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classhaskelldeclaration

How to declare a class for a calculator?

I am trying to build my own calculator in Haskell with class declaration.

Firstly, I was trying to implement the addition, but here I got the error:

Couldn't match expected type 'Exp -> Double' with actual type 'Exp'

data Exp = Number Double | Add Exp Exp 


class Calc a where
 calculate :: a -> a -> Double

instance Calc Exp where
 calculate (Add a b) = a + b

What do I have to change?

Solution

  • You don't really need, nor would I recommend, a class here. Just define an ordinary function that handles each of the data constructors separately. (Use a class only if you expect to have multiple types that can be used with calculate.)

    In either case, calculate takes a single argument, as your type represents an entire expression, operands and operator together.

    calculate :: Exp -> Double
calculate (Number n) = ...
calculate (Add a b) = ...

    Keep in mind that a and b are values of type Exp, not numbers (i.e., types with a Num instance) that you can add directly. (This suggests you need to recursively calculate the value of a and b before you can actually perform addition.)

    Some test cases for you to consider:

    n1 = Number 2.0
n2 = Add (Number 3.0) (4.0))
n3 = Add n1 n2

map calculate [n1, n2, n3] == [2.0, 7.0, 9.0]