In this paper, we report on our experience in teaching a course on concepts of programming languages at TU Delft based on Krishnamurthi's PAPL book with the definitional interpreter approach using Scala as meta-language and using the WebLab learning management system. In particular, we discuss our experience with encoding of definitional interpreters in Scala using case classes, pattern matching, and recursive functions; offering this material in the web-based learning management system WebLab; automated grading and feedback of interpreter submissions using unit tests; testing tests to force students to formulate tests, instead of just implementing interpreters; generation of tests based on a reference implementation to reduce the effort of producing unit tests; and the construction of a product line of interpreters in order to maximize reuse and consistency between reference implementations.