Start Input Union RFC document (#584)

* Start Input Union RFC document

* adopt common example types

* add order based solution; clean-up formatting

* add tagged union

rfcs/InputUnion.md

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+RFC: GraphQL Input Union
+----------
+
+The addition of an Input Union type has been discussed in the GraphQL community for many years now. The value of this feature has largely been agreed upon, but the implementation has not.
+
+This document attempts to bring together all the various solutions that have been discussed with the goal of reaching a shared understanding of the problem space.
+
+From that shared understanding, this document can then evolve into a proposed solution.
+
+## Possible Solutions
+
+Categories:
+
+* Value-based discriminator field
+* Structural discrimination
+
+### Value-based discriminator field
+
+These options rely the **value** of a specific input field to express the concrete type.
+
+#### Single `__typename` field; value is the `type`
+
+This solution was discussed in https://github.com/graphql/graphql-spec/pull/395
+
+```graphql
+input AddPostInput {
+  title: String!
+  body: String!
+}
+input AddImageInput {
+  title: String!
+  photo: String!
+  caption: String
+}
+
+inputUnion AddMediaBlockInput = AddPostInput | AddImageInput
+
+type Mutation {
+   addContent(content: AddMediaBlockInput!): Content
+}
+
+# Variables:
+{
+  content: {
+    "__typename": "AddPostInput",
+    title: "Title",
+    body: "body..."
+  }
+}
+```
+
+##### Variations:
+
+* A `default` annotation may be provided, for which specifying the `__typename` is not required. This enables a field migration from an `Input` to an `Input Union`
+
+#### Single user-chosen field; value is the `type`
+
+```graphql
+input AddPostInput {
+  kind: <AddMediaBlockInput>
+  title: String!
+  body: String!
+}
+input AddImageInput {
+  kind: <AddMediaBlockInput>
+  title: String!
+  photo: String!
+  caption: String
+}
+
+inputUnion AddMediaBlockInput = AddPostInput | AddImageInput
+
+type Mutation {
+   addContent(content: AddMediaBlockInput!): Content
+}
+
+# Variables:
+{
+  content: {
+    kind: "AddPostInput",
+    title: "Title",
+    body: "body..."
+  }
+}
+```
+
+##### Problems:
+
+* The discriminator field is non-sensical if the input is used _outside_ of an input union.
+
+#### Single user-chosen field; value is a literal
+
+This solution is derrived from one discussed in https://github.com/graphql/graphql-spec/issues/488
+
+```graphql
+enum MediaType {
+  POST
+  IMAGE
+}
+input AddPostInput {
+  kind: MediaType::POST
+  title: String!
+  body: String!
+}
+input AddImageInput {
+  kind: MediaType::IMAGE
+  title: String!
+  photo: String!
+  caption: String
+}
+
+inputUnion AddMediaBlockInput = AddPostInput | AddImageInput
+
+type Mutation {
+   addContent(content: AddMediaBlockInput!): Content
+}
+
+# Variables:
+{
+  content: {
+    kind: "POST",
+    title: "Title",
+    body: "body..."
+  }
+}
+```
+
+##### Variations:
+
+* Literal strings used instead of an `enum`
+
+```graphql
+input AddPostInput {
+  kind: 'post'
+  title: String!
+  body: String!
+}
+input AddImageInput {
+  kind: 'image'
+  title: String!
+  photo: String!
+  caption: String
+}
+```
+
+##### Problems:
+
+* The discriminator field is redundant if the input is used _outside_ of an input union.
+
+### Structural discrimination
+
+These options rely on the **structure** of the input to determine the concrete type.
+
+#### Order based type matching
+
+The concrete type is the first type in the input union definition that matches.
+
+```graphql
+input AddPostInput {
+  title: String!
+  publishedAt: Int
+  body: String
+}
+input AddImageInput {
+  title: String!
+  publishedAt: Int
+  photo: String
+  caption: String
+}
+
+inputUnion AddMediaBlockInput = AddPostInput | AddImageInput
+
+type Mutation {
+   addContent(content: AddMediaBlockInput!): Content
+}
+
+# Variables:
+{
+  content: {
+    title: "Title",
+    date: 1558066429
+    # AddPostInput
+  }
+}
+{
+  content: {
+    title: "Title",
+    date: 1558066429
+    photo: "photo.png"
+    # AddImageInput
+  }
+}
+```
+
+#### Structural uniqueness
+
+Schema Rule: Each type in the union must have a unique set of required field names
+
+```graphql
+input AddPostInput {
+  title: String!
+  body: String!
+}
+input AddImageInput {
+  photo: String!
+  caption: String
+}
+
+inputUnion AddMediaBlockInput = AddPostInput | AddImageInput
+
+type Mutation {
+   addContent(content: AddMediaBlockInput!): Content
+}
+
+# Variables:
+{
+  content: {
+    title: "Title",
+    body: "body..."
+    # AddPostInput
+  }
+}
+```
+
+An invalid schema:
+
+```graphql
+input AddPostInput {
+  title: String!
+  body: String!
+}
+input AddDatedPostInput {
+  title: String!
+  body: String!
+  date: Int
+}
+input AddImageInput {
+  photo: String!
+  caption: String
+}
+
+inputUnion AddMediaBlockInput = AddPostInput | AddDatedPostInput | AddImageInput
+
+type Mutation {
+   addContent(content: AddMediaBlockInput!): Content
+}
+```
+
+##### Problems:
+
+* Optional fields could prevent determining a unique type
+
+```graphql
+input AddPostInput {
+  title: String!
+  body: String!
+  date: Int
+}
+input AddDatedPostInput {
+  title: String!
+  body: String!
+  date: Int!
+}
+```
+
+Workaround? : Each type's set of required fields must be uniquely identifying
+
+  - A type's set of required field names must not match the set of another type's required field names
+  - A type's set of required field names must not overlap with the set of another type's required or optional field names
+
+Workaround? : Each type must have at least one unique required field
+
+  - A type must contain one required field that is not a field in any other type
+
+##### Variations:
+
+* Consider the field _type_ along with the field _name_ when determining uniqueness.
+
+#### One Of (Tagged Union)
+
+This solution was presented in https://github.com/graphql/graphql-spec/pull/395#issuecomment-361373097
+
+The type is determined by using an intermediate input type that maps field name to type.
+
+A directive has also been discussed to specify that only one of the fields may be selected. See https://github.com/graphql/graphql-spec/pull/586.
+
+```graphql
+input AddPostInput {
+  title: String!
+  body: String!
+}
+input AddImageInput {
+  photo: String!
+  caption: String
+}
+input AddMediaBlockInput @oneOf {
+  post: AddPostInput
+  image: AddImageInput
+}
+
+type Mutation {
+   addContent(content: AddMediaBlockInput!): Content
+}
+
+# Variables:
+{
+  content: {
+    post: {
+      title: "Title",
+      body: "body..."
+    }
+  }
+}
+```