContent() {
-}
-
 func InterfaceNoFragmentsQuery(
 	client graphql.Client,
 ) (*InterfaceNoFragmentsQueryResponse, error) {

generate/testdata/snapshots/TestGenerate-UnionNoFragments.graphql-UnionNoFragments.graphql.go

@@ -13,9 +13,6 @@ type UnionNoFragmentsQueryRandomLeafArticle struct {
 	Typename string `json:"__typename"`
 }
 
-func (v UnionNoFragmentsQueryRandomLeafArticle) implementsGraphQLInterfaceUnionNoFragmentsQueryRandomLeafLeafContent() {
-}
-
 // UnionNoFragmentsQueryRandomLeafLeafContent includes the requested fields of the GraphQL type LeafContent.
 // The GraphQL type's documentation follows.
 //
@@ -24,14 +21,16 @@ type UnionNoFragmentsQueryRandomLeafLeafContent interface {
 	implementsGraphQLInterfaceUnionNoFragmentsQueryRandomLeafLeafContent()
 }
 
+func (v UnionNoFragmentsQueryRandomLeafArticle) implementsGraphQLInterfaceUnionNoFragmentsQueryRandomLeafLeafContent() {
+}
+func (v UnionNoFragmentsQueryRandomLeafVideo) implementsGraphQLInterfaceUnionNoFragmentsQueryRandomLeafLeafContent() {
+}
+
 // UnionNoFragmentsQueryRandomLeafVideo includes the requested fields of the GraphQL type Video.
 type UnionNoFragmentsQueryRandomLeafVideo struct {
 	Typename string `json:"__typename"`
 }
 
-func (v UnionNoFragmentsQueryRandomLeafVideo) implementsGraphQLInterfaceUnionNoFragmentsQueryRandomLeafLeafContent() {
-}
-
 // UnionNoFragmentsQueryResponse is returned by UnionNoFragmentsQuery on success.
 type UnionNoFragmentsQueryResponse struct {
 	RandomLeaf UnionNoFragmentsQueryRandomLeafLeafContent `json:"-"`

generate/types.go

@@ -1,412 +1,188 @@
 package generate
 
-// This file is the core of genqlient: it's what generates the types into which
-// we will unmarshal.
-//
-// TODO: this file really really needs a file-comment explaining what's going
-// on.  probably writing that will help me figure out how to make it make more
-// sense!
+// This file defines the data structures from which genqlient generates types,
+// and the code to write them out as actual Go code.  The main entrypoint is
+// goType, which represents such a type, but convert.go also constructs each
+// of the implementing types, by traversing the GraphQL operation and schema.
 
 import (
 	"fmt"
+	"io"
 	"strings"
-
-	"github.com/vektah/gqlparser/v2/ast"
 )
 
-type typeBuilder struct {
-	strings.Builder
-	*generator
+// goType represents a type for which we'll generate code.
+type goType interface {
+	// WriteDefinition writes the code for this type into the given io.Writer.
+	//
+	// TODO(benkraft): Some of the implementations might now benefit from being
+	// converted to templates.
+	WriteDefinition(io.Writer, *generator) error
+
+	// Reference returns the Go name of this type, e.g. []*MyStruct, and may be
+	// used to refer to it in Go code.
+	Reference() string
 }
 
-func (g *generator) baseTypeForOperation(operation ast.Operation) (*ast.Definition, error) {
-	switch operation {
-	case ast.Query:
-		return g.schema.Query, nil
-	case ast.Mutation:
-		return g.schema.Mutation, nil
-	case ast.Subscription:
-		if !allowBrokenFeatures {
-			return nil, errorf(nil, "genqlient does not yet support subscriptions")
-		}
-		return g.schema.Subscription, nil
-	default:
-		return nil, errorf(nil, "unexpected operation: %v", operation)
+var (
+	_ goType = (*goOpaqueType)(nil)
+	_ goType = (*goSliceType)(nil)
+	_ goType = (*goPointerType)(nil)
+	_ goType = (*goEnumType)(nil)
+	_ goType = (*goStructType)(nil)
+	_ goType = (*goInterfaceType)(nil)
+)
+
+type (
+	// goOpaqueType represents a user-defined or builtin type, used to
+	// represent a GraphQL scalar.
+	goOpaqueType struct{ GoRef string }
+	// goSliceType represents the Go type []Elem, used to represent GraphQL
+	// list types.
+	goSliceType struct{ Elem goType }
+	// goSliceType represents the Go type *Elem, used when requested by the
+	// user (perhaps to handle nulls explicitly, or to avoid copying large
+	// structures).
+	goPointerType struct{ Elem goType }
+)
+
+// Opaque types are defined by the user; pointers and slices need no definition
+func (typ *goOpaqueType) WriteDefinition(io.Writer, *generator) error  { return nil }
+func (typ *goSliceType) WriteDefinition(io.Writer, *generator) error   { return nil }
+func (typ *goPointerType) WriteDefinition(io.Writer, *generator) error { return nil }
+
+func (typ *goOpaqueType) Reference() string  { return typ.GoRef }
+func (typ *goSliceType) Reference() string   { return "[]" + typ.Elem.Reference() }
+func (typ *goPointerType) Reference() string { return "*" + typ.Elem.Reference() }
+
+// goEnumType represents a Go named-string type used to represent a GraphQL
+// enum.  In this case, we generate both the type (`type T string`) and also a
+// list of consts representing the values.
+type goEnumType struct {
+	GoName      string
+	Description string
+	Values      []goEnumValue
+}
+
+type goEnumValue struct {
+	Name        string
+	Description string
+}
+
+func (typ *goEnumType) WriteDefinition(w io.Writer, g *generator) error {
+	// All GraphQL enums have underlying type string (in the Go sense).
+	writeDescription(w, typ.Description)
+	fmt.Fprintf(w, "type %s string\n", typ.GoName)
+	fmt.Fprintf(w, "const (\n")
+	for _, val := range typ.Values {
+		writeDescription(w, val.Description)
+		fmt.Fprintf(w, "%s %s = \"%s\"\n",
+			typ.GoName+goConstName(val.Name),
+			typ.GoName, val.Name)
 	}
-}
-
-func (g *generator) getTypeForOperation(operation *ast.OperationDefinition, queryOptions *GenqlientDirective) (name string, err error) {
-	name = operation.Name + "Response"
-
-	if def, ok := g.typeMap[name]; ok {
-		// TODO: check for and handle conflicts a better way
-		return "", errorf(operation.Position, "%s defined twice:\n%s", name, def)
-	}
-
-	fields, err := selections(g, operation.SelectionSet, queryOptions)
-	if err != nil {
-		return "", err
-	}
-
-	baseType, err := g.baseTypeForOperation(operation.Operation)
-	if err != nil {
-		return "", errorf(operation.Position, "%v", err)
-	}
-
-	description := fmt.Sprintf("%v is returned by %v on success.", name, operation.Name)
-
-	builder := &typeBuilder{generator: g}
-	err = builder.writeTypedef(
-		name, operation.Name, baseType, operation.Position, fields, queryOptions, description)
-	return name, err
-}
-
-var builtinTypes = map[string]string{
-	// GraphQL guarantees int32 is enough, but using int seems more idiomatic
-	"Int":     "int",
-	"Float":   "float64",
-	"String":  "string",
-	"Boolean": "bool",
-	"ID":      "string",
-}
-
-func (g *generator) typeName(prefix string, typ *ast.Definition) (name, nextPrefix string) {
-	typeGoName := upperFirst(typ.Name)
-	if typ.Kind == ast.Enum || typ.Kind == ast.InputObject {
-		// If we're an enum or an input-object, there is only one type we
-		// will ever possibly generate for this type, so we don't need any
-		// of the qualifiers.  This is especially helpful because the
-		// caller is very likely to need to reference these types in their
-		// code.
-		return typeGoName, typeGoName
-	}
-
-	name = prefix
-	if !strings.HasSuffix(prefix, typeGoName) {
-		// If the field and type names are the same, we can avoid the
-		// duplication.  (We include the field name in case there are
-		// multiple fields with the same type, and the type name because
-		// that's the actual name (the rest are really qualifiers); but if
-		// they are the same then including it once suffices for both
-		// purposes.)
-		name += typeGoName
-	}
-
-	if typ.Kind == ast.Interface || typ.Kind == ast.Union {
-		// for interface/union types, we do not add the type name to the
-		// name prefix; we want to have QueryFieldType rather than
-		// QueryFieldInterfaceType.  So we just use the input prefix.
-		return name, prefix
-	}
-
-	// Otherwise, the name will also be the prefix for the next type.
-	return name, name
-}
-
-func (g *generator) getTypeForInputType(opName string, typ *ast.Type, options, queryOptions *GenqlientDirective) (string, error) {
-	// Sort of a hack: case the input type name to match the op-name.
-	name := matchFirst(typ.Name(), opName)
-	builder := &typeBuilder{generator: g}
-	// note prefix is ignored here (see generator.typeName)
-	err := builder.writeType(name, "", typ, selectionsForInputType(g, typ, queryOptions), options)
-	return builder.String(), err
-}
-
-type field interface {
-	Alias() string
-	Options() (*GenqlientDirective, error)
-	Description() string
-	Type() *ast.Type
-	Pos() *ast.Position
-	SubFields() ([]field, error)
-}
-
-type outputField struct {
-	*generator
-	queryOptions *GenqlientDirective
-	field        *ast.Field
-}
-
-func (s outputField) Alias() string {
-	// gqlparser sets Alias even if the field is not aliased, see e.g.
-	// https://github.com/vektah/gqlparser/v2/blob/c06d8e0d135f285e37e7f1ff397f10e049733eb3/parser/query.go#L150
-	return s.field.Alias
-}
-
-func (s outputField) Options() (*GenqlientDirective, error) {
-	_, directive, err := s.generator.parsePrecedingComment(s.field, s.field.Position)
-	if err != nil {
-		return nil, err
-	}
-	return s.queryOptions.merge(directive), nil
-}
-
-func (s outputField) Description() string {
-	if s.field.Definition == nil {
-		return ""
-	}
-	return s.field.Definition.Description
-}
-
-func (s outputField) Type() *ast.Type {
-	if s.field.Definition == nil {
-		return nil
-	}
-	return s.field.Definition.Type
-}
-
-func (s outputField) Pos() *ast.Position {
-	return s.field.Position
-}
-
-func (s outputField) SubFields() ([]field, error) {
-	return selections(s.generator, s.field.SelectionSet, s.queryOptions)
-}
-
-func selections(g *generator, selectionSet ast.SelectionSet, options *GenqlientDirective) ([]field, error) {
-	retval := make([]field, len(selectionSet))
-	for i, selection := range selectionSet {
-		switch selection := selection.(type) {
-		case *ast.Field:
-			retval[i] = outputField{g, options, selection}
-		case *ast.FragmentSpread:
-			return nil, errorf(selection.Position, "not implemented: %T", selection)
-		case *ast.InlineFragment:
-			return nil, errorf(selection.Position, "not implemented: %T", selection)
-		default:
-			return nil, errorf(nil, "invalid selection type: %T", selection)
-		}
-	}
-	return retval, nil
-}
-
-type inputField struct {
-	*generator
-	field        *ast.FieldDefinition
-	queryOptions *GenqlientDirective
-}
-
-func (s inputField) Alias() string { return s.field.Name }
-func (s inputField) Options() (*GenqlientDirective, error) {
-	_, directive, err := s.generator.parsePrecedingComment(s.field, s.field.Position)
-	if err != nil {
-		return nil, err
-	}
-	return s.queryOptions.merge(directive), nil
-}
-func (s inputField) Description() string { return s.field.Description }
-func (s inputField) Type() *ast.Type     { return s.field.Type }
-func (s inputField) Pos() *ast.Position  { return s.field.Position }
-
-func (s inputField) SubFields() ([]field, error) {
-	return selectionsForInputType(s.generator, s.field.Type, s.queryOptions), nil
-}
-
-func selectionsForInputType(g *generator, typ *ast.Type, queryOptions *GenqlientDirective) []field {
-	def := g.schema.Types[typ.Name()]
-	fields := make([]field, len(def.Fields))
-	for i, field := range def.Fields {
-		fields[i] = inputField{g, field, queryOptions}
-	}
-	return fields
-}
-
-func (builder *typeBuilder) writeField(typeNamePrefix string, field field) error {
-	jsonName := field.Alias()
-	// We need an exportable name for JSON-marshaling.
-	goName := upperFirst(jsonName)
-
-	typ := field.Type()
-	if typ == nil {
-		// Unclear why gqlparser hasn't already rejected this,
-		// but empirically it might not.
-		return errorf(field.Pos(), "undefined field %v", field.Alias())
-	}
-
-	fields, err := field.SubFields()
-	if err != nil {
-		return err
-	}
-
-	options, err := field.Options()
-	if err != nil {
-		return err
-	}
-
-	typedef := builder.schema.Types[typ.Name()]
-
-	builder.writeDescription(field.Description())
-	builder.WriteString(goName)
-	builder.WriteRune(' ')
-
-	// Note we don't deduplicate suffixes here -- if our prefix is GetUser
-	// and the field name is User, we do GetUserUser.  This is important
-	// because if you have a field called user on a type called User we
-	// need `query q { user { user { id } } }` to generate two types, QUser
-	// and QUserUser.
-	// Note also this is named based on the GraphQL alias (Go name), not the
-	// field-name, because if we have `query q { a: f { b }, c: f { d } }` we
-	// need separate types for a and c, even though they are the same type in
-	// GraphQL, because they have different fields.
-	name, namePrefix := builder.typeName(typeNamePrefix+goName, typedef)
-	err = builder.writeType(name, namePrefix, typ, fields, options)
-	if err != nil {
-		return err
-	}
-
-	if typedef.IsAbstractType() {
-		// abstract types are handled in our UnmarshalJSON
-		jsonName = "-"
-	}
-
-	fmt.Fprintf(builder, " `json:\"%s\"`\n", jsonName)
+	fmt.Fprintf(w, ")\n")
 	return nil
 }
 
-func (builder *typeBuilder) writeType(name, namePrefix string, typ *ast.Type, fields []field, options *GenqlientDirective) error {
-	// gqlgen does slightly different things here, but its implementation may
-	// be useful to crib from:
-	// https://github.com/99designs/gqlgen/blob/master/plugin/modelgen/models.go#L113
-	for typ.Elem != nil {
-		// Type is a list.
-		builder.WriteString("[]")
-		typ = typ.Elem
-	}
-	if options.GetPointer() {
-		// Note this does []*T or [][]*T, not e.g. *[][]T.  See #16.
-		builder.WriteString("*")
-	}
+func (typ *goEnumType) Reference() string { return typ.GoName }
 
-	// If this is a builtin type or custom scalar, just refer to it.
-	def := builder.schema.Types[typ.Name()]
-	goName, ok := builder.Config.Scalars[def.Name]
-	if ok {
-		newName, err := builder.addRef(goName)
-		builder.WriteString(newName)
-		return err
-	}
-	goName, ok = builtinTypes[def.Name]
-	if ok {
-		builder.WriteString(goName)
-		return nil
-	}
-
-	// Else, write the name, then generate the definition.
-	builder.WriteString(name)
-
-	childBuilder := &typeBuilder{generator: builder.generator}
-	return childBuilder.writeTypedef(name, namePrefix, def, typ.Position, fields, options, "")
+// goStructType represents a Go struct type used to represent a GraphQL object
+// or input-object type.
+type goStructType struct {
+	GoName      string
+	Description string
+	GraphQLName string
+	Fields      []*goStructField
+	// Incomplete is set if this type contains only certain fields of the
+	// corresponding GraphQL type (i.e. those selected by the operation) in
+	// which case we put a note in the doc-comment saying as much.
+	Incomplete bool
 }
 
-func (builder *typeBuilder) writeTypedef(
-	typeName, typeNamePrefix string,
-	typedef *ast.Definition,
-	pos *ast.Position,
-	fields []field,
-	options *GenqlientDirective, //nolint:unparam // it is used!
-	description string, // defaults to typedef.Description
-) (err error) {
-	defer func() {
-		// Whenever we're done, add the type to the type-map.
-		// TODO: there's got to be a better way than defer.
-		if err == nil {
-			// TODO: this should also check for conflicts (except not for enums
-			// and input-objects, see above)
-			builder.typeMap[typeName] = builder.String()
-		}
-	}()
-
-	if description == "" {
-		switch typedef.Kind {
-		case ast.Object, ast.Interface, ast.Union:
-			// For types where we only have some fields, note that, along with
-			// the GraphQL documentation (if any).  We don't want to just use
-			// the GraphQL documentation, since it may refer to fields we
-			// haven't selected, say.
-			// TODO: When we implement interfaces and unions more completely,
-			// also mention the concrete types they might be.
-			description = fmt.Sprintf(
-				"%v includes the requested fields of the GraphQL type %v.",
-				typeName, typedef.Name)
-			if typedef.Description != "" {
-				description = fmt.Sprintf(
-					"%v\nThe GraphQL type's documentation follows.\n\n%v",
-					description, typedef.Description)
-			}
-		default:
-			description = typedef.Description
-		}
-	}
-	builder.writeDescription(description)
-
-	fmt.Fprintf(builder, "type %s ", typeName)
-	switch typedef.Kind {
-	case ast.Object, ast.InputObject:
-		builder.WriteString("struct {\n")
-		for _, field := range fields {
-			err := builder.writeField(typeNamePrefix, field)
-			if err != nil {
-				return err
-			}
-		}
-		builder.WriteString("}")
-
-		// If any field is abstract, we need an UnmarshalJSON method to handle
-		// it.
-		return builder.maybeWriteUnmarshal(typeName, typeNamePrefix, fields)
-
-	case ast.Interface, ast.Union:
-		if !allowBrokenFeatures {
-			return errorf(pos, "not implemented: %v", typedef.Kind)
-		}
-
-		// First, write the interface type.
-		builder.WriteString("interface {\n")
-		implementsMethodName := fmt.Sprintf("implementsGraphQLInterface%v", typeName)
-		// TODO: Also write GetX() accessor methods for fields of the interface
-		builder.WriteString(implementsMethodName)
-		builder.WriteString("()\n")
-		builder.WriteString("}")
-
-		// Then, write the implementations.
-		// TODO(benkraft): Put a doc-comment somewhere with the list.
-		for _, impldef := range builder.schema.GetPossibleTypes(typedef) {
-			name, namePrefix := builder.typeName(typeNamePrefix, impldef)
-			implBuilder := &typeBuilder{generator: builder.generator}
-			err := implBuilder.writeTypedef(name, namePrefix, impldef, pos, fields, options, "")
-			if err != nil {
-				return err
-			}
-
-			builder.typeMap[name] += fmt.Sprintf(
-				"\nfunc (v %v) %v() {}", name, implementsMethodName)
-		}
-
-		return nil
-
-	case ast.Enum:
-		// All GraphQL enums have underlying type string (in the Go sense).
-		builder.WriteString("string\n")
-		builder.WriteString("const (\n")
-		for _, val := range typedef.EnumValues {
-			builder.writeDescription(val.Description)
-			fmt.Fprintf(builder, "%s %s = \"%s\"\n",
-				typeName+goConstName(val.Name),
-				typeName, val.Name)
-		}
-		builder.WriteString(")\n")
-		return nil
-	case ast.Scalar:
-		return errorf(pos, "unknown scalar %v: please add it to genqlient.yaml", typedef.Name)
-	default:
-		return errorf(pos, "unexpected kind: %v", typedef.Kind)
-	}
+type goStructField struct {
+	GoName      string
+	GoType      goType
+	JSONName    string
+	Description string
 }
 
-func (builder *typeBuilder) writeDescription(desc string) {
+func (typ *goStructType) WriteDefinition(w io.Writer, g *generator) error {
+	description := typ.Description
+	if typ.Incomplete {
+		description = incompleteTypeDescription(typ.GoName, typ.GraphQLName, typ.Description)
+	}
+	writeDescription(w, description)
+
+	fmt.Fprintf(w, "type %s struct {\n", typ.GoName)
+	for _, field := range typ.Fields {
+		writeDescription(w, field.Description)
+		jsonName := field.JSONName
+		if _, ok := field.GoType.(*goInterfaceType); ok {
+			// abstract types are handled in our UnmarshalJSON
+			jsonName = "-"
+		}
+		fmt.Fprintf(w, "\t%s %s `json:\"%s\"`\n",
+			field.GoName, field.GoType.Reference(), jsonName)
+	}
+	fmt.Fprintf(w, "}\n")
+
+	return g.maybeWriteUnmarshal(w, typ)
+}
+
+func (typ *goStructType) Reference() string { return typ.GoName }
+
+// goInterfaceType represents a Go interface type, used to represent a GraphQL
+// interface or union type.
+type goInterfaceType struct {
+	GoName          string
+	Description     string
+	GraphQLName     string
+	Implementations []*goStructType
+}
+
+func (typ *goInterfaceType) WriteDefinition(w io.Writer, g *generator) error {
+	// TODO(benkraft): also mention the list of implementations.
+	description := incompleteTypeDescription(typ.GoName, typ.GraphQLName, typ.Description)
+	writeDescription(w, description)
+
+	// Write the interface.
+	fmt.Fprintf(w, "type %s interface {\n", typ.GoName)
+	implementsMethodName := fmt.Sprintf("implementsGraphQLInterface%v", typ.GoName)
+	// TODO(benkraft): Also write GetX() accessor methods for fields of the interface
+	fmt.Fprintf(w, "\t%s()\n", implementsMethodName)
+	fmt.Fprintf(w, "}\n")
+
+	// Now, write out the implementations.
+	for _, impl := range typ.Implementations {
+		fmt.Fprintf(w, "func (v %s) %s() {}\n",
+			impl.Reference(), implementsMethodName)
+	}
+
+	return nil
+}
+
+func (typ *goInterfaceType) Reference() string { return typ.GoName }
+
+func incompleteTypeDescription(goName, graphQLName, description string) string {
+	// For types where we only have some fields, note that, along with
+	// the GraphQL documentation (if any).  We don't want to just use
+	// the GraphQL documentation, since it may refer to fields we
+	// haven't selected, say.
+	prefix := fmt.Sprintf(
+		"%v includes the requested fields of the GraphQL type %v.",
+		goName, graphQLName)
+	if description != "" {
+		return fmt.Sprintf(
+			"%v\nThe GraphQL type's documentation follows.\n\n%v",
+			prefix, description)
+	}
+	return prefix
+}
+
+func writeDescription(w io.Writer, desc string) {
 	if desc != "" {
 		for _, line := range strings.Split(desc, "\n") {
-			builder.WriteString("// " + strings.TrimLeft(line, " \t") + "\n")
+			fmt.Fprintf(w, "// %s\n", strings.TrimLeft(line, " \t"))
 		}
 	}
 }

generate/unmarshal.go

@@ -1,5 +1,9 @@
 package generate
 
+import "io"
+
+// TODO(benkraft): We could potentially get rid of these now, and do everything
+// directly from the types.
 type templateData struct {
 	// Go type to which the method will be added
 	Type string
@@ -19,23 +23,22 @@ type concreteType struct {
 	GoName, GraphQLName string
 }
 
-func (builder *typeBuilder) maybeWriteUnmarshal(typeName, typeNamePrefix string, fields []field) error {
-	data := templateData{Type: typeName}
-	for _, field := range fields {
-		typedef := builder.schema.Types[field.Type().Name()]
-		if typedef.IsAbstractType() {
+func (g *generator) maybeWriteUnmarshal(w io.Writer, typ *goStructType) error {
+	data := templateData{Type: typ.GoName}
+	for _, field := range typ.Fields {
+		// TODO(benkraft): To handle list-of-interface fields, we should really
+		// be "unwrapping" any goSliceType/goPointerType wrappers to find the
+		// goInterfaceType.
+		if iface, ok := field.GoType.(*goInterfaceType); ok {
 			fieldInfo := abstractField{
-				GoName:   upperFirst(field.Alias()),
-				JSONName: field.Alias(),
+				GoName:   field.GoName,
+				JSONName: field.JSONName,
 			}
-			for _, typedef := range builder.schema.GetPossibleTypes(typedef) {
-				// TODO: this is slightly fragile (it needs to match the
-				// similar call in writeField)
-				goName, _ := builder.typeName(typeNamePrefix+fieldInfo.GoName, typedef)
+			for _, impl := range iface.Implementations {
 				fieldInfo.ConcreteTypes = append(fieldInfo.ConcreteTypes,
 					concreteType{
-						GoName:      goName,
-						GraphQLName: typedef.Name,
+						GoName:      impl.GoName,
+						GraphQLName: impl.GraphQLName,
 					})
 			}
 			data.Fields = append(data.Fields, fieldInfo)
@@ -46,11 +49,10 @@ func (builder *typeBuilder) maybeWriteUnmarshal(typeName, typeNamePrefix string,
 		return nil
 	}
 
-	_, err := builder.addRef("encoding/json.Unmarshal")
+	_, err := g.addRef("encoding/json.Unmarshal")
 	if err != nil {
 		return err
 	}
 
-	builder.WriteString("\n\n")
-	return builder.execute("unmarshal.go.tmpl", builder, data)
+	return g.execute("unmarshal.go.tmpl", w, data)
 }

generate/unmarshal.go.tmpl

@@ -1,3 +1,6 @@
+{{/* (the blank lines at the start are intentional, to separate
+     UnmarshalJSON from the function it follows) */}}
+
 func (v *{{.Type}}) UnmarshalJSON(b []byte) error {
 	var firstPass struct{
 		*{{.Type}}
@@ -12,6 +15,9 @@ func (v *{{.Type}}) UnmarshalJSON(b []byte) error {
         return err
     }
 
+    {{/* TODO(benkraft): split this out into a separate helper function,
+         generated for each interface type, rather than generating it inline
+         with the struct type. */}}
     {{range .Fields -}}
 	var tn struct { TypeName string `json:"__typename"` }
 	err = {{ref "encoding/json.Unmarshal"}}(firstPass.{{.GoName}}, &tn)