Pattern Guards and Transformational Patterns

We propose an extension to patterns and pattern matching in Haskell. The idea is to o er the option to perform pattern matching in two steps: values that have already matched a pattern can be further scrutinized in guards. For this we introduce a natural generalization of guard-expressions to guard-quali ers. A frequently occurring special case is that a function should be applied to a matched value and the result of this is to be matched against another pattern. For this we introduce a syntactic abbreviation that is particularly useful when dealing with views. In addition to this, we can provide this new construct with additional power to explicitly catch pattern match failure. The proposal can be implemented with very modest syntactic and implementation cost. It is upward compatible with Haskell; all existing programs will continue to work. We demonstrate the usefulness of the proposed extension by several examples, in particular, we compare our proposal with views, and we also discuss the use of the new patterns in combination with equational reasoning