Instance Read a

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Unfortunately — for historical reasons — you need to implement readsPrec:

λ> :t readsPrec
readsPrec :: Read a => Int -> ReadS a
λ> :i ReadS
type ReadS :: * -> *
type ReadS a = String -> [(a, String)]
        -- Defined in ‘Text.ParserCombinators.ReadP’

So, most likely:

instance Read Fruit where
  readsPrec _ c =
    let (s, r) = span (/= ' ') c
    in case c of
         "XYZ" -> Foo
         -- ⁝ got to buy groceries now

I wonder if that last catch-all case could come back to bite you in a creative read/show sequence in a nested data type.

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