You can use the the -ddump-deriv
GHC option to see the code for derived instances:
ghc -ddump-deriv test.hs
[1 of 1] Compiling Test ( test.hs, test.o )
==================== Derived instances ====================
Derived instances:
instance GHC.Show.Show Test.FooBar where
GHC.Show.showsPrec _ Test.Foo = GHC.Show.showString "Foo"
GHC.Show.showsPrec _ Test.Bar = GHC.Show.showString "Bar"
GHC.Show.showList = GHC.Show.showList__ (GHC.Show.showsPrec 0)
Generic representation:
Generated datatypes for meta-information:
Representation types: