Abstract: We examine the ML pattern-matching anomalies of useless clauses and non-exhaustive matches. We state the definition of these anomalies, building upon pattern matching semantics, and propose a simple algorithm to detect them. We have integrated the algorithm in the Objective Caml compiler, but we show that the same algorithm is also usable in a non-strict language such as Haskell. Or-patterns are considered for both strict and non-strict languages.

• Introduction
• Contents

• Patterns, values, etc.
• The useful clause problem
• Solving the useful clause problem
• Detecting the anomalies
• Lazy pattern matching
• Lazy pattern matching in theory
• Lazy pattern matching, à la Laville
• Pattern matching in Haskell
• Or-patterns in Haskell

• Specializing U for exhaustiveness check
• Specializing U for the useless clause problem
• Useless clause is (almost) enough
• Expansion of or-patterns
• Rules for finding  useless patterns
• Computation of useless patterns
• Performance
• Elements of complexity analysis
• Safeguards
• Measures
• Conclusion
• Acknowledgements
• References
• List of Citations
• Series of examples

This document was translated from L^AT_EX by H^EV^EA.