Add support for fragments, part 0: design sketch (#59)

## Summary:
Having implemented support for interfaces, it's time to implement
support for fragments.  And it turns out there's actually another design
decision I hadn't really thought about when thinking about interfaces!
In this commit I add a sketch of the design -- two proposed designs
really.  This one I think will be easier to change later (via a flag),
but opinions are still welcome.

Issue: #8

## Test plan:
read it


Author: benjaminjkraft

Reviewers: dnerdy, benjaminjkraft, aberkan, csilvers, MiguelCastillo

Required Reviewers: 

Approved by: dnerdy

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

Pull request URL: #59

DESIGN.md

@@ -110,13 +110,13 @@ We'll do something similar to Apollo's naming scheme.  Specifically:
 
 All of this may be configurable later.
 
-### How to support fragments and interfaces
+### How to represent interfaces
 
 Consider the following query (suppose that `a` returns interface type `I`, which may be implemented by either `T` or `U`):
 
 ```graphql
-query { a { __typename b ...f } }
-fragment f on T { c d }
+query { a { __typename b ...F } }
+fragment F on T { c d }
 ```
 
 Depending on whether the concrete type returned from `a` is `T`, we can get one of two result structures back:
@@ -128,7 +128,7 @@ Depending on whether the concrete type returned from `a` is `T`, we can get one
 
 The question is: how do we translate that to Go types?
 
-One natural option is to generate a Go type for every concrete GraphQL type the object might have, and simply inline all the fragments.  So here we would have
+**Go interfaces:** one natural option is to generate a Go type for every concrete GraphQL type the object might have, and simply inline or embed all the fragments.  So here we would have
 ```go
 type T struct{ B, C, D string }
 type U struct{ B string }
@@ -147,13 +147,18 @@ Response{A: T{B: "...", C: "...", D: "..."}}
 Response{A: U{B: "..."}}
 ```
 
-Optionally, we might define a getter-method `GetB() string` on `T` and `U`, and include it in `I`, so that if you only need `B` you don't need to type-switch.  We could also define methods `GetC() string` and `GetD() string`, and another interface
+We can also define a getter-method `GetB() string` on `T` and `U`, and include it in `I`, so that if you only need `B` you don't need to type-switch.  Or, if that's not enough, we can also define a common embedded struct corresponding to the interface, so you can extract and pass that around if you want:
+
 ```go
-type F interface { isF(); GetC() string; GetD() string }
+type IEmbed struct { B string }
+type T struct { IEmbed; C, D string }
+type U struct { IEmbed }
+type I interface { isI(); GetIEmbed() IEmbed }
 ```
-so that if you want to use the same fragment `f` to share code in several places, you can do that, because all of the relevant types will implement `F`.  To do that we'll have to have the type-names for fragment fields rooted at the fragment rather than at the query, which is probably preferable anyway since they are guaranteed to be the same for all spreads of the fragment.  (Or we can just do the methods, and let you define the interface, although you still have to root the type-names for the same reason.) 
 
-Another natural option, which looks more like the way `shurcooL/graphql` does things, is to generate a type for each fragment, and only fill in the relevant ones:
+Note that this option gives a few different ways to represent fragments specifically, discussed in the next section.
+
+**Fragment fields:** another natural option, which looks more like the way `shurcooL/graphql` does things, is to generate a type for each fragment, and only fill in the relevant ones:
 ```go
 type Response struct {
     A struct {
@@ -183,21 +188,21 @@ query { a { b } }
 using the same schema as above.  In the former approach, we still define three types (plus two receivers); and `resp.A` is still of an interface type; it might be either `T` or `U`.  In the latter approach, this looks just like any other query: `resp.A` is a `struct{ B string }`.  This has implications for how we use this data: the latter approach lets us just do `resp.A.B`, whereas the former requires we do a type-switch, or add a `GetB()` method to `I`, and do `resp.A.GetB()`.
 
 
-Pros of the first approach:
+**Pros of Go interfaces:**
 
 - it's the most natural translation of how GraphQL does things, and provides the clearest guarantees about what is set when, what is mutually exclusive, etc.
 - you always know what type you got back
 - you always know which fields are there -- you don't have to encode at the application level an assumption that if fragment A was defined, fragment B also will be, because all types that match A also match B
 
-Pros of the second approach:
+**Pros of fragment fields:**
 
 - the types are simpler, and allow the option of unnamed types
 - if you query an interface, but don't care about the types, just the shared fields, you don't even have to think about any of this stuff
 - for callers accessing shared fields (of interfaces or of fragments spread into several places) we avoid having to make them do a type switch or use getter methods
 
 Note that both approaches require that we add `__typename` to every selection set which has fragments (unless the types match exactly).  This seems fine since Apollo client also does so for all selection sets.  We also need to define `UnmarshalJSON` on every type with fragment-spreads; in the former case Go doesn't know how to unmarshal into an interface type, while in the latter the Go type structure is too different from that of the JSON.  (Note that `shurcooL/graphql` actually has its own JSON-decoder to solve this problem.)
 
-A third non-approach is to simplify define all the fields on the same struct, with some optional:
+**Flatten everything:** a third non-approach is to simplify define all the fields on the same struct, with some optional:
 
 ```go
 type Response struct {
@@ -211,7 +216,7 @@ type Response struct {
 
 Apart from being semantically messy, this doesn't have a good way to handle the case where there are types with conflicting fields, e.g.
 
-```
+```graphql
 interface I {}
 type T implements I { f: Int }
 type U implements I { f: String }
@@ -226,15 +231,157 @@ query {
 
 What type is `resp.A.F`?  It has to be both `string` and `int`.
 
-In other libraries:
-- Apollo does basically option 1, except with TypeScript/Flow's better support for sum types.
-- GraphQL Code Generator does basically option 1 (except with sum types instead of interfaces), except with unnamed types.  It definitely generates some ugly types, even with TypeScript/Flow's better support for sum types!
-- Khan's mobile autogen basically does option 1 (again with unnamed types, and sum types).
+**In other libraries:**
+- Apollo does basically the interface approach, except with TypeScript/Flow's better support for sum types.
+- GraphQL Code Generator does basically the interface approach (except with sum types instead of interfaces), except with unnamed types.  It definitely generates some ugly types, even with TypeScript/Flow's better support for sum types!
+- Khan's mobile autogen basically does the interface approach (again with unnamed types, and sum types).
 - gqlgen doesn't have this problem; on the server, fragments and interfaces are handled entirely in the framework and need not even be visible in user-land.
-- shurcooL/graphql uses option 2.
-- protobuf has a similar problem, and uses basically option 1 in Go (even though in other languages it uses something more like option 3).
+- shurcooL/graphql uses fragment fields.
+- protobuf has a similar problem, and uses basically the interface approach in Go (even though in other languages it uses something more like flattening everything).
+
+**Decision:** In general, it seems like the GraphQL Way, and perhaps also the Go Way is to use Go interfaces; I've always found the way shurcooL/graphql handles this to be a bit strange.  So I think it has to be at least the default.  In principle we could allow both though, since fragment fields are legitimately convenient for some cases, especially if you are querying shared fields of interfaces or using fragments as a code-sharing mechanism, since using Go interfaces handles those only through getter methods (although see below).
+
+### How to support fragments
+
+The previous section leaves some parts of how we'll handle fragments ambiguous.  Even within the way it lays things out, we generally have two options for how to represent a fragment.  Consider a query
+
+```graphql
+query MyQuery { a { b ...F } }
+fragment F on T { c d }
+```
+
+We'll have some struct (potentially several structs, one per implementation) representing the type of the value of `a`.  We can handle the fragment one of two ways: we can either flatten it into that type, such that the code is equivalent to writing `query Q { a { b c d } }` (except that `c` and `d` might be included for only some implementations, if `a` returns an interface), or we can represent it as a separate type `F` and embed it in the relevant structs.  (Or it could be a named field of said structs, but this seems to have little benefit, and it's nice to be able to access fields without knowing what fragment they're on.)  When the spread itself is abstract, there are a few sub-cases of the embed option.
+
+To get more concrete, there are [four cases](https://spec.graphql.org/June2018/#sec-Fragment-spread-is-possible) depending on the types in question.  (In all cases below, the methods, and other potential implementations of the interfaces, are elided.  Note also that I'm not sure I got all the type-names right exactly as genqlient would, but they should match approximately.)
+
+**Object spread in object scope:** The simplest spread is when we have an object-typed fragment spread into an object-typed selection.  (The two object types must be the same.)  This is typically used as a code-sharing mechanism.
+
+```graphql
+type Query { a: A }
+type A { b: String, c: String, d: String }
+
+query MyQuery { a { b ...F } }
+fragment F on A { c d }
+```
+
+```go
+// flattened:
+type MyQueryA struct { B, C, D string }
+
+// embedded
+type MyQueryA struct { B string; F }
+type F struct { C, D string }
+```
+
+**Abstract spread in object scope:** We can also spread an interface-typed fragment into an object-typed selection, again as a code-sharing mechanism.  (The object must implement the interface.)
+
+```graphql
+type Query { a: A }
+type A implements I { b: String, c: String, d: String }
+interface I { c: String, d: String }
+
+query MyQuery { a { b ...F } }
+fragment F on I { c d }
+```
+
+```go
+// flattened:
+type MyQueryA struct { B, C, D string }
+
+// embedded:
+type MyQueryA struct { B string; FA }
+type F interface { isF(); GetC() string; GetD() string } // for code-sharing purposes
+type FA struct { C, D string } // implements F
+```
+
+**Object spread in abstract scope:** This is the most common spread, perhaps, where you spread an object-typed fragment into an interface-typed selection in order to request some fields defined on a particular implementation of the interface.  (Again the object must implement the interface.)
+
+```graphql
+type Query { a: I }
+type A implements I { b: String, c: String, d: String }
+type T implements I { b: String, u: String, v: String }
+interface I { b: String }
+
+query MyQuery { a { b ...F ...G } }
+fragment F on A { c d }
+fragment G on A { u v }
+```
+
+```go
+// flattened:
+type MyQueryAI interface { isMyQueryAI(); GetB() string }
+type MyQueryAIA struct { B, C, D string } // implements MyQueryAI
+type MyQueryAIT struct { B, U, V string } // implements MyQueryAI
+
+// embedded:
+type MyQueryAI interface { isMyQueryAI(); GetB() string }
+type MyQueryAIA struct { B string; F } // implements MyQueryAI
+type MyQueryAIT struct { B string; G } // implements MyQueryAI
+type F struct { C, D string }
+type G struct { U, V string }
+```
+
+**Abstract spread in abstract scope:** This is a sort of combination of the last two, where you spread one interface's fragment into another interface, and can be used for code-sharing and/or to conditionally request fields.  Perhaps surprisingly, this is legal any time the two interfaces share an implementation, and neither need implement the other; this means there are arguably four cases of spreading a fragment of type `I` into a scope of type `J`: `I = J` (similar to object-in-object), `I implements J` (similar to abstract-in-object), `J implements I` (similar to object-in-abstract), and none of the above (which is quite rare).
+
+```graphql
+type Query { a: I }
+type A implements I & J { b: String, c: String, d: String }
+type T implements I { b: String }
+type U implements J { c: String, d: String, v: String }
+interface I { b: String }
+interface J { c: String, d: String }
+
+query MyQuery { a { b ...F } }
+fragment F on J { c d }
+```
+
+```go
+// flattened:
+type MyQueryAI interface { isMyQueryAI(); GetB() string }
+type MyQueryAIA struct { B, C, D string } // implements MyQueryAI (and MyQueryAJ if generated)
+type MyQueryAIT struct { B }              // implements MyQueryAI
+
+// embedded:
+type MyQueryAI interface { isMyQueryAI(); GetB() string }
+type MyQueryAIA struct { B string; FA } // implements MyQueryAI
+type MyQueryAIT struct { B string } // implements MyQueryAI
+type F interface { isF(); GetC() string; GetD() string }
+type FA struct { C, D string } // implements F
+type FU struct { C, D string } // implements F (never used; might be omitted)
+// if I == J or I implements J, perhaps additionally:
+type MyQueryAI interface { isMyQueryAI(); GetB() string; F }
+```
+
+Note in this case a third non-approach is
+
+```go
+// does not work:
+type MyQueryAI interface { isMyQueryAI(); GetB() string }
+type MyQueryAIA struct { B string; F } // implements MyQueryAI
+type F struct { C, D string }
+```
+
+This doesn't work because the fragment F might itself embed other fragments of object type.
+
+**Inline and named fragments:** Another complication is that in each of these cases, the fragment might be inline (`... on T { ... }`) or defined (`...F` where `fragment F on T { ... }`).  The latter is useful for sharing code, whereas the former may be more preferable when we just want request fields from a particular type.  Note that inline fragments have no natural name of their own; in the embedding approach we'd need to generate one.  (This is extra awkward because there's no prohibition on having several inline fragments of the same type in the same selection, e.g. `{ ... on T { id }, ... on T { name } }`, so even adding the type-name doesn't produce a unique name.)
+
+**Pros of flattening:**
+
+- Makes for the simplest resulting types, by far; the fragment adds no further complexity to the types.
+- Especially a simplification when embedding abstract-typed fragments, since you don't have to go through an extra layer of interface when you already have a concrete type.
+- More efficient when multiple fragments spread into the same selection contain the same field: we need only store it once whereas embedding must copy it once for each fragment.  Also easier to use in the same case, since if you have both `val.FragmentOne.MyField` and `val.FragmentTwo.MyField`, you can't access either via `val.MyField`.  (Empirically in the Khan Academy codebase this appears to be quite rare.)
+- If you need to manually construct values of genqlient-generated types, flattening will be a lot easier, but I don't really recommend doing that.
+
+**Pros of embedding:**
+
+- Results in cleaner type-names, since each (named) fragment can act as its own "root" for naming purposes.  (In principle we could do this even when flattening, although it might read somewhat awkwardly.)
+- Much more usable for deduplication; if you spread the fragment in several places in your query you can write a function which accepts an `F`, and pass it any of the data.  (Again we might be able to get this benefit, awkwardly, when flattening by generating an interface corresponding to each fragment, assuming we are also rooting the type-names at the fragment.)
+- It's easier to tell what fields go where; the casework for abstract-in-abstract embeds gets somewhat complex.
+- Arguably the most philosophically faithful representation of the GraphQL types in Go.
+
+Note in principle we could apply some of those benefits 
 
-**Decision:** In general, it seems like the GraphQL Way, and perhaps also the Go Way is Option 1; I've always found the way shurcooL/graphql handles this to be a bit strange.  So I think it has to be at least the default.  In principle we could allow both though, since option 2 is legitimately convenient for some cases, especially if you are querying shared fields of interfaces or using fragments as a code-sharing mechanism, since option 1 handles those only through getter methods.
+**Decision:** There are pros and cons both ways here; in general it seems embedding is the most natural where your goal is deduplication, whereas flattening is best for inline fragments; for named fragments used only once there's maybe a slight benefit to flattening but it's not a big difference either way.  If we have to go with one or the other, probably flattening is better.  But I think the best thing, unless it turns out to be too much work to implement, is probably just to flatten inline fragments and embed named ones.  (In principle we could also have a flag flatten named fragments, if we find a need.)
 
 ## Configuration and runtime