Query level nullability review and drafting thread

rfcs/QueryLevelNullability.md

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+# RFC: Query Level Nullability 
+
+**Proposed by:** 
+- [Liz Jakubowski](https://github.com/lizjakubowski) - Yelp iOS
+- [Sanae Rosen](<social or github link here>) - Yelp Android
+- [Mark Larah](https://github.com/magicmark) - Yelp Web
+- [Alex Reilly](<social or github link here>) - Yelp iOS
+- [Wei Xue](<social or github link here>) - Yelp iOS
+
+This RFC proposes a syntactical construct for GraphQL clients to express the **nullability** of schema fields requested in a query.
+
+## Definitions
+
+Nullability: A concept that exists across many programming languages (eg [Swift](https://developer.apple.com/documentation/swift/optional), [Kotlin](https://kotlinlang.org/docs/null-safety.html#nullable-types-and-non-null-types), [SQL](https://www.w3schools.com/sql/sql_notnull.asp))
+that is used to express when users can be certain that a value can or can never be `null` 
+(or the language equivalent). Nullability language constructs (eg `?` in Swift/Kotlin)
+have become popular due to their ability to solve ergonomic problems in languages
+such as those surrounding `NullPointerException` in Java.
+
+Codegen: Short for Code Generation, tools that exist for working with GraphQL which accept a schema and a set of documents as
+input, and output code in a language of choice that represents the data returned by those 
+operations. GraphQL codegen tools exist for many platforms. As an example, [here's some info about Apollo's offerings](https://github.com/apollographql/apollo-tooling#code-generation).
+
+## 📜 Problem Statement
+
+GraphQL is a "nullable by default" language meaning that all properties are allowed to be `null`.
+This is in contrast to the two modern languages used on mobile clients, Swift and Kotlin,
+which are both non-null by default. In Swift and Kotlin, unless developers otherwise
+specify it, properties cannot be `null`.
+
+This mismatch creates some dissonance for developers who are currently forced to deal with
+problems that commonly surround nullablility. This makes large numbers of null fields tedious and
+time-consuming to work with.
+
+It is often unclear how to handle partial results.
+- What elements are essential to having an adequate user experience? How many elements can fail before you 
+  should just show an error message instead?
+- When any combination of elements can fail, it is very hard to anticipate all edge cases and how the app 
+  might look and work, including transitions to other screens.
+
+In Yelp's GraphQL schema, almost all object fields are nullable except for those with ID type. 
+This adheres to what seems to be the [official best practice](https://graphql.org/learn/best-practices/#nullability).
+
+This poses a problem for GraphQL clients that use codegen. The codegen provides 
+Swift/Kotlin types to represent queries used in the app. Nullable fields in the schema are represented
+as nullable properties on the resulting type, represented by `?` following the type name:
+```graphql
+query GetBusinessName($encid: String!) {
+  business(encid: $encid) {
+    name
+  }
+}
+```
+```swift
+struct GetBusinessNameQuery.Data.Business {
+  let name: String?
+}
+```
+In many cases, the client should error if the business `name` is null. If codegen were out of the picture,
+we would be able to throw an error at JSON response-parsing time if it's missing, or otherwise instantiate
+a hand-written business object with a non-nullable `name` property. From that point on, all feature code
+can happily assume that it has a non-null business name to work with.
+
+## 🧑‍💻 Proposed syntax
+
+It would make more sense if the client could express that `name` must be non-null _in the query itself_:
+```graphql
+query GetBusinessName($encid: String!) {
+  business(encid: $encid) {
+    name!  <-- this!
+  }
+}
+```
+Semantically the GraphQL `!` operator is nearly identical to its counterpart in Swift (also represented by `!`) which is
+referred to as the "force unwrap operator". In Swift, for example, you can cast a string to an integer with `Int("5")` 
+but the string being cast may not be a valid number, so that statement will return `null` rather than an integer if the
+string cannot be turned into an integer. If you want to ensure that the statement does not return `null` you can instead 
+write `Int("5")!`. If you do that, an exception will be thrown if the statement would return `null`.
+
+In GraphQL, the `!` operator will act similarly. In the case that `name` does not exist, the query will return an
+error to the client rather than any data.
+
+On web where codegen is not used, the client no longer needs to handle the case where expected fields are missing.
+On mobile platforms where codegen is used, clients have full control over the nullability of the properties on the
+generated types. Since nullability is expressed in the query rather than the schema, it's flexible enough to accommodate
+various use-cases (e.g., where the business `name` _is_ allowed to be nullable).
+
+### `!`
+
+We have chosen `!` because `!` is already being used in the GraphQL spec to indicate that a field is non-nullable.
+Incidentally the same precedent exists in Swift (`!`) and Kotlin (`!!`) which both use similar syntax to indicate
+"throw an exception if this value is `null`". 
+
+### Use cases
+
+#### When a field is necessary to the function of the client
+There are times where a field can be `null`, but a feature that queries for the field will not function
+if it is `null`. For example if you are trying to render an information page for a movie, you won't
+be able to do that if the name field of the movie is missing. In that case it would be preferable
+to fail as early as possible.
+
+### ✨ Examples
+
+#### Non-nullable field
+```graphql
+query GetBusinessName($encid: String!) {
+  business(encid: $encid) {
+    name!
+  }
+}
+```
+would codegen to the following type on iOS.
+
+```swift
+struct GetBusinessNameQuery.Data.Business {
+  let name: String // lack of `?` indicates that `name` will never be `null`
+}
+```
+
+#### Non-nullable object with nullable fields
+```graphql
+query {
+  business(id: 4) {
+    reviews! {
+      rating
+      text
+    }
+  }
+}
+```
+
+## ✅ RFC Goals
+- Non-nullable syntax that is based off of syntax that developers will already be familiar with
+
+## 🚫 RFC Non-goals
+
+## 🗳️ Alternatives considered
+
+### Make Schema Fields Non-Nullable Instead
+Discussion on [this topic can be found here](https://medium.com/@calebmer/when-to-use-graphql-non-null-fields-4059337f6fc8)
+// We definitely want to flesh this out
+
+### Use a directive such as `@non-null` instead
+This is the alternative being used at some of the companies represented in this proposal for the time being.
+However this feels like a common enough need to call for a language feature. A single language feature
+rather than using directives as a workaround would enable more unified public tooling around GraphQL.
+
+### Write wrapper types that null-check fields
+This is the alternative being used at some of the companies represented in this proposal for the time being.
+It's quite labor intensive and the work is quite rote. It more or less undermines the purpose of
+having code generation.
+
+### Alternatives to `!`
+#### `!!`
+This would follow the precedent set by Kotlin.
+
+### Make non-nullability apply recursively
+For example, everything in this tree would be non-nullable
+```graphql
+query! {
+  business(id: 4) {
+    name
+  }
+}
+```
+// TODO: This was rejected when it was proposed. Looking for more info as to why
+
+## 🙅 Syntax Non-goals
+
+This syntax consciously does not cover the following use cases:
+
+- **Default Values**
+  The syntax being used in this proposal causes queries to error out in the case that
+  a `null` is found. As an alternative, some languages provide syntax (eg `??` for Swift)
+  that says "if a value would be `null` make it some other value instead". We are
+  not interested in covering that in this proposal.
+
+## Implementation
+GraphQL.js: https://github.yelpcorp.com/wxue/graphql-js