Support null literals as a value for arguments and input object fields

[mpenick]

graphql-go doesn't support null literals, but it's supported by the the graphql-spec: graphql/graphql-spec@3ce8b79.

There's an existing issue and PR for graphql-go:

• Support null literal graphql-go/graphql#178
• Add support for null literals graphql-go/graphql#401 (This PR is a bit rough and doesn't work fully)

Using the existing PR I've (mostly) fixed support for null: https://github.com/mpenick/graphql/tree/null_support

[mpenick]

This led me to wonder if there's a better graphql server implementation out there. The candidates (so far):

• https://github.com/graph-gophers/graphql-go
  • Doesn't support null: Support for Null Values in mutations graph-gophers/graphql-go#210
• https://github.com/samsarahq/thunder/tree/master/graphql
  • Doesn't support null: Uses graphql-go's parser:
    • Import: https://github.com/samsarahq/thunder/blob/master/graphql/parser.go#L8
    • Vendored version: https://github.com/samsarahq/thunder/blob/master/vendor/github.com/graphql-go/graphql/language/parser/parser.go#L659-L662

[mpenick]

I'm currently using a value:

type nullValue struct {}

// Used to detect the difference between a "null" literal and not present
var NullValue = nullValue{}

I'm not sure if using a value is better or worse than using a type type NullValue struct{}. I need to dig into this more.

Ideally, I'd like to use Go's nil instead and cleanup graphql-go's handling of nil values. My current impression is that using nil might only break using default values for input variables. I think this could be fixed though. "Not present" should be used for default values and explicit nil values should be used for null.

[mpenick]

The implementation for null support in graphql-go lives here: https://github.com/riptano/graphql-go and that implementation was added to the service in this PR: #9

Externally, the implementation uses nil as the representation for "null" in golang, and internally it differentiates between "not present" and "null" by using nil and a special value type nullValue, respectively.

inputs := map[string]interface{}{}

if input, ok := inputs["field1"]; ok {
	if input == nil {
		// Use nullValue{}
	} else {
		// Use value provided
	}
} else {
	// Not present, use `nil`
}