Question

How can I find the type of a value in Haskell?

I want something like this:

data Vegetable = 
  Und Under
 |Abv Above

is_vegetable ::a->Bool  
is_vegetable a = if (a is of type Vegetable) then True else False

---

Update:

I want a datastructure to model the above tree.

I would also like to have some functions (is_drink, is_vegetable,is_wine,is_above) so that I can apply some filters on a list.

Answer 1

You cannot do this in Haskell. All function arguments have concrete types (like Int and String) or they are type variables (like the a in your example). Type variables can be restricted to belong to a certain type class.

When you use an unrestricted type variable, then you cannot do anything interesting with the values of that type. By restricting the type variable to a type class, you get more power: if you have Num a, then you know that a is a numeric type and so you can add, multiple, etc.

From your comment, it sounds like you need a (bigger) data type to hold the different types of elements in your tree. The Either a b type may come in handy here. It is either Left a or Right b and so you can have a function like

is_vegetable :: Either Vegetable Drink -> Bool
is_vegetable (Left _) = True
is_vegetable (Right _) = False

Your tree nodes will then be Either Vegetable Dring elements.