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Note the custom error message.

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Rob Rix 2019-10-08 15:40:23 -04:00
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commit 31ba6be9d8
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semantic-tags/src/AST/Element.hs
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@ -7,7 +7,7 @@ module AST.Element
import GHC.Generics ((:+:)(..)) import GHC.Generics ((:+:)(..))
import GHC.TypeLits (ErrorMessage(..), TypeError) 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. This provides custom type errors when the tree is known statically, rather than provided via a constraint passed on to callers.
class Element sub sup where class Element sub sup where
-- | Project one element out of a sum type. -- | Project one element out of a sum type.
prj :: sup a -> Maybe (sub a) prj :: sup a -> Maybe (sub a)