WhichIsB.hs

class MyNum a where
    my_plus :: a -> a -> a
    my_fromInteger :: Integer -> a

class MyNum a => MyIntegral a where
    my_toInteger :: a -> Integer

my_fromIntegral :: (MyIntegral a, MyNum b) => a -> b
my_fromIntegral = my_fromInteger . my_toInteger

-- this is silly
plus :: (MyIntegral a, MyIntegral b) => a -> b -> a
plus a b = my_plus a (my_fromIntegral b)

plus2 a b = plus b (plus b a) -- reverse

result = plus2 1 2

{-

    • No instance for (MyIntegral b1) arising from a use of ‘plus2’
    • In the expression: plus2 1 2
      In an equation for ‘result’: result = plus2 1 2
   |
30 | result = plus2 1 2
   |          ^^^^^^^^^

bar.hs:30:16: error:
    • Ambiguous type variable ‘b0’ arising from the literal ‘1’


-}