Rework error messages, see #45.

internal/util/reflect.go

@@ -0,0 +1,10 @@
+package util
+
+import "reflect"
+
+func ReflectType(v reflect.Value) reflect.Type {
+	if v.Kind() != reflect.Invalid {
+		return v.Type()
+	}
+	return nil
+}

internal/util/reflect_test.go

@@ -0,0 +1,21 @@
+package util
+
+import (
+	"fmt"
+	"math"
+	"reflect"
+	"testing"
+)
+
+func TestReflectType(t *testing.T) {
+	tests := []any{nil, 1, math.Pi, "abc"}
+	for _, tt := range tests {
+		t.Run(fmt.Sprint(tt), func(t *testing.T) {
+			want := fmt.Sprintf("%T", tt)
+			got := fmt.Sprintf("%v", ReflectType(reflect.ValueOf(tt)))
+			if got != want {
+				t.Errorf("ReflectType() = %v, want %v", got, want)
+			}
+		})
+	}
+}