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russmatney/lens-op-ob-composedviewexamples.hs

Created May 12, 2018 03:34
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lens-op-ob-composedviewexamples.hs
composedViewExamples :: IO ()
composedViewExamples = do
let
bob = User (UserName "bob") 42 Nothing HM.empty
fitzgerald = Pet (PetName "Fitzgerald")
jeff = User (UserName "jeff") 42 (Just fitzgerald) HM.empty
print "Bob's pet's name is: "
print $ preview (pet . _Just . petName) bob
-- Nothing
print $ bob ^? pet . _Just . petName
-- Nothing
print "Jeff's pet's name is: "
print $ preview (pet . _Just . petName) jeff
-- Just (PetName "Fitzgerald")
print $ jeff ^? pet . _Just . petName
-- Just (PetName "Fitzgerald")
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