Others have already pointed this out, but it is worth saying again:
Maybe is not the only monadic effect which makes sense during
pattern-matching. Wolfram Kahl and I have explored some of these things
as part of the Pattern Matching Calculus,
http://sqrl.mcmaster.ca/~kahl/PMC/
[If you want to jump to the most recent, most complete version, see
http://www.cas.mcmaster.ca/~kahl/Publications/TR/Kahl-Carette-Ji-2006b/]
Various other monads can be used for pattern-matching-effects. While
Maybe is quite classical, List and LogicT give extremely useful
alternatives.
Jacques
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