Add a new option to treat an interface like an object (#97)

## Summary:
The basic idea here is if you only request interface fields (no
fragments) you may not care about the concrete type, and so we could
just generate a struct as if it were an object.  I don't think it's a
good idea to do that by default, because then if you later add a
fragment all your code totally changes, but it's quite reasonable as an
option!

Most of the code involved is just wiring and validation; the
core implementation is literally just: treat it like an object.

Issue: #85

## Test plan:
make check


Author: benjaminjkraft

Reviewers: csilvers, StevenACoffman, benjaminjkraft, aberkan, dnerdy, jvoll, mahtabsabet, MiguelCastillo

Required Reviewers: 

Approved By: StevenACoffman

Checks: ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Lint, ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Lint

Pull Request URL: #97

Author: benjaminjkraft

docs/FAQ.md

@@ -183,6 +183,28 @@ if novel, ok := resp.Favorite.(*GetBooksFavoriteNovel); ok {
 
 The interface-type's GoDoc will include a list of its implementations, for your convenience.
 
+If you only want to request shared fields of the interface (i.e. no fragments), this may seem like a lot of ceremony.  If you prefer, you can instead add `# @genqlient(struct: true)` to the field, and genqlient will just generate a struct, like it does for GraphQL object types.  For example, given:
+
+```graphql
+query GetBooks {
+  # @genqlient(struct: true)
+  favorite {
+    title
+  }
+}
+```
+
+genqlient will generate just:
+
+```go
+type GetBooksFavoriteBook struct {
+  Title string
+}
+```
+
+Keep in mind that if you later want to add fragments to your selection, you won't be able to use `struct` anymore; when you remove it you may need to update your code to replace `.Title` with `.GetTitle()` and so on.
+
+
 ### … documentation on the output types?
 
 For any GraphQL types or fields with documentation in the GraphQL schema, genqlient automatically includes that documentation in the generated code's GoDoc.  To add additional information to genqlient entrypoints, you can put comments in the GraphQL source:

docs/genqlient_directive.graphql

@@ -54,6 +54,22 @@ directive genqlient(
   # zero value and null (for nullable fields).
   pointer: Boolean
 
+  # If set, this field will use a struct type in Go, even if it's an interface.
+  #
+  # This is useful when you have a query like
+  #  query MyQuery {
+  #    myInterface { myField }
+  #  }
+  # where you are requesting only shared fields of an interface.  By default,
+  # genqlient still generates an interface type, for consistency.  But this
+  # isn't necessary: a struct would do just fine since there are no
+  # type-specific fields.  Setting `struct: true` tells genqlient to do that.
+  #
+  # Note that this is only allowed when there are no fragments in play, such
+  # that all fields are on the interface type.  Note that if you later add a
+  # fragment, you'll have to remove this option, and the types will change.
+  struct: Boolean
+
   # If set, this argument or field will use the given Go type instead of a
   # genqlient-generated type.
   #

generate/convert.go

@@ -175,7 +175,14 @@ func (g *generator) convertDefinition(
 		GraphQLName:        def.Name,
 	}
 
-	switch def.Kind {
+	// The struct option basically means "treat this as if it were an object".
+	// (It only applies if valid; this is important if you said the whole
+	// query should have `struct: true`.)
+	kind := def.Kind
+	if options.GetStruct() && validateStructOption(def, selectionSet, pos) == nil {
+		kind = ast.Object
+	}
+	switch kind {
 	case ast.Object:
 		name := makeTypeName(namePrefix, def.Name)
 
@@ -463,7 +470,7 @@ func (g *generator) convertInlineFragment(
 	containingTypedef *ast.Definition,
 	queryOptions *genqlientDirective,
 ) ([]*goStructField, error) {
-	// You might think fragmentTypedef would be a fragment.ObjectDefinition, but
+	// You might think fragmentTypedef is just fragment.ObjectDefinition, but
 	// actually that's the type into which the fragment is spread.
 	fragmentTypedef := g.schema.Types[fragment.TypeCondition]
 	if !fragmentMatches(containingTypedef, fragmentTypedef) {

generate/generate.go

@@ -209,6 +209,8 @@ func (g *generator) preprocessQueryDocument(doc *ast.QueryDocument) {
 	// needed).  Note this does mean abstract-typed fragments spread into
 	// object-typed scope will *not* have access to `__typename`, but they
 	// indeed don't need it, since we do know the type in that context.
+	// TODO(benkraft): We should omit __typename if you asked for
+	// `# @genqlient(struct: true)`.
 	observers.OnField(func(_ *validator.Walker, field *ast.Field) {
 		// We are interested in a field from the query like
 		//	field { subField ... }

generate/genqlient_directive.go

@@ -14,11 +14,13 @@ type genqlientDirective struct {
 	pos       *ast.Position
 	Omitempty *bool
 	Pointer   *bool
+	Struct    *bool
 	Bind      string
 }
 
 func (dir *genqlientDirective) GetOmitempty() bool { return dir.Omitempty != nil && *dir.Omitempty }
 func (dir *genqlientDirective) GetPointer() bool   { return dir.Pointer != nil && *dir.Pointer }
+func (dir *genqlientDirective) GetStruct() bool    { return dir.Struct != nil && *dir.Struct }
 
 func setBool(dst **bool, v *ast.Value) error {
 	ei, err := v.Value(nil) // no vars allowed
@@ -44,15 +46,15 @@ func setString(dst *string, v *ast.Value) error {
 	return errorf(v.Position, "expected string, got non-string value %T(%v)", ei, ei)
 }
 
-func fromGraphQL(dir *ast.Directive) (*genqlientDirective, error) {
+func fromGraphQL(dir *ast.Directive, pos *ast.Position) (*genqlientDirective, error) {
 	if dir.Name != "genqlient" {
 		// Actually we just won't get here; we only get here if the line starts
 		// with "# @genqlient", unless there's some sort of bug.
-		return nil, errorf(dir.Position, "the only valid comment-directive is @genqlient, got %v", dir.Name)
+		return nil, errorf(pos, "the only valid comment-directive is @genqlient, got %v", dir.Name)
 	}
 
 	var retval genqlientDirective
-	retval.pos = dir.Position
+	retval.pos = pos
 
 	var err error
 	for _, arg := range dir.Arguments {
@@ -62,10 +64,12 @@ func fromGraphQL(dir *ast.Directive) (*genqlientDirective, error) {
 			err = setBool(&retval.Omitempty, arg.Value)
 		case "pointer":
 			err = setBool(&retval.Pointer, arg.Value)
+		case "struct":
+			err = setBool(&retval.Struct, arg.Value)
 		case "bind":
 			err = setString(&retval.Bind, arg.Value)
 		default:
-			return nil, errorf(arg.Position, "unknown argument %v for @genqlient", arg.Name)
+			return nil, errorf(pos, "unknown argument %v for @genqlient", arg.Name)
 		}
 		if err != nil {
 			return nil, err
@@ -74,7 +78,7 @@ func fromGraphQL(dir *ast.Directive) (*genqlientDirective, error) {
 	return &retval, nil
 }
 
-func (dir *genqlientDirective) validate(node interface{}) error {
+func (dir *genqlientDirective) validate(node interface{}, schema *ast.Schema) error {
 	switch node := node.(type) {
 	case *ast.OperationDefinition:
 		if dir.Bind != "" {
@@ -90,24 +94,69 @@ func (dir *genqlientDirective) validate(node interface{}) error {
 			return errorf(dir.pos, "bind is not implemented for named fragments")
 		}
 
+		if dir.Struct != nil {
+			return errorf(dir.pos, "struct is only applicable to fields")
+		}
+
 		// Like operations, anything else will just apply to the entire
 		// fragment.
 		return nil
 	case *ast.VariableDefinition:
 		if dir.Omitempty != nil && node.Type.NonNull {
 			return errorf(dir.pos, "omitempty may only be used on optional arguments")
 		}
+
+		if dir.Struct != nil {
+			return errorf(dir.pos, "struct is only applicable to fields")
+		}
+
 		return nil
 	case *ast.Field:
 		if dir.Omitempty != nil {
 			return errorf(dir.pos, "omitempty is not applicable to fields")
 		}
+
+		if dir.Struct != nil {
+			typ := schema.Types[node.Definition.Type.Name()]
+			if err := validateStructOption(typ, node.SelectionSet, dir.pos); err != nil {
+				return err
+			}
+		}
+
 		return nil
 	default:
 		return errorf(dir.pos, "invalid @genqlient directive location: %T", node)
 	}
 }
 
+func validateStructOption(
+	typ *ast.Definition,
+	selectionSet ast.SelectionSet,
+	pos *ast.Position,
+) error {
+	if typ.Kind != ast.Interface && typ.Kind != ast.Union {
+		return errorf(pos, "struct is only applicable to interface-typed fields")
+	}
+
+	// Make sure that all the requested fields apply to the interface itself
+	// (not just certain implementations).
+	for _, selection := range selectionSet {
+		switch selection.(type) {
+		case *ast.Field:
+			// fields are fine.
+		case *ast.InlineFragment, *ast.FragmentSpread:
+			// Fragments aren't allowed. In principle we could allow them under
+			// the condition that the fragment applies to the whole interface
+			// (not just one implementation; and so on recursively), and for
+			// fragment spreads additionally that the fragment has the same
+			// option applied to it, but it seems more trouble than it's worth
+			// right now.
+			return errorf(pos, "struct is not allowed for types with fragments")
+		}
+	}
+	return nil
+}
+
 func (dir *genqlientDirective) merge(other *genqlientDirective) *genqlientDirective {
 	retval := *dir
 	if other.Omitempty != nil {
@@ -116,6 +165,9 @@ func (dir *genqlientDirective) merge(other *genqlientDirective) *genqlientDirect
 	if other.Pointer != nil {
 		retval.Pointer = other.Pointer
 	}
+	if other.Struct != nil {
+		retval.Struct = other.Struct
+	}
 	if other.Bind != "" {
 		retval.Bind = other.Bind
 	}
@@ -141,11 +193,11 @@ func (g *generator) parsePrecedingComment(
 			if err != nil {
 				return "", nil, err
 			}
-			genqlientDirective, err := fromGraphQL(graphQLDirective)
+			genqlientDirective, err := fromGraphQL(graphQLDirective, pos)
 			if err != nil {
 				return "", nil, err
 			}
-			err = genqlientDirective.validate(node)
+			err = genqlientDirective.validate(node, g.schema)
 			if err != nil {
 				return "", nil, err
 			}

generate/testdata/errors/StructOptionOnObject.graphql

@@ -0,0 +1,6 @@
+query StructOptionOnObject {
+  # @genqlient(struct: true)
+  myObject {
+    f
+  }
+}

generate/testdata/errors/StructOptionOnObject.schema.graphql

@@ -0,0 +1,7 @@
+type Query {
+  myObject: MyObject
+}
+
+type MyObject {
+  f: String!
+}

generate/testdata/errors/StructOptionWithFragments.graphql

@@ -0,0 +1,9 @@
+query StructOptionOnObject {
+  # @genqlient(struct: true)
+  myInterface {
+    f
+    ... on MyObject {
+      g
+    }
+  }
+}

generate/testdata/errors/StructOptionWithFragments.schema.graphql

@@ -0,0 +1,16 @@
+type Query {
+  myInterface: MyInterface
+}
+
+interface MyInterface {
+  f: String!
+}
+
+type MyObject implements MyInterface {
+  f: String!
+  g: String!
+}
+
+type OtherObject implements MyInterface {
+  f: String!
+}

generate/testdata/queries/StructOption.graphql

@@ -0,0 +1,24 @@
+fragment VideoFields on Video { duration }
+
+# @genqlient(struct: true)
+query StructOption {
+  root {
+    id
+    children {
+      id
+      parent {
+        id
+        children {
+          id
+        }
+        # (it won't apply to this)
+        interfaceChildren: children {
+          id
+          ...VideoFields
+        }
+      }
+    }
+  }
+  # (nor this)
+  user { roles }
+}