The type signature […] lacks an accompanying binding

Question

Why do I end up with "binding" errors in the following program?

wheels :: Int

cars :: Int

carpark wheels cars   
  | wheels   == odd      = error wheels "is not even"
  | cars     <= 0        = error cars "is an invalid number"
  | (2*cars) >= wheels   = error "the number of wheels is invalid"
  | wheels   >= (4*cars) = error "the number of wheels is invalid"
  | otherwise            = "There are " (wheels-(2 * cars)) `div` 2 "cars and " (cars - ((wheels - (2 * cars)) div 2)) "motorcycles on the parking lot"

This is the error:

 aufgabe1.lhs:6:3:
The type signature for ‘wheels’ lacks an accompanying binding

aufgabe1.lhs:7:3:
The type signature for ‘cars’ lacks an accompanying binding

How can I get rid of it?

Answer 1

What are missing bindings?

There are many problems with your program, but lets focus on the "binding" one first. You're probably accustomed to Pascal or C, where you have to specify the type of the argument at the argument:

string carpark(int wheels, int cars);

However, Haskell doesn't work like this. If you write

wheels :: Int

in your document, you're telling the compiler that the value wheels will have the type Int. The compiler now expects a definition somewhere. This definition—it's binding—is missing. The type of wheels is known, but it's not known what value wheels should be bound to.

If you were to add

wheels = 1 * 2 + 12312

the compiler wouldn't complain about that particular binding anymore.

What's the actual problem?

As I've conclused above, you want to specify the arguments' types of carpark, right? However, this concludes that you specify carpark's type:

carpark :: Int -> Int -> String
carpark wheels cars 
   | -- omitted

This will get rid of the "missing bindings" errors.

What's missing?

Well, after this, you will still have a non-compiling piece of software, for example error wheels "is not even" isn't valid. Have a look at error's type:

error :: String -> a

Since wheels isn't a String, this won't compile. Instead, you have to show wheels:

error (show wheels ++ " is not even")

Note that error (show wheels) " is not even" will happily compile, but won't give you the error message you're actually looking for, so beware of parenthesis and string concatenation.

Exercises

• Write a function whatNumber that returns "Is Odd" if the number is odd and "Is Even" if the number is even, e.g.

  whatNumber 2 == "Is Even"
  

• Write a function whatNumberId, that returns "<x> is odd" if the number is odd and "<x> is even" if the number is even, where <x> should be the number, e.g.

  whatNumberId 123 == "123 is odd"
  

Both exercises should help you to accomplish your original task.