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14 lines
419 B
Idris
14 lines
419 B
Idris
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interface Decidable p where
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decide : Either p (Not p)
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{m, n : Nat} -> Decidable (m === n) where
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decide = go m n where
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go : (m, n : Nat) -> Either (m === n) (Not (m === n))
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go 0 0 = Left Refl
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go 0 (S k) = Right (\case Refl impossible)
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go (S k) 0 = Right (\case Refl impossible)
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go (S k) (S j) = case go k j of
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Left eq => Left (cong S eq)
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Right neq => Right (\case Refl => neq Refl)
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