reflective: Add Reflection{} placeholder and the New() factory

Signed-off-by: Alejandro Mery <amery@jpi.io>

reflective/docs.go

@@ -0,0 +1,2 @@
+// Package reflective assists reflection-driven unmarshalling
+package reflective

reflective/error.go

@@ -0,0 +1,69 @@
+package reflective
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+)
+
+// An InvalidUnmarshalError describes an invalid argument passed to New.
+// (It must be a non-nil pointer.)
+type InvalidUnmarshalError struct {
+	Method string
+	Prefix string
+
+	reflect.Type
+}
+
+func (e *InvalidUnmarshalError) Error() string {
+	var buf bytes.Buffer
+
+	if e.Prefix != "" {
+		_, _ = fmt.Fprintf(&buf, "%s: ", e.Prefix)
+	}
+
+	method := e.Method
+	if method == "" {
+		method = "New"
+	}
+
+	_, _ = fmt.Fprintf(&buf, "%s(", method)
+
+	switch {
+	case e.Type == nil:
+		_, _ = buf.WriteString("nil")
+	case e.Type.Kind() != reflect.Pointer:
+		_, _ = fmt.Fprintf(&buf, "%s %s", "non-pointer", e.Type.String())
+	default:
+		_, _ = fmt.Fprintf(&buf, "%s %s", "nil", e.Type.String())
+	}
+
+	_, _ = buf.WriteString(")")
+
+	return buf.String()
+}
+
+// An UnmarshalTypeError tells something went wrong while processing
+// a type.
+type UnmarshalTypeError struct {
+	Prefix string
+	Err    error
+
+	reflect.Type
+}
+
+func (e UnmarshalTypeError) Unwrap() error {
+	return e.Err
+}
+
+func (e UnmarshalTypeError) Error() string {
+	var buf bytes.Buffer
+
+	if e.Prefix != "" {
+		_, _ = fmt.Fprintf(&buf, "%s: ", e.Prefix)
+	}
+
+	_, _ = fmt.Fprintf(&buf, "%s: %s", e.Type.String(), e.Err)
+
+	return buf.String()
+}

reflective/reflection.go

@@ -0,0 +1,27 @@
+package reflective
+
+import "reflect"
+
+// Reflection provides Marshalling/Unmarshalling oriented view
+// of a value
+type Reflection struct {
+	v reflect.Value
+}
+
+// New creates a Reflection of the given pointer
+func New(v any) (*Reflection, error) {
+	rv := reflect.ValueOf(v)
+
+	if rv.Kind() != reflect.Pointer || rv.IsNil() {
+		err := &InvalidUnmarshalError{
+			Type: rv.Type(),
+		}
+		return nil, err
+	}
+
+	r := &Reflection{
+		v: rv,
+	}
+
+	return r, nil
+}