@hackage OrPatterns0.1

A quasiquoter for or-patterns. It allows one additional form for patterns:

f [o| p1 | p2 | p3 |] = rhs

Above, p1, p2 and p3 are three arbitrary patterns that bind the same variables. These variables are available in the expression rhs. Nesting of or-patterns is not supported yet.

See also:

• http://hackage.haskell.org/package/first-class-patterns supports \/ (or). However, variables bound with those patterns are not named. This means:

g :: Either (x, y) (y, x) -> (x, y)
g [o| Left (x,y) | Right (y,x) |] = (x,y)

-- ends up slightly longer
g = elim $ left (pair var var) \/ right flipped ->> (,)
 where
  flipped = (\(a,b) -> (b,a)) --> pair var var

• http://hackage.haskell.org/trac/ghc/ticket/3919 is the feature request for or-patterns in ghc