Document and improve support for binding non-scalars to a specific type (#69)

## Summary:
We had this setting called "scalars", which said: bind this GraphQL type
to this Go type, rather than the one you would normally use.  It's
called that because it's most useful for custom scalars, where "the one
you would normally use" is "error: unknown scalar".  But nothing ever
stopped you from using it for a non-scalar type.  I was planning on
removing this functionality, because it's sort of a rough edge, but a
discussion with Craig found some good use cases, so instead, in this
commit, I document it better and add some slightly nicer ways to specify
it.

Specifically, here are a few potential non-scalar use cases:
- bind a GraphQL enum to a nonstandard type (or even `string`)
- bind an input type to some type that has exactly the fields you want;
  this acts as a sort of workaround for issues #14 and #44
- bind an object type to your own struct, so as to add methods to it
  (this is the use case Craig raised)
- bind an object type to your own struct, so as to share it between
  multiple queries (I believe named fragments will address this case
  better, but it doesn't hurt to have options)
- bind a GraphQL list type to a non-slice type in Go (presumably one
  with an UnmarshalJSON method), or any other different structure
The latter three cases still have the sharp edge I was originally
worried about, which is that nothing guarantees that the fields you
request in the query are the ones the type expects to get.  But I think
it's worth having the option, with appropriate disclaimers.

The main change to help support that better is that you can now specify
the type inline in the query, as an alternative to specifying it in the
config file; this means you might map a given object to a given struct,
but only in some cases, and when you do you have a chance to look at the
list of fields you're requesting.

Additionally, I renamed the config field from "scalars" to "bindings"
(but mentioned it in a few places where you might go looking for how to
map scalars, most importantly the error message you get for an unknown
(custom) scalar).  While I was making a breaking change, I also changed
it to be a `map[string]<struct>` instead of a `map[string]string`,
because I expect to add more fields soon, e.g. to handle issue #38.

Finally, since the feature is now intended/documented, I added some
tests, although it's honestly quite simple on the genqlient side.

## Test plan:
make tesc


Author: benjaminjkraft

Reviewers: csilvers, aberkan, dnerdy, MiguelCastillo

Required Reviewers: 

Approved by: csilvers

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: https://github.com/Khan/genqlient/pull/69

generate/config.go

@@ -65,13 +65,25 @@ type Config struct {
 	// TODO(#5): This is a bit broken, fix it.
 	ClientGetter string `yaml:"client_getter"`
 
-	// A map from GraphQL scalar type name to Go fully-qualified type name for
-	// the types to use for any custom or builtin scalars.  By default, builtin
-	// scalars are mapped to the obvious Go types (String and ID to string, Int
-	// to int, Float to float64, and Boolean to bool), but this setting will
-	// extend or override those mappings.  These types must define MarshalJSON
-	// and UnmarshalJSON methods, or otherwise be convertible to JSON.
-	Scalars map[string]string `yaml:"scalars"`
+	// A map from GraphQL type name to Go fully-qualified type name to override
+	// the Go type genqlient will use for this GraphQL type.
+	//
+	// This is primarily used for custom scalars, or to map builtin scalars to
+	// a nonstandard type.  By default, builtin scalars are mapped to the
+	// obvious Go types (String and ID to string, Int to int, Float to float64,
+	// and Boolean to bool), but this setting will extend or override those
+	// mappings.
+	//
+	// genqlient does not validate these types in any way; they must define
+	// whatever logic is needed (MarshalJSON/UnmarshalJSON or JSON tags) to
+	// convert to/from JSON.  For this reason, it's not recommended to use this
+	// setting to map object, interface, or union types, because nothing
+	// guarantees that the fields requested in the query match those present in
+	// the Go type.
+	//
+	// To get equivalent behavior in just one query, use @genqlient(bind: ...);
+	// see GenqlientDirective.Bind for more details.
+	Bindings map[string]*TypeBinding `yaml:"bindings"`
 
 	// Set to true to use features that aren't fully ready to use.
 	//
@@ -84,6 +96,16 @@ type Config struct {
 	configFilename string
 }
 
+// A TypeBinding represents a Go type to which genqlient will bind a particular
+// GraphQL type.  See Config.Bind, above, for more details.
+type TypeBinding struct {
+	// The fully-qualified name of the Go type to which to bind.  For example:
+	//	time.Time
+	//  map[string]interface{}
+	//	github.com/you/yourpkg/subpkg.MyType
+	Type string `yaml:"type"`
+}
+
 // baseDir returns the directory of the config-file (relative to which
 // all the other paths are resolved).
 func (c *Config) baseDir() string {