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copied!<p>This uses <code>GHC.Generics</code> to provide a <code>showsPrecDefault</code>, which can be easily used to define a <code>Show</code> instance.</p> <pre><code>data Person a = Person { name :: a, age :: Int } deriving Generic instance Show a => Show (Person a) where showsPrec = showsPrecDefault >>> Person "Alihuseyn" 20 Person "Alihuseyn" 20 </code></pre> <p>The definition of <code>showsPrecDefault</code> is below.</p> <pre><code>{-# LANGUAGE DeriveGeneric , FlexibleContexts , FlexibleInstances , KindSignatures , TypeOperators , TypeSynonymInstances #-} import GHC.Generics class GShow f where gshowsPrec :: Int -> f a -> ShowS instance GShow U1 where gshowsPrec _ U1 = id instance Show c => GShow (Rec0 c) where gshowsPrec p = showsPrec p . unK1 instance GShow f => GShow (D1 d f) where gshowsPrec p = gshowsPrec p . unM1 instance Constructor c => GShow (C1 c U1) where gshowsPrec _ c@(M1 U1) = showParen (isInfix c) (showString (conName c)) instance (Constructor c, GShow (M1 i c f)) => GShow (C1 c (M1 i c f)) where gshowsPrec = gshowsPrec' instance (Constructor c, GShow (f :+: g)) => GShow (C1 c (f :+: g)) where gshowsPrec = gshowsPrec' instance (Constructor c, GShow (f :*: g)) => GShow (C1 c (f :*: g)) where gshowsPrec = gshowsPrec' gshowsPrec' :: (Constructor c, GShow f) => Int -> C1 c f p -> ShowS gshowsPrec' p c@(M1 f) = showParen (p > 10) $ showParen (isInfix c) (showString (conName c)) . showChar ' ' . gshowsPrec 11 f isInfix :: Constructor c => t c (f :: * -> *) a -> Bool isInfix c = case conFixity c of Infix _ _ -> True _ -> False instance GShow f => GShow (S1 s f) where gshowsPrec p = gshowsPrec p . unM1 instance (GShow a, GShow b) => GShow (a :+: b) where gshowsPrec p (L1 a) = gshowsPrec p a gshowsPrec p (R1 b) = gshowsPrec p b instance (GShow a, GShow b) => GShow (a :*: b) where gshowsPrec p (a :*: b) = gshowsPrec (p + 1) a . showChar ' ' . gshowsPrec (p + 1) b showsPrecDefault :: (Generic a, GShow (Rep a)) => Int -> a -> ShowS showsPrecDefault p = gshowsPrec p . from </code></pre>

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