ds041.hs:1:14: warning:
    -XDatatypeContexts is deprecated: It was widely considered a misfeature, and has been removed from the Haskell language.

ds041.hs:16:7: warning: [-Wmissing-fields (in -Wdefault)]
    • Fields of ‘Foo’ not initialised: x
    • In the expression: Foo {}
      In an equation for ‘foo’: foo = Foo {}