[Haskell-beginners] Filtering lists

[Daniel Fischer]

Am Dienstag 09 Februar 2010 11:53:00 schrieb 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,

Couldn't that at least be the first 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?

data Rec
    = A a1 a2 a3 a4
    | B b1 b2 b3 b4 b5 b6 b7 b8 b9 b10
      deriving (whatever)

isA :: Rec -> Bool
isA (A _ _ _ _) = True
isA _ = False

breakIntoFields :: String -> [[String]]
breakIntoFields file = map (splitOn ',') $ lines file

recordToRec :: [String] -> Rec
recordToRec (a1 : "A" : a2 : a3 : : a4 []) 
    = (A (read a1) (read a2) (read a3) (read a4))
recordToRec (b1 : "B" : ... )
    = (B (read b1) ...)
recordToRec _ = error "invalid record format"

records :: String -> [Rec]
records = map recordToRec . breakIntoFields

solution = Data.List.partition isA . records

>
> Thanks,
> Didier