Thank you very much for your reply! It is really helpful!

But I just found another 'problem', I just realize that the list does not support the user-defined data type?
the list is also depending on the Eq function?

For example,

data Shape = Square | Triangle | Circle

when I type either

[Square, Triangle, Circle]

This is perfectly legal, but GHCi won't be able to print it, because there is no Show instance for Shape. You can declare one:

instance Show Shape where
    show Square = "Square"
    show Triagle = "Triangle"
    show Circle = "Circle"

This can be generated automatically when you declare the type, by using:

data Shape = Square | Triangle | Circle
    deriving (Show)

or

Square == Square

Similarly, to use (==), you need an Eq instance, which can be defined much in the same way as the Show instance above (deriving also works on Eq -- don't generalize too hastily; not all classes work with deriving).

Luke