variants fragment

variants.hs

--------------------------------------------------------------------------------
-- Variants

-- NB: By the types here, you can't construct a @Variant NilL@.
data Variant xs where
  LeftV :: forall k a xs. a -> Variant (ConsL k a xs)
  RightV :: Variant xs -> Variant (ConsL k a xs)

-- Construction:
bar :: Variant (ConsL "String" Text (ConsL "Number" Double NilL))
bar = RightV (LeftV 123)

-- Case analysis:
foo :: Variant (ConsL "String" Text (ConsL "Number" Double NilL)) -> Text
foo = \v ->
  case v of
    LeftV text -> text
    RightV (LeftV double) -> Text.pack (show @Double double)