Prelude says:
data Either a b = Left a | Right b
deriving (Eq, Ord, Read, Show)
^^^^
Interactive session:
Prelude> Left 3
ERROR - Cannot find "show" function for:
*** Expression : Left 3
*** Of type : Either Integer a
Test script:
> data Foo a b = Bar a | Baz b
> showFoo (Bar x) = "Bar: " ++ show x
> showFoo (Baz x) = "Baz: " ++ show x
Interactive session:
Main> showFoo (Bar 3)
ERROR - Unresolved overloading
*** Type : Show a => [Char]
*** Expression : showFoo (Bar 3)
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