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 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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