Question

I am trying to define a function that will concatenate two tuples of (Int, Char) like this:

tupleCat :: (Integral a, Char b )=> (a, b) ->  (a, b) -> (a, [Char])
tupleCat (x1, y1) (x2, y2) =(x1+ x2, [y1] ++ [y2])

However, I get the following error message:

Type constructor `Char' used as a class ...

What am I doing wrong?

Answer 1

Char is no type class, it is a type:

tupleCat :: (Integral a) => (a, Char) ->  (a, Char) -> (a, [Char])
tupleCat (x1, y1) (x2, y2) =(x1 + x2, [y1] ++ [y2])

And if you really want Ints and not an Integral, they are types, too:

tupleCat :: (Int, Char) ->  (Int, Char) -> (Int, [Char])
tupleCat (x1, y1) (x2, y2) =(x1+ x2, [y1] ++ [y2])

Further, you might consider to make this a new type and to implement the Monoid type class (as suggested in the comments). One possibility would be

newtype Cat = Cat (Int, String)

instance Monoid Cat where
  mempty = Cat (0, [])
  mappend (Cat (i1,s1)) (Cat (i2,s2)) = Cat (i1 + i2, s1 ++ s2)

With this definition tupleCat becomes simply mappend. Then you can e.g. concatenate the Cats in every Foldable (e.g. a list). Of course I don’t know your intentions, so this is just a educated guess.