Extract ID scalar implmentation

This change moves the ID scalar implementation out of `graphql.go` and
into its own file `id.go` for consistency with the Time scalar
implementation.

graphql.go

@@ -6,8 +6,6 @@ import (
 
 	"encoding/json"
 
-	"strconv"
-
 	"github.com/neelance/graphql-go/errors"
 	"github.com/neelance/graphql-go/internal/common"
 	"github.com/neelance/graphql-go/internal/exec"
@@ -21,27 +19,6 @@ import (
 	"github.com/neelance/graphql-go/trace"
 )
 
-// ID represents GraphQL's "ID" type. A custom type may be used instead.
-type ID string
-
-func (_ ID) ImplementsGraphQLType(name string) bool {
-	return name == "ID"
-}
-
-func (id *ID) UnmarshalGraphQL(input interface{}) error {
-	switch input := input.(type) {
-	case string:
-		*id = ID(input)
-		return nil
-	default:
-		return fmt.Errorf("wrong type")
-	}
-}
-
-func (id ID) MarshalJSON() ([]byte, error) {
-	return strconv.AppendQuote(nil, string(id)), nil
-}
-
 // ParseSchema parses a GraphQL schema and attaches the given root resolver. It returns an error if
 // the Go type signature of the resolvers does not match the schema. If nil is passed as the
 // resolver, then the schema can not be executed, but it may be inspected (e.g. with ToJSON).

id.go

@@ -0,0 +1,30 @@
+package graphql
+
+import (
+	"errors"
+	"strconv"
+)
+
+// ID represents GraphQL's "ID" scalar type. A custom type may be used instead.
+type ID string
+
+func (_ ID) ImplementsGraphQLType(name string) bool {
+	return name == "ID"
+}
+
+func (id *ID) UnmarshalGraphQL(input interface{}) error {
+	var err error
+	switch input := input.(type) {
+	case string:
+		*id = ID(input)
+	case int32:
+		*id = ID(strconv.Itoa(int(input)))
+	default:
+		err = errors.New("wrong type")
+	}
+	return err
+}
+
+func (id ID) MarshalJSON() ([]byte, error) {
+	return strconv.AppendQuote(nil, string(id)), nil
+}