Haskell tuple concatenation function definition issue

I am trying to define a function that will concatenate two tuples of (In开发者_StackOverflow中文版t, 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?

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.

Good points all around so far, but I think its worth pointing out that YOUR FUNCTION CODE IS OK! You did, however, mess up the type signature. Its not so much that it doesn't match your function, but that Char is a concrete type, not a typeclass like Integral, Ord, Show etc.

Others have shared the matching type signature, I want to help you get there. Often a good, (at least) first step is to let Haskell determine the type for you.

Prelude> let tupleCat (x1, y1) (x2, y2) =(x1+ x2, [y1] ++ [y2])
Prelude> :t tupleCat
tupleCat :: (Num t) => (t, t1) -> (t, t1) -> (t, [t1])

This is a little more general than you wanted, but replace Num with Integral and t1 with Char and your there.

PS - the Monoid idea above is a great idea and allows a lot of higher order uses. But don't be worried if it confuses you, it would've thrown me for a loop pretty badly just a few months back.