[Haskell-beginners] Filtering lists

[legajid at free.fr]

Hi, 
i get a .csv file with two different record formats (too bad).
The first one, identified by a "A" in the second field, contains 4 fields.
The second one, identified by a "B", contains 10 fields.

In order to put them in lists, i use lines then break to get a list of lists.
Then i want to filter this list, one for the A list, the other one for the B list.
Unfortunately, the  pattern for the A type contains 4 fields; when a type B list occurs (10 fields), i get a message concerning the pattern : Non-exhaustive patterns in lambda

How can i separate the two record formats?

Thanks, 
Didier