diff :: [(a,b)] -> [c]
diff [] = []
diff ((a,b):xs) = (a-b):(diff xs)

GHCI ERROR MESSAGES:

addprime.hs:3:19: error:
    • Couldn't match type ‘a’ with ‘c’
      ‘a’ is a rigid type variable bound by
        the type signature for:
          diff :: forall a b c. [(a, b)] -> [c]
        at addprime.hs:1:1-22
      ‘c’ is a rigid type variable bound by
        the type signature for:
          diff :: forall a b c. [(a, b)] -> [c]
        at addprime.hs:1:1-22
      Expected type: [c]
        Actual type: [a]
    • In the expression: (a - b) : (diff xs)
      In an equation for ‘diff’: diff ((a, b) : xs) = (a - b) : (diff xs)
    • Relevant bindings include
        xs :: [(a, b)] (bound at addprime.hs:3:13)
        a :: a (bound at addprime.hs:3:8)
        diff :: [(a, b)] -> [c] (bound at addprime.hs:2:1)
  |
3 | diff ((a,b):xs) = (a-b):(diff xs)
  |                   ^^^^^^^^^^^^^^^

addprime.hs:3:22: error:
    • Couldn't match expected type ‘a’ with actual type ‘b’
      ‘b’ is a rigid type variable bound by
        the type signature for:
          diff :: forall a b c. [(a, b)] -> [c]
        at addprime.hs:1:1-22
      ‘a’ is a rigid type variable bound by
        the type signature for:
          diff :: forall a b c. [(a, b)] -> [c]
        at addprime.hs:1:1-22
    • In the second argument of ‘(-)’, namely ‘b’
      In the first argument of ‘(:)’, namely ‘(a - b)’
      In the expression: (a - b) : (diff xs)
    • Relevant bindings include
        xs :: [(a, b)] (bound at addprime.hs:3:13)
        b :: b (bound at addprime.hs:3:10)
        a :: a (bound at addprime.hs:3:8)
        diff :: [(a, b)] -> [c] (bound at addprime.hs:2:1)
  |
3 | diff ((a,b):xs) = (a-b):(diff xs)
  |                      ^
Failed, no modules loaded.