The lambdas must flow.
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Changed directory: /home/hugo/Academic/Teaching/2022.2/TCPF/Aulas/
λ> x = Par 1 2 
λ> y = Par 1 3
λ> x
Par 1 2
λ> y
Par 1 3
λ> x == y
False
λ> pi_a x
1
λ> :i (,)
type (,) :: * -> * -> *
data (,) a b = (,) a b
  	-- Defined in ‘GHC.Tuple’
instance Monoid a => Applicative ((,) a) -- Defined in ‘GHC.Base’
instance (Eq a, Eq b) => Eq (a, b) -- Defined in ‘GHC.Classes’
instance Functor ((,) a) -- Defined in ‘GHC.Base’
instance Monoid a => Monad ((,) a) -- Defined in ‘GHC.Base’
instance (Monoid a, Monoid b) => Monoid (a, b)
  -- Defined in ‘GHC.Base’
instance (Ord a, Ord b) => Ord (a, b) -- Defined in ‘GHC.Classes’
instance (Semigroup a, Semigroup b) => Semigroup (a, b)
  -- Defined in ‘GHC.Base’
instance (Show a, Show b) => Show (a, b) -- Defined in ‘GHC.Show’
instance (Read a, Read b) => Read (a, b) -- Defined in ‘GHC.Read’
instance Foldable ((,) a) -- Defined in ‘Data.Foldable’
instance Traversable ((,) a) -- Defined in ‘Data.Traversable’
instance (Bounded a, Bounded b) => Bounded (a, b)
  -- Defined in ‘GHC.Enum’
λ> :t fst
fst :: (a, b) -> a
λ> :t snd
snd :: (a, b) -> b
λ> (1,3) < (2,2)
True
λ> x' = Par' {pi'_a = 1, pi'_b = 2}
λ> x'
Par' {pi'_a = 1, pi'_b = 2}
λ> :t Par'
Par' :: a -> b -> Prod' a b
λ> :t pi'_a
pi'_a :: Prod' a b -> a
λ> :t pi'_b
pi'_b :: Prod' a b -> b
λ> x

<interactive>:37:1: error: Variable not in scope: x
λ> x'
Par' {pi'_a = 1, pi'_b = 2}
λ> pi'_a x'
1
λ>