I can see why you’re a bit confused here, the question is poorly written. You can not use the Prelude filter function to do this, but you do need to write a function that filters and maps over the data to do this transformation (onlyDateTimes below is an example type signature for such a function).

mapMaybe is the sort of function you may to use for this purpose.

Here are some type signatures as a hint for one way to implement it:

dbDateTime :: DatabaseItem -> Maybe UTCTime

onlyDateTimes :: [DatabaseItem] -> [UTCTime]

There are of course other ways to implement onlyDateTimes such as directly using pattern matching and recursion, or using foldr, concatMap, etc. I would go with whichever method you’ve learned from the textbook so far. 

-bob

On Tue, Jun 2, 2020 at 09:06 Alexander Chen <alexander@chenjia.nl> wrote:
Hi,

given this:
import Data.Time

data DatabaseItem = DbString String
                  | DbNumber Integer
                  | DbDate   UTCTime
                  deriving  (Eq, Ord, Show)

theDatabase :: [DatabaseItem]
theDatabase =
    [ DbDate (UTCTime
             (fromGregorian 1911 5 1)
      (secondsToDiffTime 34250))
    , DbNumber 9001
    , DbString "Hello, world!"
    , DbDate (UTCTime
             (fromGregorian 1921 5 1)
             (secondsToDiffTime 34123))
    ]


question from textbook is : write a function that filters for DbDate values and returns a list of the UTCTime values inside them.


my question could you give me an example of a working function, I don't get how i use the filter function on a data type in a list. Hence i am kinda stuck.

thanks in advance.

best,


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