Abstract

. Active patterns apply preprocessing functions to data type values before they are matched. This is of use for unfree data types where more than one representation exists for an abstract value: in many cases there is a specific representation for which function definitions become very simple, and active patterns just allow to assume this specific representation in function definitions. We define the semantics of active patterns and describe their implementation. 1 Introduction Pattern matching is a well-appreciated concept of (functional) programming. It contributes to concise function definitions by implicitly decomposing data type values. In many cases, a large part of a function's definition is only needed to prepare for recursive application and to finally lead to a base case for which the definition itself is rather simple. Such function definitions could be simplified considerably if these preparatory computations could be factorized and given in a separate place. Recognizing t...

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