无法将预期的类型'Integer-> t'与实际类型匹配

Question

I'm trying to run this textbook example but I keep getting this error:

* Couldn't match expected type `Integer -> t'
              with actual type `[Char]'
* The function `showPerson' is applied to two arguments,
  but its type `People -> [Char]' has only one
  In the expression: showPerson "charles" 18
  In an equation for `it': it = showPerson "charles" 18
* Relevant bindings include it :: t (bound at <interactive>:38:1)

I don't understand why I'm getting this error. I'm inputting the correct types.
My code is:

type Name = String 
type Age = Int
data People = Person Name Age

showPerson :: People -> String
showPerson (Person a b) = a ++ " -- " ++ show b

Answer 1

Your showPerson function, as declared, has only one argument. This argument is of type People .

However, judging by the error you're quoting, you are trying to call this function with two arguments - "charles" and 18 . The first argument is of type String , second - of type Int .

This is what the compiler is trying to tell you when it says:

The function `showPerson' is applied to two arguments, 
but its type `People -> [Char]' has only one

To correctly call your function, you need to first create a value of type People , then pass that value to the function as argument:

p = Person "charles" 18
showPerson p

Or the same thing in one line:

showPerson (Person "charles" 18)