Support constructor rules

It would be really great to be able to deconstruct something based on its type. For example in Haskell

``` haskell
 h :: Maybe Int -> Int
 h x = case x of
         Just n -> n * 100
         Nothing -> 0
```

Not sure it is possible tho as we don't have an universal method to access the value inside a type.

Maybe some kind of interface the type must implement ?
