haskell understanding data type

Question

Assume that we have next data:

data Value =
IntVal Int
| BoolVal Bool

and function

f :: Value -> Int

How can I separate cases on different constructors on argument of type Value? So f (IntVal 1) has one behaviour and f (BoolVal True) has some another behaviour.

Answer 1

There are a few different methods a popular one is to use pattern matching in a function parameter.

negateValue :: Value -> Value
negateValue (IntVal  n) = IntVal  (-n)
negateValue (BoolVal p) = BoolVal (not p)

Another method would be to use case patterns.

Using case:

negateValue val = case val of
    IntVal  n -> IntVal  (-n)
    BoolVal p -> BoolVal (not p)