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Canonical Monoid instance & UndecidableInstances: Do I really need to turn on UndecidableInstances?
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| newtype CdRoot a = CdRoot (IO a) | |
| cdRoot :: CdRoot a -> IO a | |
| cdRoot (CdRoot x) = changeWorkingDirectory "/" >> x | |
| instance Semigroup a => Semigroup (CdRoot a) where | |
| CdRoot x <> CdRoot y = CdRoot $ x <> y | |
| -- The constraint ‘Semigroup (CdRoot a)’ is no smaller than the instance head (Use UndecidableInstances to permit this) | |
| instance (Semigroup (CdRoot a), Monoid a) => Monoid (CdRoot a) where | |
| mempty = CdRoot mempty | |
| mappend = (<>) |
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