Evaluating Haskell expressions in a tutoring environment

A number of introductory textbooks for Haskell use calculations right from the start to give the reader insight into the evaluation of expressions and the behavior of functional programs. In fact, many programming concepts that are considered to be important by the functional programming paradigm, such as recursion, higher-order functions, pattern-matching, and lazy evaluation, can be explained by showing a step-wise computation. We think that students can get a better understanding of these concepts if they are trained to perform these evaluation steps on their own. We also think that tool support is lacking for experimenting with the evaluation of Haskell expressions. In this paper we present a prototype implementation of a step-wise evaluator for Haskell expressions that supports multiple evaluation strategies, and which is specifically targeted at education. Besides performing these evaluation steps, the tool can also diagnose steps that are submitted by students and provide feedback.