Dmitri Pissarenko wrote:
Hello!
I have a function activityIndicator, which has an argument of class Customer and should return a numeric value.
The module is defined as follows.
<module-definition> data Purchase = Purchase { price, rebate :: Double } deriving (Show, Eq)
data Customer = Customer { id :: Int, purchases :: [Purchase] }
activityIndicator :: Customer -> Num activityIndicator (Customer id purchases) = length purchases </module-definition>
What is wrong in the signature above?
'Num' is not a type. It is a type class. You can think of it as a set of types; it includes, for example, Int, Float and the other usual suspects. If you want a type in the class Num the signature looks something like this: Num a => a Which reads like "the type a, where a is in the class Num" However, the problem here is that length returns an Int; so your signature should be: activityIndicator :: Customer -> Int