i want a list of numerical values in my data type
data Polynomial = Polynomial {xs ::(Num a) => [a] } deriving (Show)
but i'm still getting this Error
error: Not in scope: type variable ‘a’
According to the Haskell wiki
In Haskell 98, only functions can have type constraints.
In order to do what you want, you can declare Polynomial with a type parameter. Then you write functions with the appropriate type constraints.
data Polynomial a = Polynomial xs
deriving (Show)
This allows you to construct concrete polynomial types such as Polynomial Int or Polynomial Float or even Polynomial String. A function which operates on your Polynomial type can declare constraints on the type parameter. For example a function to add two polynomials can have the following signature:
(+) :: (Num a) => Polynomial a -> Polynomial a -> Polynomial a
Rinse and repeat as appropriate for each function.
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