Add UnmarshalJSON implementations. (#232)

This commit adds custom UnmarshalJSON implementations around the
SelectionSet type, so that it is now possible to directly deserialize an
appropriately-formed JSON object into an AST document struct.

Signed-off-by: Philip Conrad <philipaconrad@gmail.com>

Signed-off-by: Philip Conrad <philipaconrad@gmail.com>

ast/decode.go

@@ -0,0 +1,216 @@
+package ast
+
+import (
+	"encoding/json"
+)
+
+func UnmarshalSelectionSet(b []byte) (SelectionSet, error) {
+	var tmp []json.RawMessage
+
+	if err := json.Unmarshal(b, &tmp); err != nil {
+		return nil, err
+	}
+
+	var result = make([]Selection, 0)
+	for _, item := range tmp {
+		var field Field
+		if err := json.Unmarshal(item, &field); err == nil {
+			result = append(result, &field)
+			continue
+		}
+		var fragmentSpread FragmentSpread
+		if err := json.Unmarshal(item, &fragmentSpread); err == nil {
+			result = append(result, &fragmentSpread)
+			continue
+		}
+		var inlineFragment InlineFragment
+		if err := json.Unmarshal(item, &inlineFragment); err == nil {
+			result = append(result, &inlineFragment)
+			continue
+		}
+	}
+
+	return result, nil
+}
+
+func (f *FragmentDefinition) UnmarshalJSON(b []byte) error {
+	var tmp map[string]json.RawMessage
+	if err := json.Unmarshal(b, &tmp); err != nil {
+		return err
+	}
+	for k := range tmp {
+		switch k {
+		case "Name":
+			err := json.Unmarshal(tmp[k], &f.Name)
+			if err != nil {
+				return err
+			}
+		case "VariableDefinition":
+			err := json.Unmarshal(tmp[k], &f.VariableDefinition)
+			if err != nil {
+				return err
+			}
+		case "TypeCondition":
+			err := json.Unmarshal(tmp[k], &f.TypeCondition)
+			if err != nil {
+				return err
+			}
+		case "Directives":
+			err := json.Unmarshal(tmp[k], &f.Directives)
+			if err != nil {
+				return err
+			}
+		case "SelectionSet":
+			ss, err := UnmarshalSelectionSet(tmp[k])
+			if err != nil {
+				return err
+			}
+			f.SelectionSet = ss
+		case "Definition":
+			err := json.Unmarshal(tmp[k], &f.Definition)
+			if err != nil {
+				return err
+			}
+		case "Position":
+			err := json.Unmarshal(tmp[k], &f.Position)
+			if err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+func (f *InlineFragment) UnmarshalJSON(b []byte) error {
+	var tmp map[string]json.RawMessage
+	if err := json.Unmarshal(b, &tmp); err != nil {
+		return err
+	}
+	for k := range tmp {
+		switch k {
+		case "TypeCondition":
+			err := json.Unmarshal(tmp[k], &f.TypeCondition)
+			if err != nil {
+				return err
+			}
+		case "Directives":
+			err := json.Unmarshal(tmp[k], &f.Directives)
+			if err != nil {
+				return err
+			}
+		case "SelectionSet":
+			ss, err := UnmarshalSelectionSet(tmp[k])
+			if err != nil {
+				return err
+			}
+			f.SelectionSet = ss
+		case "ObjectDefinition":
+			err := json.Unmarshal(tmp[k], &f.ObjectDefinition)
+			if err != nil {
+				return err
+			}
+		case "Position":
+			err := json.Unmarshal(tmp[k], &f.Position)
+			if err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+func (f *OperationDefinition) UnmarshalJSON(b []byte) error {
+	var tmp map[string]json.RawMessage
+	if err := json.Unmarshal(b, &tmp); err != nil {
+		return err
+	}
+	for k := range tmp {
+		switch k {
+		case "Operation":
+			err := json.Unmarshal(tmp[k], &f.Operation)
+			if err != nil {
+				return err
+			}
+		case "Name":
+			err := json.Unmarshal(tmp[k], &f.Name)
+			if err != nil {
+				return err
+			}
+		case "VariableDefinitions":
+			err := json.Unmarshal(tmp[k], &f.VariableDefinitions)
+			if err != nil {
+				return err
+			}
+		case "Directives":
+			err := json.Unmarshal(tmp[k], &f.Directives)
+			if err != nil {
+				return err
+			}
+		case "SelectionSet":
+			ss, err := UnmarshalSelectionSet(tmp[k])
+			if err != nil {
+				return err
+			}
+			f.SelectionSet = ss
+		case "Position":
+			err := json.Unmarshal(tmp[k], &f.Position)
+			if err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+func (f *Field) UnmarshalJSON(b []byte) error {
+	var tmp map[string]json.RawMessage
+	if err := json.Unmarshal(b, &tmp); err != nil {
+		return err
+	}
+	for k := range tmp {
+		switch k {
+		case "Alias":
+			err := json.Unmarshal(tmp[k], &f.Alias)
+			if err != nil {
+				return err
+			}
+		case "Name":
+			err := json.Unmarshal(tmp[k], &f.Name)
+			if err != nil {
+				return err
+			}
+		case "Arguments":
+			err := json.Unmarshal(tmp[k], &f.Arguments)
+			if err != nil {
+				return err
+			}
+		case "Directives":
+			err := json.Unmarshal(tmp[k], &f.Directives)
+			if err != nil {
+				return err
+			}
+		case "SelectionSet":
+			ss, err := UnmarshalSelectionSet(tmp[k])
+			if err != nil {
+				return err
+			}
+			f.SelectionSet = ss
+		case "Position":
+			err := json.Unmarshal(tmp[k], &f.Position)
+			if err != nil {
+				return err
+			}
+		case "Definition":
+			err := json.Unmarshal(tmp[k], &f.Definition)
+			if err != nil {
+				return err
+			}
+		case "ObjectDefinition":
+			err := json.Unmarshal(tmp[k], &f.ObjectDefinition)
+			if err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}