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📝 prj.

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Rob Rix 2019-10-08 15:08:37 -04:00
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semantic-tags/src/AST/Element.hs
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@ -8,6 +8,7 @@ import GHC.TypeLits (ErrorMessage(..), TypeError)
-- | Projection of an element out of a tree of sums, particularly suitable for use with highly branching (e.g. balanced) trees. -- | Projection of an element out of a tree of sums, particularly suitable for use with highly branching (e.g. balanced) trees.
class Element sub sup where class Element sub sup where
-- | Project one element out of a sum type.
prj :: sup a -> Maybe (sub a) prj :: sup a -> Maybe (sub a)
instance (Element' elem sub sup, elem ~ Elem sub sup) => Element sub sup where instance (Element' elem sub sup, elem ~ Elem sub sup) => Element sub sup where