pwm · December 8, 2020 15:31 · Dec 8, 2020
diff --git a/A.hs b/A.hs
@@ -0,0 +1,32 @@
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE OverloadedLabels #-}
+
+module A where
+
+import Control.Lens
+import Data.Generics.Labels ()
+import GHC.Generics (Generic)
+import Prelude
+
+data Person = MkPerson
+  { name :: String,
+    age :: Int
+  }
+  deriving stock (Show, Generic)
+
+getName :: Person -> String
+getName p = p ^. #name
+
+getAge :: Person -> Int
+getAge p = p ^. #age
+
+dAdams :: Person
+dAdams = MkPerson "Douglas Adams" 42
+
+{-
+λ> getName dAdams
+"Douglas Adams"
+λ> getAge dAdams
+42
+-}
diff --git a/B.hs b/B.hs
@@ -0,0 +1,46 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE DuplicateRecordFields #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE OverloadedLabels #-}
+
+module B where
+
+import Control.Lens
+import Data.Generics.Labels ()
+import Data.Generics.Product
+import GHC.Generics (Generic)
+import Prelude
+
+data Person = MkPerson
+  { name :: String,
+    age :: Int
+  }
+  deriving stock (Show, Generic)
+
+data Group = MkGroup
+  { name :: String,
+    members :: [Person]
+  }
+  deriving stock (Show, Generic)
+
+_name :: (HasField "name" s t a b) => Lens s t a b
+_name = #name
+
+dAdams :: Person
+dAdams = MkPerson "Douglas Adams" 42
+
+greatWriters :: Group
+greatWriters =
+  MkGroup
+    { name = "Great writers",
+      members = [dAdams]
+    }
+
+{-
+λ> view _name dAdams
+"Douglas Adams"
+λ> view _name greatWriters
+"Great writers"
+-}