[Haskell-cafe] Signature of a function

[Dmitri Pissarenko]

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>

When I try to load this module into GHCi, I get this error:

<error-message>
    Class `Num' used as a type
    In the type: Customer -> Num
    While checking the type signature for `activityIndicator'
</error-message>

When I remove the line

activityIndicator :: Customer -> Num

the error disappears.

What is wrong in the signature above?

TIA

Dmitri Pissarenko