Add "generic" option to the "optional" configuration for handling nullable types (#252)

This is an implementation for #251, it adds a new `"generic"` option for
the `"optional"` configuration, and a companion type
`"optional_generic_type"` which is a fully qualified type with a
placeholder `%` for the generic parameter.

Co-authored-by: Dylan R. Johnston <[email protected]>
Co-authored-by: Ben Kraft <[email protected]>

docs/CHANGELOG.md

@@ -24,6 +24,8 @@ When releasing a new version:
 
 ### New features:
 
+- The new `optional: generic` allows using a generic type to represent optionality. See the [documentation](genqlient.yaml) for details.
+
 ### Bug fixes:
 
 ## v0.6.0

docs/genqlient.yaml

@@ -109,8 +109,19 @@ use_extensions: boolean
 #   pointers-to-slices, so the GraphQL type `[String]` will map to the Go
 #   type `[]*string`, not `*[]*string`; GraphQL null and empty list simply
 #   map to Go nil- and empty-slice.
+# - generic: optional fields are generated as type parameters to a generic type
+#   specified by `optional_generic_type`. E.g. fields with GraphQL type `String`
+#   will map to the Go type `generic.Type[string]`. This is useful if you have a
+#   type that mimics the behavior of Option<A> or Maybe<A> in other languages like
+#   Rust, Java, or Haskell.
 optional: value
 
+# Only used when `optional: generic` is set. `example.Type` must be a fully qualified
+# generic type with only one generic parameter e.g. atomic.Value[string]. 
+# It must also implement the `encoding/json.Marshaler` and `encoding/json.Unmarshaler`
+# interface if you want it to serialize / deserialize properly.
+optional_generic_type: github.com/organisation/repository/example.Type
+
 # A map from GraphQL type name to Go fully-qualified type name to override
 # the Go type genqlient will use for this GraphQL type.
 #