Data.Profunctor.Product.Default

For some ProductProfunctors p and types a, a' there is a unique most sensible value of p a a'. Default exists to automatically generate that unique most sensible value for a product given unique most sensible values for the base types. If the unique most sensible values of type p a a', p b b' and p c c' are

sensible_a :: p a a'
sensible_b :: p b b'
sensible_c :: p c c'

then the unique most sensible value of type p (a, b, c) (a', b', c') is

p3 (sensible_a, sebsible_b, sensible_c)
     :: p (a, b, c) (a', b', c')
 

Therefore there is an instance

instance
  ( Default p a a'
  , Default p b b'
  , Default p c c'
  )
=> Default p (a, b, c) (a', b', c')
    where def = p3 (def, def, def)

which can be read as "if the unique most sensible values of types ... are ... then the unique most sensible value of the 3-tuple is given by composing them with p3". Naturally each different product type has a different composition function.