F#. If you're working in .NET, you might also be interested in [PEX](http://research.microsoft.com/en-us/projects/pex/ http://research.microsoft.com/en-us/proje

F#. If you're working in .NET, you might also be interested in [PEX](http://research.microsoft.com/en-us/projects/pex/ http://research.microsoft.com/en-us/projects/pex/) - it can catch edge cases like these, automatically generate high coverage unit tests, etc. and integrates nicely into VS.