Refactor argument-handling to use a struct (#103)

## Summary:
In this commit I refactor the argument-generation logic to move most of
the code out of the template and into the type-generator.  This logic
predates #51, and I didn't think to update it there, but I think it
benefits from similar treatment, for similar reasons.

Specifically, the main change is to treat variables as another struct
type we can generate, rather than handling them inline as a
`map[string]interface{}`.  Users still pass them the same way, but
instead of putting them into a `map[string]interface{}` and JSONifying
that, we generate a struct and put them there.

This turns out to simplify things quite a lot, because we already have a
lot of code to generate types.  Notably, the omitempty code goes from a
dozen lines to basically two, and fixes a bug (#43) in the process,
because now that we have a struct, `json.Marshal` will do our work for
us! (And, once we have syntax for it (#14), we'll be able to handle
field-level omitempty basically for free.)  More importantly, it will
simplify custom marshalers (#38, forthcoming) significantly, since we do
all that logic at the containing-struct level, but will need to apply it
to arguments.

It does require two breaking changes:

1. For folks implementing the `graphql.Client` API (rather than just
   calling `NewClient`): we now pass them variables as an `interface{}`
   rather than a `map[string]interface{}`.  For most callers, including
   Khan/webapp, this is basically a one-line change to the signature of
   their `MakeRequest`, and it should be a lot more future-proof.
2. genqlient's handling of the `omitempty` option has changed to match
   that of `encoding/json`, in particular it now never considers structs
   "empty".  The difference was never intentional (I just didn't realize
   that behavior of `encoding/json`); arguably our behavior was more
   useful but I think that's outweighed by the value of consistency with
   `encoding/json` as well as the simpler and more correct
   implementation (fixing #43 is actually quite nontrivial otherwise).
   Once we have custom unmarshaler support (#38), users will be able to
   map a zero value to JSON null if they wish, which is mostly if not
   entirely equivalent for GraphQL's purposes.

Issue: https://github.com/Khan/genqlient/issues/38
Issue: https://github.com/Khan/genqlient/issues/43

## Test plan:
make check

Author: benjaminjkraft

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

Required Reviewers: 

Approved By: StevenACoffman, dnerdy

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

Pull Request URL: https://github.com/Khan/genqlient/pull/103

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

@@ -8,6 +8,12 @@ import (
 	"github.com/Khan/genqlient/graphql"
 )
 
+// __convertTimezoneInput is used internally by genqlient
+type __convertTimezoneInput struct {
+	Dt time.Time `json:"dt"`
+	Tz string    `json:"tz"`
+}
+
 // convertTimezoneResponse is returned by convertTimezone on success.
 type convertTimezoneResponse struct {
 	Convert time.Time `json:"convert"`
@@ -18,11 +24,10 @@ func convertTimezone(
 	dt time.Time,
 	tz string,
 ) (*convertTimezoneResponse, error) {
-	variables := map[string]interface{}{
-		"dt": dt,
-		"tz": tz,
+	__input := __convertTimezoneInput{
+		Dt: dt,
+		Tz: tz,
 	}
-
 	var err error
 
 	var retval convertTimezoneResponse
@@ -35,7 +40,7 @@ query convertTimezone ($dt: DateTime!, $tz: String) {
 }
 `,
 		&retval,
-		variables,
+		&__input,
 	)
 	return &retval, err
 }

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

@@ -38,14 +38,18 @@ const (
 	RoleTeacher Role = "TEACHER"
 )
 
+// __InputEnumQueryInput is used internally by genqlient
+type __InputEnumQueryInput struct {
+	Role Role `json:"role"`
+}
+
 func InputEnumQuery(
 	client graphql.Client,
 	role Role,
 ) (*InputEnumQueryResponse, error) {
-	variables := map[string]interface{}{
-		"role": role,
+	__input := __InputEnumQueryInput{
+		Role: role,
 	}
-
 	var err error
 
 	var retval InputEnumQueryResponse
@@ -60,7 +64,7 @@ query InputEnumQuery ($role: Role!) {
 }
 `,
 		&retval,
-		variables,
+		&__input,
 	)
 	return &retval, err
 }

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

@@ -56,14 +56,18 @@ type UserQueryInput struct {
 	HasPokemon testutil.Pokemon `json:"hasPokemon"`
 }
 
+// __InputObjectQueryInput is used internally by genqlient
+type __InputObjectQueryInput struct {
+	Query UserQueryInput `json:"query"`
+}
+
 func InputObjectQuery(
 	client graphql.Client,
 	query UserQueryInput,
 ) (*InputObjectQueryResponse, error) {
-	variables := map[string]interface{}{
-		"query": query,
+	__input := __InputObjectQueryInput{
+		Query: query,
 	}
-
 	var err error
 
 	var retval InputObjectQueryResponse
@@ -78,7 +82,7 @@ query InputObjectQuery ($query: UserQueryInput) {
 }
 `,
 		&retval,
-		variables,
+		&__input,
 	)
 	return &retval, err
 }

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

@@ -27,14 +27,18 @@ type ListInputQueryUser struct {
 	Id testutil.ID `json:"id"`
 }
 
+// __ListInputQueryInput is used internally by genqlient
+type __ListInputQueryInput struct {
+	Names []string `json:"names"`
+}
+
 func ListInputQuery(
 	client graphql.Client,
 	names []string,
 ) (*ListInputQueryResponse, error) {
-	variables := map[string]interface{}{
-		"names": names,
+	__input := __ListInputQueryInput{
+		Names: names,
 	}
-
 	var err error
 
 	var retval ListInputQueryResponse
@@ -49,7 +53,7 @@ query ListInputQuery ($names: [String]) {
 }
 `,
 		&retval,
-		variables,
+		&__input,
 	)
 	return &retval, err
 }

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

@@ -72,6 +72,15 @@ type UserQueryInput struct {
 	HasPokemon testutil.Pokemon `json:"hasPokemon"`
 }
 
+// __OmitEmptyQueryInput is used internally by genqlient
+type __OmitEmptyQueryInput struct {
+	Query         UserQueryInput   `json:"query,omitempty"`
+	Queries       []UserQueryInput `json:"queries,omitempty"`
+	Dt            time.Time        `json:"dt,omitempty"`
+	Tz            string           `json:"tz,omitempty"`
+	TzNoOmitEmpty string           `json:"tzNoOmitEmpty"`
+}
+
 func OmitEmptyQuery(
 	client graphql.Client,
 	query UserQueryInput,
@@ -80,29 +89,13 @@ func OmitEmptyQuery(
 	tz string,
 	tzNoOmitEmpty string,
 ) (*OmitEmptyQueryResponse, error) {
-	variables := map[string]interface{}{
-		"tzNoOmitEmpty": tzNoOmitEmpty,
+	__input := __OmitEmptyQueryInput{
+		Query:         query,
+		Queries:       queries,
+		Dt:            dt,
+		Tz:            tz,
+		TzNoOmitEmpty: tzNoOmitEmpty,
 	}
-
-	var zero_query UserQueryInput
-	if query != zero_query {
-		variables["query"] = query
-	}
-
-	if len(queries) > 0 {
-		variables["queries"] = queries
-	}
-
-	var zero_dt time.Time
-	if dt != zero_dt {
-		variables["dt"] = dt
-	}
-
-	var zero_tz string
-	if tz != zero_tz {
-		variables["tz"] = tz
-	}
-
 	var err error
 
 	var retval OmitEmptyQueryResponse
@@ -122,7 +115,7 @@ query OmitEmptyQuery ($query: UserQueryInput, $queries: [UserQueryInput], $dt: D
 }
 `,
 		&retval,
-		variables,
+		&__input,
 	)
 	return &retval, err
 }

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

@@ -79,18 +79,24 @@ type UserQueryInput struct {
 	HasPokemon *testutil.Pokemon `json:"hasPokemon"`
 }
 
+// __PointersQueryInput is used internally by genqlient
+type __PointersQueryInput struct {
+	Query *UserQueryInput `json:"query"`
+	Dt    time.Time       `json:"dt"`
+	Tz    *string         `json:"tz"`
+}
+
 func PointersQuery(
 	client graphql.Client,
-	query UserQueryInput,
+	query *UserQueryInput,
 	dt time.Time,
-	tz string,
+	tz *string,
 ) (*PointersQueryResponse, error) {
-	variables := map[string]interface{}{
-		"query": query,
-		"dt":    dt,
-		"tz":    tz,
+	__input := __PointersQueryInput{
+		Query: query,
+		Dt:    dt,
+		Tz:    tz,
 	}
-
 	var err error
 
 	var retval PointersQueryResponse
@@ -113,7 +119,7 @@ query PointersQuery ($query: UserQueryInput, $dt: DateTime, $tz: String) {
 }
 `,
 		&retval,
-		variables,
+		&__input,
 	)
 	return &retval, err
 }

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

@@ -79,18 +79,24 @@ type UserQueryInput struct {
 	HasPokemon testutil.Pokemon `json:"hasPokemon"`
 }
 
+// __PointersQueryInput is used internally by genqlient
+type __PointersQueryInput struct {
+	Query *UserQueryInput `json:"query"`
+	Dt    *time.Time      `json:"dt"`
+	Tz    string          `json:"tz"`
+}
+
 func PointersQuery(
 	client graphql.Client,
 	query *UserQueryInput,
 	dt *time.Time,
 	tz string,
 ) (*PointersQueryResponse, error) {
-	variables := map[string]interface{}{
-		"query": query,
-		"dt":    dt,
-		"tz":    tz,
+	__input := __PointersQueryInput{
+		Query: query,
+		Dt:    dt,
+		Tz:    tz,
 	}
-
 	var err error
 
 	var retval PointersQueryResponse
@@ -113,7 +119,7 @@ query PointersQuery ($query: UserQueryInput, $dt: DateTime, $tz: String) {
 }
 `,
 		&retval,
-		variables,
+		&__input,
 	)
 	return &retval, err
 }

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

@@ -37,14 +37,18 @@ type GetPokemonSiblingsUserGenqlientPokemon struct {
 	Level   int    `json:"level"`
 }
 
+// __GetPokemonSiblingsInput is used internally by genqlient
+type __GetPokemonSiblingsInput struct {
+	Input testutil.Pokemon `json:"input"`
+}
+
 func GetPokemonSiblings(
 	client graphql.Client,
 	input testutil.Pokemon,
 ) (*GetPokemonSiblingsResponse, error) {
-	variables := map[string]interface{}{
-		"input": input,
+	__input := __GetPokemonSiblingsInput{
+		Input: input,
 	}
-
 	var err error
 
 	var retval GetPokemonSiblingsResponse
@@ -69,7 +73,7 @@ query GetPokemonSiblings ($input: PokemonInput!) {
 }
 `,
 		&retval,
-		variables,
+		&__input,
 	)
 	return &retval, err
 }

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

@@ -36,14 +36,18 @@ type RecursiveInput struct {
 	Rec []RecursiveInput `json:"rec"`
 }
 
+// __RecursionInput is used internally by genqlient
+type __RecursionInput struct {
+	Input RecursiveInput `json:"input"`
+}
+
 func Recursion(
 	client graphql.Client,
 	input RecursiveInput,
 ) (*RecursionResponse, error) {
-	variables := map[string]interface{}{
-		"input": input,
+	__input := __RecursionInput{
+		Input: input,
 	}
-
 	var err error
 
 	var retval RecursionResponse
@@ -64,7 +68,7 @@ query Recursion ($input: RecursiveInput!) {
 }
 `,
 		&retval,
-		variables,
+		&__input,
 	)
 	return &retval, err
 }

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

@@ -27,14 +27,18 @@ type SimpleInputQueryUser struct {
 	Id testutil.ID `json:"id"`
 }
 
+// __SimpleInputQueryInput is used internally by genqlient
+type __SimpleInputQueryInput struct {
+	Name string `json:"name"`
+}
+
 func SimpleInputQuery(
 	client graphql.Client,
 	name string,
 ) (*SimpleInputQueryResponse, error) {
-	variables := map[string]interface{}{
-		"name": name,
+	__input := __SimpleInputQueryInput{
+		Name: name,
 	}
-
 	var err error
 
 	var retval SimpleInputQueryResponse
@@ -49,7 +53,7 @@ query SimpleInputQuery ($name: String!) {
 }
 `,
 		&retval,
-		variables,
+		&__input,
 	)
 	return &retval, err
 }

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

@@ -24,6 +24,11 @@ type SimpleMutationResponse struct {
 	CreateUser SimpleMutationCreateUser `json:"createUser"`
 }
 
+// __SimpleMutationInput is used internally by genqlient
+type __SimpleMutationInput struct {
+	Name string `json:"name"`
+}
+
 // SimpleMutation creates a user.
 //
 // It has a long doc-comment, to test that we handle that correctly.
@@ -32,10 +37,9 @@ func SimpleMutation(
 	client graphql.Client,
 	name string,
 ) (*SimpleMutationResponse, error) {
-	variables := map[string]interface{}{
-		"name": name,
+	__input := __SimpleMutationInput{
+		Name: name,
 	}
-
 	var err error
 
 	var retval SimpleMutationResponse
@@ -51,7 +55,7 @@ mutation SimpleMutation ($name: String!) {
 }
 `,
 		&retval,
-		variables,
+		&__input,
 	)
 	return &retval, err
 }

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

@@ -36,6 +36,11 @@ type UserQueryInput struct {
 	HasPokemon testutil.Pokemon `json:"hasPokemon"`
 }
 
+// __unexportedInput is used internally by genqlient
+type __unexportedInput struct {
+	Query UserQueryInput `json:"query"`
+}
+
 // unexportedResponse is returned by unexported on success.
 type unexportedResponse struct {
 	// user looks up a user by some stuff.
@@ -60,10 +65,9 @@ func unexported(
 	client graphql.Client,
 	query UserQueryInput,
 ) (*unexportedResponse, error) {
-	variables := map[string]interface{}{
-		"query": query,
+	__input := __unexportedInput{
+		Query: query,
 	}
-
 	var err error
 
 	var retval unexportedResponse
@@ -78,7 +82,7 @@ query unexported ($query: UserQueryInput) {
 }
 `,
 		&retval,
-		variables,
+		&__input,
 	)
 	return &retval, err
 }