lambda cube

content/posts/the-lambda-cube/DataKinds.hs

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+{-# LANGUAGE GADTs, DataKinds, KindSignatures #-}
+-- explicitly enable the warning that would say the pattern match below is
+-- nonexhaustive, to show it does not appear
+{-# OPTIONS_GHC -Wincomplete-uni-patterns #-}
+module DataKinds where
+
+-- Perhaps this could be a database enum, with the tag of the object, along
+-- with an untyped value field
+data SettingTag = TOne | TTwo | TThree
+
+-- GADT to staple a type to the return value of the 'decode' function below
+data Setting (tag :: SettingTag) where
+    SettingOne :: Int -> Setting 'TOne
+    SettingTwo :: Bool -> Setting 'TTwo
+    SettingThree :: String -> Setting 'TThree
+
+-- Typeclasses are functions from type to value (in this case, the value is a
+-- value-level function)
+class Decode (tag :: SettingTag) where
+    decode :: String -> Setting tag
+
+instance Decode 'TOne where
+    decode = SettingOne . read
+
+instance Decode 'TTwo where
+    decode = SettingTwo . read
+
+instance Decode 'TThree where
+    decode = SettingThree
+
+test :: Int
+test = let
+    SettingOne a = decode @'TOne "blah"
+    in a
+